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Endangered and Threatened Species of the Platte River 4 SCIENTIFIC DATA FOR THE PLATTE RIVER ECOSYSTEM Background discussions in Chapter 2 and the analyses of environmental law and science in Chapter 3 lead to an assessment of our understanding of the ecological foundations for threatened and endangered species in the Platte River Basin. The agencies of the U.S. Department of the Interior (DOI) have evaluated research into the habitat requirements of the whooping crane, piping plover, interior least tern, and pallid sturgeon for survival and recovery and have based their recommendations for instream flows and river management on the relevant studies. Critics question the studies. The charge to the committee regarding the Platte River ecosystem and its management generally takes the form of assessing the “scientific validity” of DOI decisions. For the purposes of this report, a management decision and the conclusions that led to that decision have scientific validity if the scientific knowledge that existed when the decision was made is identifiable and verifiable and was the product of research methods and techniques that were generally accepted by the scientific community at the time of the decision. The data and the information that resulted from processing them must be identifiable and archived so that they are recoverable by subsequent investigators if needed. The conclusions drawn and the theories and models used to understand and explain the conclusions must be verifiable, that is, subsequent investigators must be able to replicate the research and arrive at the same conclusions. The methods and techniques must be similar to those used by other workers in similar applications and be commonly found in the scientific literature or discourse of professional
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Endangered and Threatened Species of the Platte River meetings. The committee assessed the science used by DOI agencies as it existed when the agencies made their decisions. Since those decisions were made, there have been advances in science and engineering that may improve management. This chapter evaluates the types, relevance, and quality of science used by DOI to understand and manage the individual endangered species and the ecosystem associated with the river, and it assesses the validity of the science for policy decisions. The scientific basis of listing a species and designating critical habitat is discussed in Chapters 5, 6, and 7. This chapter begins with a consideration of how science is connected with the goals of restoration of the Platte River for the benefit of threatened and endangered species. It then describes the basic connections that sustain the Platte River ecosystem (including its hydrology and geomorphology) and the habitats important for its threatened and endangered species. Next, it addresses specifically the validity of the science underlying DOI decisions related to instream flows and ecosystem connections that managers use to preserve and enhance habitat for threatened and endangered species. The chapter concludes with some special scientific considerations for decision makers that have not yet been fully explored. SCIENCE AND MANAGEMENT TARGETS FOR SPECIES Management of the Platte River for the benefit of threatened and endangered species entails a preliminary decision that deeply involves science and the state of our knowledge. It is likely to require restoration of the physical system of the river, its hydrology and geomorphology, to create habitats useful for sustaining the species. Restoration in this sense implies managed and designed changes to alter the existing river to some other target condition. The target of restoration in most applications is the presettlement condition because those arrangements supported in relative abundance the species that are now endangered or threatened. It is rarely possible to completely attain such a restoration because of human effects such as land-use changes in the watershed and water-control infrastructure, but as a general objective the presettlement conditions represent the end-point of a spectrum of possibilities. There are two fundamental approaches to restoration: first, through knowledge of the presettlement conditions, and second, through knowledge of the present connections among physical systems, habitats, and species. In the case of the Platte River, restoration of the river to its prehuman or pre-European-settlement condition is faced with three issues: first, knowledge about prehistoric systems is sparse; second, they were always changing; and third, it is not possible to reconstitute them. First, knowledge about the prehuman ecosystem of the Platte River is highly limited by the lack of direct observations. Proxy measures of the
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Endangered and Threatened Species of the Platte River environmental conditions are sparse because of the general dryland nature of the northern Great Plains. Although geologic evidence can inform us about the long-term general environments and their adjustments on a scale of tens of thousands of years, the evidence is not detailed enough to reconstruct specific environmental conditions along reaches of the river a few kilometers long. These issues commonly face restoration efforts, and they are not unique to the Platte River case. We have considerable knowledge about the nature of the Platte River system’s general characteristics for the period between the arrival of early settlers and the twentieth century. By the time of official Government Land Office (GLO) surveys, the most exacting early assessments of landforms and vegetation that were conducted in the 1860s, substantial alteration of the vegetation by Europeans had already occurred. By the 1890s, the water system was also subject to human-induced adjustments. As a result, there are three distinct time periods of our knowledge base: the presettlement period for which we have little direct information during which the now listed species were abundant; the post-settlement period of the nineteenth century for which we have some knowledge during which the species were under considerable pressure, and the twentieth century period for which we have the most information when the species populations declined to very low levels. Restoration of the central and lower Platte River ecosystems to their presettlement conditions is not possible, even if the prehistoric target conditions could be specified. The central river and lower river are at the downstream end of a far-flung drainage basin and river system, and they exhibit characteristics that are determined by processes throughout the watershed. Land use and land cover are now substantially different from the prehistoric conditions, and the watershed hosts several large dams and many smaller control structures. Those features change runoff and stream flows and could not sustain a prehistoric reconstruction in the central and lower Platte River. A second approach to defining restoration goals (in addition to knowledge about undisturbed ancient conditions) is to establish the sorts of conditions that we know from research in present environments favor the threatened and endangered birds and fish but are also consistent with our knowledge of presettlement conditions. We can then create an environment that contains those conditions (Box 4-1). This approach has the advantage of working from an observable premise: the connections among river flows, geomorphology, vegetation, and wildlife. Those connections are complex and are not completely understood; but given our partial knowledge about them, restoration for species is possible. As this normative approach to restoration proceeds, corrections and adjustments, particularly in the flow regimes of the river, can provide for experimentation through adaptive management.
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Endangered and Threatened Species of the Platte River BOX 4-1 Restoration for the Future Platte River The primary route for managing the Platte River to benefit threatened and endangered species is to “restore” the river flows. The basic hydrological question, however, is to restore the flows to what target condition? One choice might be entirely “natural” flow regimes, such as ones that might have existed before the arrival of humans or at least before the imposition of numerous water-control structures that occurred after European settlement. It is not possible to return the flow regime of the Platte River to either of those “natural” models, for three reasons: knowledge, limitations of the present system, and social considerations. First, we lack detailed knowledge about the true nature of prehuman flow conditions, and we have only minimal understanding of flows before the construction of dams and diversion works. We can compensate somewhat for this lack of direct knowledge by application of theory. For example, we know how modern braided rivers work, and we know the prehistoric Platte River was a braided stream, so we can make some useful generalizations about the presettlement river. Second, the present hydrological system includes widespread changes in hydrology throughout the watershed. For example, widespread intensive grazing of many areas may or may not mimic grazing intensity and patterns of the original native animals that once roamed the region. Mountain watersheds now include storage reservoirs that did not exist in presettlement periods. Therefore, even if adjustments could be made for some “natural” target in the central and lower Platte River, those adjustments are not likely to be easily coupled with changed runoff and storage conditions upstream of those reaches. Finally, the water-control infrastructure of the river is in place as a result of public decision processes that sought to create the present hydrology, characterized by suppressed flood peaks and a dependable water supply even in dry months. A complete return to the presettlement flows would mean the abandonment of social and economic goals that have driven substantial investment in the system—an unlikely scenario. A more likely alternative for the target of flow restoration is the blending of objectives, whereby some flow characteristics benefit key wildlife species and attempt to mimic presettlement conditions to the extent possible. Other hydrological characteristics also remain in place to serve additional (such as social) needs. Such a compromise arrangement represents a more “normalized” flow regime that mimics natural rhythms, magnitudes, and durations but within constraints that recognize the changed nature of the basin and other competing economic demands on the water resource. In that way, restoration of the river flows is restoration part of the way toward the “natural” objective. Any restoration and management program will have to go forward with incomplete knowledge, but decision makers can use the best available scientific knowledge in guiding their choices. Researchers and decision-makers can provide education for a public that may expect higher levels of scientific certainty than are possible. Ecosystem research is not as controlled or exacting as a bench science, so its input to public decisions is accompanied by more uncertainty than is the case for laboratory sciences.
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Endangered and Threatened Species of the Platte River BASIC CONNECTIONS IN THE PLATTE RIVER ECOSYSTEM Issues defining the current conflicts in the central Platte River are related to seven interconnected elements: threatened and endangered species, other species (not regulated by the Endangered Species Act), flow of water through the river, sediment transported by that flowing water, groundwater, agriculture (and other human systems and uses), and riparian vegetation. Habitat is controlled by those elements, and they are intricately linked by connections that ultimately respond to stream flow and other ecological processes (such as fire, grazing, and human use). Water flowing through the river constitutes mass and energy, and changes in stream discharge trigger changes in sediment, morphology, and vegetation. The mass of water, for example, supplies sustenance for vegetation, and the energy that water expends as it descends through a channel contributes to erosion, transport, and redeposition of sediment. The energy expended by water flowing downgradient through the river performs geomorphologic work. It rearranges sediment, building such landforms as islands and bars attached to the banks and erodes previously existing forms. The significance of those features is that they are the foundations of vegetation and species habitats. Sediment, in turn, influences the morphologic characteristics of the channel. As outlined in Chapter 2, the central and lower Platte River have a braided channel, but the variety of forms is shaped by interactions between the volume of sediment to be transported and the water, with its accompanying energy, that is available for the work of transport. When the available energy is less than that required to transport the available sediment, deposition occurs, bars develop, islands expand, and beaches encroach into the channel area. When the available energy is sufficient to entrain and transport the available sediment that is temporarily stored in bars, islands, and beaches, erosion results, and these features become smaller. Riparian forests are influenced by abiotic ecological processes, such as the dynamics of water, sediment, and morphology of the river. Surfaces created by deposition are potential seedbeds for young plants. In the absence of fire and grazing, if flows do not sweep away or bury new plants and their substrate, the vegetation is likely to become relatively permanent and be in the form of a cottonwood and willow woodland. The resulting vegetation then has a feedback effect on the physical processes: the trees introduce hydraulic roughness to the channel and its nearby surfaces and thus induce additional deposition, which leads to higher floodplain surfaces that further influence vegetation. Under low natural flows, a broad, braided channel (with many subchannels) may be converted to a narrower system of fewer channels. That chain of events also can be reversed by higher natural flows, or clearing of vegetation, which can expose surfaces to energetic flows that might entrain sediment and erode the surface.
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Endangered and Threatened Species of the Platte River Those hydrological connections are particularly relevant for threatened and endangered species. DOI’s perspective on the connections is outlined by the U.S. Fish and Wildlife Service (USFWS, unpublished material, June 16, 2000). Whooping cranes prefer roost sites that include shallow water bars that are surrounded by deeper channels and that have long sight lines (unvegetated areas)—a set of conditions common along the Platte River of a century ago. Piping plovers and interior least terns prefer a habitat that has unvegetated areas with sandy surfaces exposed by receding river flows during the breeding season of late spring. If nests are to produce fledglings, they cannot be inundated by pulse flows. Pallid sturgeon appear to prefer streams with sandy bottoms for foraging and a series of annual flows that have natural fluctuations, with high spring flows and lower flows in late summer (USFWS, unpublished material, June 16, 2000). The causal chain of adjustments in the Platte River ecosystem begins with alterations of ecological processes, such as hydrology, and perhaps fire, grazing, and invasive species. Flows respond to two primary influences: climate and human regulation. Climatic influences through variation in the rainfall and snowpack that supplies river discharge change on time scales of a decade or more. Because much of the water flowing through the central and lower Platte River originates as precipitation over the Rocky Mountains, climatic changes in the mountainous areas are more relevant to flow in the central and lower Platte River than are climatic changes in Nebraska. Human influences on Platte River hydrology are long term and short term. Large storage dams in the upper reaches of the Platte River Basin through the 1900s changed river discharges by lowering flood peaks and by releasing more water during summer months than was released during the summers before the dams were built. Other major changes include the installation of diversion works on the river system that divert some or all of the flow during portions of the year. Water often returns to the channel from return canals or groundwater seepage. The short-term effects of human regulation of the river include management of water delivery from storage sites to downstream water-users, adjustments in flow to generate hydropower during periods of high demand for electricity, and scheduling of return flows. Groundwater, which is influenced by pumping and seepage from fields and unlined canals, also contributes to stream flow. Human-induced changes in the controlling flows of water in the Platte River Basin are larger and more important than climate-induced changes in controlling the Platte River ecosystem and habitats for endangered species. The storage reservoirs in the Platte River Basin store about 6 million acre-ft of water, and they have reduced flood peaks by 80% or more in the central Platte River (Figure 2-6). Those changes are at least half an order of magnitude greater than any changes envisioned for river systems as a result
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Endangered and Threatened Species of the Platte River of climatic change from global warming (Arnell 1996). Simpson (2003) observed a similarly important role for human activities in changing channel conditions on the South Platte River in Colorado. Although storage reservoirs, diversions, power houses, and return flows have apparently caused changes in habitats along the river, these same engineered features offer an opportunity for ecosystem restoration. If the water-control infrastructure can be operated in such a manner as to partly mimic the natural flows that once dominated the river and if other ecological processes can be re-established, it is likely that habitats will respond by reverting to conditions that are more like those preferred by the listed species (Murphy and Randle 2001). Given the magnitude of engineered changes and the economic value derived from them (such as flood control and water supply for agricultural or urban uses), it is not possible to completely restore predevelopment flows and the habitats they created. Bison will never again mass along the shores of the Platte. Even so, it is possible to partially recreate the presettlement conditions, given appropriate knowledge about the connections among the various subsystems that make up the Platte River ecosystem. In geographic locations where more extensive restoration is not possible, some management actions (such as wetland creation) may be necessary to complement restoration activities. In general, because habitat requirements of individual endangered species that use the central Platte are linked to more natural functions of the entire ecosystem, it is possible to pursue ecosystem restoration goals even when the presettlement conditions cannot be fully attained. In contrast with ecosystem restoration, simply combining habitat-management goals for individual species quickly leads to conflicting management options that may be mutually exclusive, especially when other species are affected. Management of songbird habitat as opposed to crane habitat is one contentious example that was illustrated in presentations to the committee. Compared with individual-species management, ecosystem-restoration approaches tend to be more stable, will include more (but not all) species successfully, and provide a larger geographic and temporal scale on which work can be accomplished. Restoration of conditions more similar to presettlement conditions than the present arrangements will be likely to benefit a wide range of species, including valuable waterfowl. Such restoration is also likely to result in more stable conditions and populations. If the hydrological regime is adjusted to become more like the original, natural regime with the influence of dams, native species are likely to be favored because they established communities in a river regime without dams. Often, regimes that are highly unnatural eliminate the advantages of native species, and invasive species are more successful. DOI research and investigations by others supported two types of management decisions targeted to maintain habitat and enhance benefits to the
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Endangered and Threatened Species of the Platte River listed species. First, DOI identified a series of operating rules for the flow of water in the river; these rules became the agency’s recommendations for instream flows. Recommended instream flows are important because they represent DOI’s vision of the kind of river hydrology that most directly affects habitat of individual species, particularly the movement of sediment, the forming of the channel, and periodic high or low flows. According to the DOI view, instream flows can benefit the threatened and endangered species by hydrologically creating or maintaining preferred habitat conditions. Second, DOI identified connections among water, sediment, morphology, and vegetation that predict how the Platte system may respond to recommended instream flows. Besides mechanical removal of vegetation and some wetland manipulations, DOI did not appear to consider restoring any ecological processes. Available Data on the Platte River Ecosystem One purpose of this report is to evaluate the scientific validity of DOI’s conclusions about what the instream flows should be and how they influence other aspects of the Platte River ecosystem. Part of that evaluation entails a review of the available data that DOI used to reach its conclusions. The following paragraphs assess the data available to DOI for its instream-flow and ecological research. Assessments of the data pertaining to the threatened and endangered species are presented in later chapters, where each species is considered in detail. The most important data used as input for explanations and predictions of Platte River habitat needs of the listed species are related to water, sediment, channel morphology, and riparian vegetation. The water-discharge data on the Platte River are from a set of gaging stations that have varied lengths of record (Figure 4-1). The gages with the longest records in the central and lower Platte River are on the mainstem Platte River above the Loup River confluence and near Duncan, Nebraska, where measurements began in 1890. Both records are discontinuous, so their value for analysis and modeling is diminished. The longest continuous record (1925+) for the central and lower Platte River is from gages on the river near Overton, Duncan (Figure 4-2), and Ashland, Nebraska. Data from a nearby location at Lexington extends the Overton gage record back an additional decade. As the twentieth century progressed, the U.S. Geological Survey (USGS) installed additional gages on the river; by 1955, there were six recording sites in the central and lower Platte River, three on local tributaries, and six more in the upstream reaches of the North and South Platte Rivers. Examination of the stream gage data for the central and lower Platte River shows that before 1925 measurements were sporadic and accom-
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Endangered and Threatened Species of the Platte River FIGURE 4-1 Periods of stream-flow record from gaging stations in Platte River Basin. (1) Includes four sites: Saratoga, WY; upstream of Seminoe Dam; upstream of Pathfinder Dam (historical); and downstream of current Pathfinder Dam site (historical). (2) Includes data from records for Camp Clarke (1896-1900), Mitchell (1901; 1907-1911), Scottsbluff (1912), Oshkosh (1929-1930), and Bridgeport (1902-1906; 1915-1928; 1931-1998). (3) Includes data from historical record for Platte River near Lexington, NE, before 1925. (4) Includes data from records for Platte River near Central City (1922-1927) and Columbus (1895-1914; June 15-Oct. 31, 1928). (5) Sum of Platte River at Columbus, Loup River near Genoa, and Loup River Power Canal. (6) Consists partly (Oct. 1, 1960-July 2, 1988) of record synthesized by combining Platte River at Louisville with Salt Creek near Ashland and Platte River at North Bend with Elkhorn River at Waterloo. (7) Includes data from record for Loup River at Columbus, NE, before Oct. 11, 1978, with Loup River Power Canal diversion added beginning Jan. 1, 1937. (8) Includes data from historical record for Elkhorn River at Arlington, NE. (9) Consists partly (Sept. 30, 1969-Dec. 31, 1998) of record synthesized by regression based on Salt Creek at Greenwood, NE. Source: Adapted from Stroup et al. 2001.
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Endangered and Threatened Species of the Platte River plished with techniques that would be considered unreliable today. In addition, the Platte River is a difficult stream to gage because its channel banks and bed are unstable; this complicates the calculations necessary to produce a stream-discharge measurement. That situation comes about because early calculations involved only two variables: channel width and depth of flow. Such an approach was incapable of taking into account the rapid changes in channel geometry that are common in sand-bed braided rivers. Methods became more sophisticated and discharge measurements more reliable after about 1925. The post-1925 stream-gage record is useful for supporting explanations of channel dynamics and interactions with sediment and vegetation, and the record documents short-term changes of a few years in duration. DOI analysts can specify with confidence the process connections between stream-flow and sediment transport, riparian vegetation, and inundation of various surfaces in and near the channel. Long-term changes in discharge are still difficult to specify, however. In searches of the stream-gage record for cyclic changes in discharge, for example, a common rule of thumb for hydrological analysis is to require a record that has 2-4 times the length of the suspected cycle; in this way, the cycle is repeated often enough in the record to be confidently identified. Hydrological adjustments to climatic changes occurring in cycles of several decades to a century, therefore, are not yet identifiable in the stream-flow record for the Platte River. The additional complications of variations in exchanges between surface water and groundwater are also difficult to discern in the stream-flow record. The long-term gaging data from the North Platte, South Platte, and Platte Rivers show that river discharge has changed (Murphy and Randle
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Endangered and Threatened Species of the Platte River FIGURE 4-2 Daily stream-flow record for Platte River near Duncan, Nebraska, showing length of record and variability of flow. Source: USGS 2003. 2003). The flow record can be divided into four periods, separated by the gaging record and installation of engineering structures in the system: 1895-1909, 1910-1935, 1936-1969, and 1970-1999. There was a steady decline in the mean daily discharge of the rivers at a variety of measurement points from 1895 to 1969. Since 1969, however, flows have increased (Table 4-1), probably because of transmountain diversions into the river from outside the basin and return flows from high groundwater. The increased annual discharges through the Platte River in the later parts of the record have not TABLE 4-1 Annual Mean Platte River Flows, cubic feet per second Gaging Station 1895-1909 1910-1935 1936-1969 1970-1999 North Platte River at North Platte, NE 3,190 2,750 646 862 South Platte River at North Platte, NE 582 492 322 619 Platte River at North Platte, NE 3,780 3,240 968 1,480 Platte River near Cozad, NE 3,550 3,020 461 981 Platte River near Overton, NE 3,660 3,160 1,140 2,100 Platte River near Grand Island, NE 3,580 2,950 1,080 2,110 Source: Randle and Samad 2003.
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Endangered and Threatened Species of the Platte River cies for stopover, resting, breeding, or feeding. The guidelines aid in connecting knowledge about species needs with management strategies designed to enhance or create habitat to support them. USFWS based habitat suitability guidelines on the best available information, even in the cases of the whooping crane and pallid sturgeon, for which available data were particularly sparse. USFWS’s specifications for suitable habitat relied on data collected by DOI personnel, research by them and other professionals, and wide-ranging expert opinion. Chapters 5, 6, and 7 provide complete reviews of the background literature on each of the species. In reaching its conclusions, USFWS relied on 54 pieces of published literature (including many standard reference books and articles in peer-reviewed journals) that form a convincing basis of habitat suitability findings. The agency incorporated several measurable physical-habitat elements—including depth of water, vegetation density, size of sandbars, and river-level variability (water-level fluctuation)—as physical measurable dimensions that are relevant to the listed species. The resulting habitat suitability guidelines offer an estimate of the degree to which particular physical environments can serve as viable habitat for a species. The guidelines can be broadly defined or sensitive, depending on how much is known about the selected species and the degree to which the physical environment can be codified or quantified. Habitat suitability guidelines for the listed species on the central and lower Platte River emerge from processes outlined by USFWS (unpublished material, June 16, 2000). USFWS defined specific optimal conditions for the whooping crane, piping plover, interior least tern, and pallid sturgeon beginning with the hydrological behavior of the river as related to habitats for the species. For example, in the case of cranes, USFWS identified common characteristics of known roosting sites, such as a wide channel, lack of forest vegetation, sandy substrate, unobstructed long views, and shallow water nearby. For the piping plover and interior least tern, USFWS deduced common characteristics among river-channel habitat, nesting sites, and feeding locations for each species. For the pallid sturgeon, USFWS identified favorable river conditions, including the presence of sandy bottoms, islands or bars, and sediment-rich waters. USFWS extended the earlier study by exploring restoration measures that could re-establish suitable habitat conditions where they had been lost. Additional Scientific Data Issues for Decision Makers In addition to the issues of interconnections among ecosystem components, instream flows, and habitat suitability, several issues remain to be addressed by DOI researchers: how species other than the listed ones that use the Platte River corridor are affected by management, alternative
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Endangered and Threatened Species of the Platte River approaches to measuring and managing the effects of water-control infrastructure on river flows and other ecosystem components, the physical implications of present restoration efforts, the other ecological characteristics of the system that need to be re-established, and evaluation of water-quality and climate-change issues. THE PLATTE RIVER AS AN ECOSYSTEM The Platte River ecosystem is one of the most diverse in the Great Plains, with an especially rich assemblage of vertebrates and higher plants. For example, 58 species of fish live in the central Platte River (Chadwick et al. 1997), and more than 300 species of vascular plants grow on the Platte’s floodplain (Currier 1982). About 50 species of birds nest in the Platte’s floodplain woodlands, nearly half of which are neotropical migrants, a group of birds in decline in other parts of their ranges (Robinson et al. 1995). Perhaps as many as several hundred bird species, including large numbers of waterbirds, use the Platte’s channel and woodland communities twice each year during transcontinental migration (Chapter 2). This bountiful biodiversity is maintained by substantial habitat heterogeneity on the Platte’s floodplain, adjacent wet meadows, and nearby agricultural fields. Heterogeneity in the river system is now controlled largely by the hydrological regime. The elements of USFWS’s target flows are based on perceived needs of the listed species. For example, a pulse flow is recommended to scour vegetation to provide more suitable sandbar habitat for terns, plovers, and cranes; higher spring and fall flows are proposed to favor whooping crane use during migration; and higher summer flow minimums are proposed to reduce the mortality of forage fish used by terns. However, developing a single set of target flows for one river to favor four very different species (one large migratory bird, two small nesting birds, and one large fish) that have divergent threats, different ecological optima, and different space-related and time-related uses of habitat comes with costs to other species (Clark and Harvey 2002). In addition, specific flow prescriptions that benefit one listed species may actually cause harm or preclude benefits to other listed species. For example, using more water from reservoir storage in fall for whooping cranes may reduce the opportunity to provide adequate spring pulse flows to build higher sandbars for tern and plover nesting. Criticism of species-focused approaches to develop flow targets for rivers has stimulated the development of alternatives. Most of them emphasize a holistic or ecosystem-level approach that involves returning rivers closer to their predevelopment hydrograph in an effort to regain natural stream-flow variability. Shifting the current hydrograph toward the “natural flow regime” should simultaneously benefit large numbers of native
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Endangered and Threatened Species of the Platte River riverine species adapted to preregulation conditions, including those currently protected. This approach should have a lower probability of unintended losses or shifts in biodiversity than reshaping the hydrograph on the basis of the expected response of a small number of rare species. A related approach to systemic river restoration is that of the “normative river” (Stanford et al. 1996). Normative habitat conditions are those established from what is possible in a natural-cultural context, as opposed to striving for pristine conditions. The primary goal is to regain as much as possible of the former structure of the hydrograph (peaks, pulses, base flows, and timing) given system constraints (storage capacity, water rights, and property damage). The difference between a normative-river approach and that currently proposed by USFWS for the Platte River is that the latter may produce a disarticulated target hydrograph that differs in fundamental ways from predevelopment river flows. USFWS recommended peak-flow target for the Platte, for example, is scheduled earlier than when most predevelopment peaks occurred, possibly to prevent flooding of interior least tern and piping plover nests. The primary advantage of the development of a normative hydrograph is that the ecosystem-based approach emphasizes the rebuilding of key physical processes and high biodiversity of the preregulated river. The Yakima River Basin Enhancement Project is working toward the establishment of normative flows for salmon (NRC 2002b); this project is a good example of a basinwide riparian restoration project that couples basic research with clear management objectives and stakeholder participation, and it may serve as an alternative approach to that recommended by USFWS for the Platte River. Another river-restoration project after which the Platte could be modeled is that of the Colorado River (Patten et al. 2001). The restoration of the Florida Everglades, which involves reversing the undesired effects of low dams and water-control structures, is based on the acknowledged first step, which DOI agencies label “Get the water right.” A similar overriding objective as a first step also fits well for the Platte River. In addition to the different outcomes of applying the PHABSIM-IFIM and normative models, the two approaches are underlain by fundamentally different philosophies. IFIM deals with the issue of connecting river flows to physical characteristics of the river one species at a time. It defines the flows needed to create and maintain habitat for one species, then begins again to define the flows needed by a second species, and so on. The problem with that approach is that it obscures the complexity of the real world and instead treats the river as a machine that can be analyzed and “tooled” for the benefit of each species in turn. The normative-flow regime, in contrast, has as its philosophical basis the view of the river as an integrated system. The approach is to control water flows and pulses in a manner that is as close to the “natural” (in the case of the Platte River, the
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Endangered and Threatened Species of the Platte River predam or pre-1909) discharges as possible. The normative approach recognizes that the existing river, its aquatic habitats, and its riparian systems are different partly as a result of river engineering (such as bridges) and partly as a result of upstream watershed changes. The idea underlying the normative approach is that if the more “normal” flows are restored, more “normative” habitats will logically follow, and these habitats will have a complexity sufficient to sustain a wide variety of species, including species that are endangered, threatened, and not listed. The committee recognizes that the DOI agencies have used IFIM because at the time of their decisions it constituted the best available science—a circumstance that lends credibility to their management decisions. To maintain that credibility, however, the DOI agencies must shift their approach to one based on the normative flow regime because it now (2004) constitutes the best available science. In a recent, authoritative review of the relationships among stream flow, sediment, and habitat from the physical science perspective, Pitlick and Wilcock (2001) conclude that “restoration efforts that focus on site-specific issues or single-species enhancement are likely to fall short of their objectives.” Similar sentiments come from the biological community (Poff et al. 1997). Poff et al. (2003) and Richter et al. (2003) provided the most recent statements on the normative-flow approach that can be employed on the Platte River. The normative-flow approach has also seen successful applications in river-restoration efforts in Australia and South Africa (Postel and Richter 2003). One important aspect of restoration of the Platte River concerns the woodland cover in and along the river. The Platte River presents a management conundrum. Riparian woodland established in the central Platte River between the 1930s and 1960s is now in its most productive and diverse stage and supports the majority of species on the floodplain. The clearing of large tracts of this woodland is recommended by USFWS to recover the target bird species. In contrast with other western U.S. rivers where river dewatering has stimulated the expansion of invasive trees of low wildlife value, the Platte’s woodlands are dominated by native species, primarily cottonwood and willow, with high wildlife value. Because riparian areas are positioned at the convergence of terrestrial and aquatic ecosystems, they are hotspots of biodiversity and exhibit high rates of biological productivity in marked contrast with the larger landscape (NRC 2002a). USFWS has chosen a conservation strategy that includes the removal of riparian woodland in the Platte River to produce more open-channel habitat for the three listed bird species (Figure 4-18). The clearing of wooded islands followed by periodic disking and mowing to keep vegetation short was begun in about 1980 by conservation organizations that used privately raised funds (Lillian Rowe Sanctuary–Audubon Society) and trust-fund earnings (Platte River Whooping Crane Trust). In addition, many wooded
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Endangered and Threatened Species of the Platte River FIGURE 4-18 Cleared area along central Platte River. Removal of woodland cover designed to improve long sight lines and open areas to benefit whooping cranes. Source: Photograph by W.L. Graf, May 2003. islands were removed by being bulldozed to riverbed level. The practice has been effective in attracting larger flocks of roosting sandhill cranes (Faanes and LeValley 1993). Clearing was expanded more recently through expenditure of large amounts of public funds primarily from the Partners in Wildlife Program (a landowner-federal cost-sharing program) and the requirement that power companies clear woodlands to acquire new operating licenses. Exact measures are not available for the extent of woodland clearing in the restoration of the Platte River under present plans. As much as one-third of the 45-mile long intensive management segment may be cleared, with additional areas cleared by Partners in Wildlife. The context of this clearing is that it is focused in a particular segment of the 310-mile river. Clearing smaller parcels for Partners in Wildlife projects and private duck blinds may amount to 25% of the woodland in the less intensively managed river sections. Clearing as a restoration strategy has both beneficial and adverse outcomes. The adverse consequences of clearing include loss of wooded nesting and migratory habitat for many species of songbirds; proliferation of invasive purple loosestrife due to soil disturbance and chopping of mature plants; possible oversupplying of sediment to downstream reaches, a cause
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Endangered and Threatened Species of the Platte River of woodland expansion; loss of cottonwood and willow ingrowth needed to replace future senescent stands (cottonwood and willow are pioneer species that do not reproduce in established woodlands); and reduction in patch size of remaining woodlands. The expected beneficial consequences of clearing wooded areas include improved habitat for the three federally listed bird species of the central Platte as well as many other bird species that require more open habitat (Chapter 2, Appendix B). Other less defined benefits include re-establishing lost or reduced ecological processes that may be important to the proper function of a more natural river system. Restoration sometimes focuses on benefits to the threatened and endangered species, but unintended detrimental consequences for other species ought to be minimized. In the case of the central Platte River, clearing of woodland to improve habitat for whooping cranes, for example, entails removal of forest environments for some songbirds. The “cost” to songbirds versus the “benefit” to cranes has not been carefully studied or determined. The effects of woodland clearing on other bird species are discussed in Chapter 2. No quantitative assessments of the response of the listed bird species to clearing over the last 2 decades have been published, but some qualitative patterns are apparent. Sandhill cranes concentrate in wide, unobstructed channels, including areas that have been cleared. For example, in 1999, 66% of the night roosts for about 300,000 sandhill cranes were on reaches of river where forests had been removed (Platte River Whooping Crane Maintenance Trust 1999). The proportion of the whooping crane population that stops on the Platte River during its spring and fall migrations has increased since 1976 (Chapter 5); when cranes stop on the Platte, they often settle near or in cleared areas. In contrast, tern and plover populations have declined steadily in the central Platte since the late 1980s despite increasing cleared area. The reported downstream shifting of sandhill crane populations from the upper to the central Platte in the last 25 years (Faanes and LeValley 1993) has been attributed to upstream channel narrowing. Whooping cranes have also shifted their use patterns to the east (Stehn 2003). The channel area in the upper Platte has increased steadily since the 1950s (although not necessarily in the areas that the cranes have used or to the degree that roosting cranes will find useful), so there may be other factors in the downstream shifting. Other factors include the clearing projects that attract cranes, and the proximity to abundant supplies of waste corn located near suitable roost sites. The issue of shifting use patterns needs more study and analysis. Few data have been collected in vegetation removal projects that illuminate the effectiveness of clearing. Thus, exactly what is lost and gained through woodland removal is often poorly known. Studies were initiated in the late 1990s at Cottonwood Ranch and Jeffrey Island to monitor the effects of clearing on vegetation, sediment supply, and channel structure. Several years of preclearing measurements, including an inventory of plants
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Endangered and Threatened Species of the Platte River and birds, were made before the first clearing began in winter of 2003. It may be 5-10 years before results will be available from these studies to evaluate the practice of clearing. General studies are needed to develop broad standards to define the expected benefits of clearing to open-channel species; to assess the relative success of past clearing; to propose and test alternative woodland-removal methods; and to develop effective approaches to habitat management that best support ecological restoration to the extent possible. Historical data indicate a range of tree densities along the central and lower Platte River before settlement (see cover painting of this book). Therefore, when viewed as a whole, the river in its entirety should reflect a variety of tree densities. Approaches to restoration in the central Platte have been under way for over 2 decades. Thus far, crane populations have increased during this period, and there is strong evidence that they followed cleared areas for roosting purposes (Faanes and LeValley 1993). Other factors, including availability of upland food sources, may also influence this geographic distribution of birds but to a lesser extent than does roost site availability (Su 2003; Iverson et al. 1987). Management has not stimulated increases in piping plover or interior least tern populations, and their numbers on the Platte have continued to decline. Restoration efforts have also benefited other waterbird species but are likely to have affected woodland species adversely. Restoration of the central and lower Platte River in the future can provide a context for a biologically diverse ecosystem that includes a variety of gradients from forest to open areas. In an adaptive management approach, the maintenance of some woodland along with the open areas will permit an assessment of the effects of restoration on all the species in the ecosystem. A monitoring system is essential to the success of such an ecosystem restoration. The management of the central and lower Platte River through a partially restored flow regime (a normalized flow pattern) and reduced forest cover in some locations may cause a reduction in some native vegetation species (such as red cedar and hackberry) and some nonnative species (such as honeysuckle and buckthorn). Management of the ecosystem to benefit particular fauna inevitably will affect some species adversely, but the objectives of adaptive management, with its constant monitoring and redefinition of strategies, can minimize the unwanted effects. This approach will allow exploration of the responses of a variety of species to the managed changes and will provide an opportunity to learn more about the role of control factors other than the river, such as fire, predators, and human activities. SUMMARY AND CONCLUSIONS This section summarizes the committee’s observations and conclusions about DOI’s approach to understanding the physical processes and forms
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Endangered and Threatened Species of the Platte River that underlie the habitats of threatened and endangered species in the Platte River Basin. It begins with a brief summary of the origins and major points in DOI’s approach to river-flow management and habitat connections and then presents the committee’s observations and suggestions. The section concludes with a list of specific recommendations. USFWS has developed instream-flow recommendations through literature reviews, field observations, data collection and analysis, numerical modeling, workshops, and other approaches. Those processes and methods are scientifically valid, and the techniques applied in the Platte River continue to be used for many other rivers. DOI-recommended flow values appear reasonable, but their effects on this river system require further analysis based on empirical data collection and field observations. USFWS has already expended a great deal of effort to develop an effective flow-management plan, and more investigations are planned. According to USFWS (2002b), “these flow recommendations are intended to achieve the flow-dependent goal of rehabilitating and maintaining the structure and function, patterns and processes, and habitat of the central Platte River Valley ecosystem.” Four types of flow recommendations were made: for species flows, annual pulse flows, peak flows, and program target flows. The values of the species flows for dry, normal, and wet years were based on a consultation process initiated in the 1980s and concluded with the discussions in the March 8-10, 1994, workshop, summarized by David Bowman (1994). The values of the annual pulse flows and peak flows for dry, normal, and wet years were presented by Bowman and Carlson (1994) and based on a workshop held on May 16-20, 1994. Flow values for the Platte River were based on expert opinions summarized in the two reports. According to USFWS (2002b), target flows are the discharges that the program activity seeks to establish through the water-control infrastructure to alter magnitude and timing of flows. The central Platte River contains both meandering and braided reaches that represent complex hydrological and geomorphic conditions. The current state of knowledge is insufficient to predict its precise morphologic change due to flow volumes with confidence, and a great deal of field observation is needed to support the analysis. The analysis should experiment with series of flows designed to meet the variety of requirements related to vegetation growth, channel maintenance, sediment mobility, and ecosystem stability. It is essential that the field data be collected and analyzed to evaluate the actual effects of USFWS-recommended flows on the central Platte River, should they be implemented. Monitoring river behavior requires careful design of field data collection. Specific data-collection characteristics—location, timing, goals, and level of detail—should be planned well before the occurrence of the targeted flow events. Field data can be expensive to collect, but timing is
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Endangered and Threatened Species of the Platte River important because large flows are highly uncommon. If a research program does not collect data during and after a large flow, the next opportunity may not occur for many years. Perhaps USFWS could use its workshop technique to discuss various field needs of different disciplines. DOI flow recommendations would be most helpful if they were evaluated for suitability for listed and other regionally important species. Regarding DOI interpretations of the interrelationships among flow, sediment, morphology, and vegetation, the committee has two sets of observations, one concerning the channel and history work by Murphy and Randle (2003) and the other concerning the sediment and vegetation work being undertaken by Murphy et al. (2001). First, undoubtedly, there are strong relationships among sediment, flow, vegetation, and channel morphology. Flow is also directly related to climate, water needs, reservoir storage, and diversion. Since the construction of the first large dam (Pathfinder in 1910), various water-resources developments have altered flow distribution and water consumption substantially, especially in the upper Platte River system. Active discussions between the Environmental Impact Statement team and the Parson team occurred in our August committee meeting in Grand Island, Nebraska, about the relative importance of climate and the water-resources developments for the change of river characteristics (for examples see Parsons 2003; Lewis 2003; Woodward 2003; Yang 2003; Murphy and Randle 2003). Water-resources developments have diverted and returned flows into the North Platte River, South Platte River, and upper Platte River. Murphy and Randle (2003) have estimated consumptive uses of water, including sewage, and evaporation of reservoir water. Because the return flows from diversions (except Kearney Canal return) occur upstream near Overton, Nebraska, the relative flow effect of water-resources development is considerably greater on the upper Platte River than on the central Platte River. Regardless of climate change, water-resources development will continue to affect Platte River flows as long as there is a net irrigation water consumption and reservoir evaporation. The human controls on flows in the river are the most important controls on a daily, monthly, or annual basis, but the longer-term effects of climate change are a background control worthy of further investigation. The mathematical modeling by Murphy et al. (2001) has some shortcomings that will challenge DOI investigators. Some of the required data are unavailable, and some of the modeling techniques are still in the development stage. The success of a numerical model depends on knowledge of flow roughness in the flow-momentum equation and of sediment transport rate in the sediment-continuity equation. Methods in field data collection to improve that knowledge have yet to be fully developed. The relationships of
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Endangered and Threatened Species of the Platte River lateral variations of flow properties among the different subsections in a braided river are also difficult to determine. There are four dimensions in any river system: the longitudinal direction, which is the main flow direction; the lateral direction, across the channel into the floodplain; the vertical direction, including surface-groundwater exchanges; and time. In a one-dimensional model, we investigate only the change in flow properties in the longitudinal direction with time and space by assuming no change in the other dimensions. Thus, in a strictly one-dimensional model, the lateral cross section is uniform because we assume no change of flow in the lateral direction. In a braided channel, a river cross section (perpendicular to the main flow direction) may include two flow channels with an island in between. Thus, Murphy et al. (2001) use a pseudo-one-dimensional model to include the possibility of having a channel cross section with nonuniform shapes. That approach is generally accepted, and many models use different schemes to represent sediment and lateral flow distribution among the various lateral cross sections. Several unproven assumptions have been used for the lateral distributions of flow and sediment in the current pseudo-one-dimensional model. The vegetation resistance should be determined from the field data instead of from other references. Some of the longitudinal intervals between two cross-sectional stations are too long to yield any reliable hydrogeomorphic relationships. If properly calibrated and validated, this model can give qualitative impressions of sediment and flow analyses, including the evaluation of the effect of vegetation removal and management. Only a few two-dimensional mathematical models that can include sediment movements are under development. Two-dimensional models have limited application because two-dimensional flow data are often unavailable to calibrate them. The committee recognizes six approaches for potential improvement of DOI investigations into ecosystem dynamics on the central and lower Platte River: Field data collection and methods for the monitoring of the effects of various flow recommendations and mechanical removal of vegetation must be carefully designed long before the occurrence of the targeted flow events or vegetation manipulations. Some kinds of data are also essential for calibrating and verifying the mathematical model. A risk-based hydrological model should be explored with various penalty functions (water not diverted to users as previously has been the case) at the water-demand points for optimization analysis of the flow-management plan of the river system. The effects of mechanical removal of vegetation should be included in the flow-management plan.
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Endangered and Threatened Species of the Platte River For more detailed analysis, variations in flow velocity and flow depth are more important than flow discharge for evaluating ecological requirements because it is the depths and velocities that create habitats. Climate and water-resources developments can have strong influences on river flow distributions. Water-resources development affects river flows substantially in the upper Platte River, and its effects extend to the central Platte River. The relative importance of climate influences and water-resources development on channel characteristics should be analyzed and should encompass a record of several decades. Restoration of the central Platte River should include water processes and forms, control of invasive species, and some grazing and fire if research shows these phenomena to be important aspects of the pre-European river. More emphasis should be placed on the management of the Platte River as an ecosystem, rather than keeping the focus exclusively on listed species. In summary, the committee’s review of DOI’s efforts to explain and model the connections among ecosystem components of water, sediment, morphology, and vegetation leads us to conclude that these efforts are underlain by valid science. Likewise, DOI’s instream-flow requirements are grounded in scientific understanding of the system and in the technology of model construction that was state-of-the-art when the decisions and recommendations appeared. Science and engineering are making progress, however, and new technology is becoming available. New advances are needed because of the braided, complex nature of the Platte River, a configuration that is unlike that of the streams where others often apply the models. Current DOI model developments, including the emerging SEDVEG model, are likely to be helpful and useful in both understanding and managing the Platte River. DOI’s determination of suitable habitat rests on the best available science. The committee also recognizes, however, that there has been no substantial testing of the predictions of DOI’s modeling work,1 and we urge that calibration of the models be improved and that monitoring of the effects of recommended flows and vegetation management be built into a continuing program of adaptive management. In such a system, monitoring can indicate whether recommendations and determinations are valid and can suggest further adjustments to the recommendations and determinations on the basis of observations. 1 The committee did not consider USGS’s in-progress evaluation of the models and data used by USFWS to set flow recommendations for whooping cranes.
Representative terms from entire chapter: