Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 32
Endangered and Threatened Species of the Platte River 2 REGIONAL CONTEXT FOR WATER AND SPECIES The validity of science for decision-making related to threatened and endangered species of the Platte River Basin depends on a fundamental understanding of the resources of the region, the nature of science, and the policies developed to address species concerns. Any consideration of environmental concerns must begin with the recognition that various natural processes and human activities interact to create complex conditions in the environment. The landscape and resources of the central Platte River Basin are no exception, and connections among land, water, wildlife, and human activities have shaped the present conditions. Understanding the central Platte ecosystem requires knowledge of habitat needs of the species and of the complexity of their interactions with natural and human controls. This chapter provides an overview of the central Platte region—its geography; human history; hydrological, geomorphic, and vegetation changes; and population trends in key groups of organisms. The overview in this chapter identifies several key issues that are threads through later chapters: interactions among components of the hydrological cycle, including surface water and groundwater; river behavior, including floods, droughts, and annual flow patterns; climatic and human influences on environmental change; and complex connections between the physical environment and vegetation and wildlife.
OCR for page 33
Endangered and Threatened Species of the Platte River GEOGRAPHY The Platte River Basin has three distinct geographic sub-basins: the North Platte, the South Platte, and the Platte River Valley of Nebraska. The North Platte River rises in the Medicine Bow Mountains of northern Colorado and flows northward and then eastward through Wyoming; it continues eastward into western Nebraska near Scottsbluff. The South Platte River originates in the Front Range of the Rocky Mountains around the valley of South Park, Colorado, and flows northeastward past Denver; it leaves the state at the northeast corner and joins the North Platte in west central Nebraska. The combined streams form the Platte River, which flows generally eastward through Nebraska; its valley has a distinctive southward bend, sometimes referred to locally as the “Big Bend.” The Platte River joins the Missouri River south of Omaha at Plattsmouth and is its largest tributary (Figure 2-1). For the purposes of this report, the central Platte River is the segment of the river extending from Lexington, Nebraska, downstream (eastward) to Columbus, Nebraska, although most of the interest related to habitat areas for the listed birds extends only to Denman, Nebraska (Figure 2-1). Also for the purposes of this report, the lower Platte River is the segment of the river from Columbus to the confluence with the Missouri River (Figure 2-1). The points along the Platte River that define those segments are related to FIGURE 2-1 General location and places of interest in Platte River Basin, including its position across 100th meridian. Central Platte designated in yellow, lower Platte in Orange. Source: Adapted from DOI 2003.
OCR for page 34
Endangered and Threatened Species of the Platte River the modified hydrology and geomorphology of the river. The upstream boundary of the central Platte River is the Johnson-2 (also known as J-2) return (the point at which water may either be returned to the south channel of the Platte River or be diverted into the Phelps County Canal). Upstream of that point, the channel is substantially dewatered by diversions. Downstream of that point, Johnson-2 return flows, local storm runoff, and returns from irrigated fields provide continuous flows even during nonflood periods. The division between the central and lower Platte River at Columbus corresponds to the entry of the Loup River. The western mountains of the Platte River Basin have peaks above 14,000 ft (4,200 m), and the precipitation they intercept creates runoff, which, with groundwater base flow, provides most of the water that flows through the Platte River. The Great Plains form a ramp that slopes gently eastward from the base of the mountains, descending from almost 6,000 ft (1,800 m2) on the Colorado Piedmont in the West to less than 1,000 ft (300 m) at the Missouri River. This plains region—characterized by sand and loess (fine silt) sheets, sand hills, and planar surfaces—is slightly dissected by the regional rivers (Figure 2-2). The Platte River Valley, bounded by bluffs of unconsolidated sands and silts, includes a flat floor and the river, a wide sandy ribbon of intersecting channels, and numerous vegetated islands (Figure 2-3). HUMAN USE OF PLATTE RIVER BASIN European perceptions of the geographic area encompassing the Platte River have always focused on its water. Until the twentieth century, the region east of the Rockies was described as high plains and prairie, with the division somewhere near the 96th to 98th meridian. However, much of the national perception of the region was based on the Great American Desert label derived from the reports of the expeditions of Zebulon Pike in 1806-1807, Stephen H. Long in 1819-1820, and others. Long’s 1823 map labeled the central plains as the Great American Desert (Long and James 1823), helping to perpetuate a perception that had already become firmly established (Webb 1931). Pre-European occupants of the western part of the region were nomadic plains tribes, including the Arapaho, Cheyenne, and Dakotah Sioux (Figure 2-4). These were hunting cultures that depended particularly on bison. In eastern and central parts of the region, tribes were more settled. The Pawnee, for example, were farmers; they occupied lodges for about 10 years at a time (Holmgren et al. 1993). They occupied the central Platte in Nebraska and other areas, including the Loup River, cultivating crops, gathering wild fruits and vegetables, and hunting bison and other wildlife. Early Euro-American reports indicated that Indians used cottonwood bark and leaves as food for horses and used wood for fuel, poles, and stakes.
OCR for page 35
Endangered and Threatened Species of the Platte River FIGURE 2-2 Landforms of Platte River Basin and nearby regions. Source: Raisz and Atwood 1957. Reprinted with permission; copyright 1957, E. Raisz, Cambridge, MA.
OCR for page 36
Endangered and Threatened Species of the Platte River FIGURE 2-3 Channel of Platte River showing its broad, shallow, braided nature. Source: Photograph by W.L. Graf, May 2003. Euro-Americans followed the Platte River westward, and much of the route remained in use as the Mormon Trail (on the north side of the Platte) and the Oregon Trail (on the south side) from the 1840s to the late 1860s (Brown and Whitaker 1948; Holmgren et al. 1993). The width of the river, its shifting channels, and its soft bed meant that there were few places where pioneers could easily cross. As westward movement of settlers proceeded, so did conflicts with aboriginal Americans. To protect pioneers and control Indian activities, a number of military encampments and forts were established along the Platte and its tributaries, including Fort Kearny, Fort Laramie, and Fort Collins. By 1855, there were violent conflicts between Plains Indians and Euro-Americans, which continued through the Civil War. After the Civil War, General Philip H. Sheridan ordered the destruction of their subsistence resources with the statement “Kill the buffalo and you kill the Indians.” The federal government had removed the tribes from the Platte region by the end of the 1870s. Trapping of beaver and intensive hunting of bison for both strategic purposes and sport characterized the beginning of substantial Euro-American-induced changes in the fauna of the Platte River Basin. Even before gaining complete control over the Platte River Basin, Euro-Americans were gaining an understanding of the region’s environmental conditions and increasing the use of its resources. The earliest settlers followed the trails to areas in California and Oregon. Perceptions of the region changed as railroads made the high plains and prairies more accessible and more attractive for settlement. John C. Frémont’s 1842 expedition along
OCR for page 37
Endangered and Threatened Species of the Platte River FIGURE 2-4 Portion of Platte River Basin as shown on Map of the Trans-Mississippi of the United States During the Period of the American Fur Trade as Conducted from St. Louis Between the Years 1807-1843. Oregon Trail alignment along central Platte River westward and then along North Platte River is shown by heavy black line. Source: Chittenden 1902.
OCR for page 38
Endangered and Threatened Species of the Platte River the Oregon Trail coincided with a period of high rainfall, and settlers traveling in the 1840s experienced relatively wet conditions (Lawson 1974). Terminology shifted from “desert” to “subhumid” and “semiarid” to describe the region (Brown and Whitaker 1948). More settlers came to the Platte River Basin and stayed after the end of the Civil War in 1865 following passage of the Homestead Act of 1862. The Union Pacific Railroad, with a vast amount of granted land along the Platte, contributed to the settlement of the region. The Transcontinental Railroad was completed across Nebraska in 1867 and finished in May 1869. Railroad construction, housing, and fencing stimulated the rapid cutting of available timber. Settlement and land use in the Platte region were affected by environmental perception and environmental reality. The Great American Desert came to be regarded as a potentially productive agricultural region, perhaps with trends toward increasing rainfall (Brown and Whitaker 1948). The idea that “rain follows the plow,” popular in the 1870s and 1880s, seemed accurate when settlers experienced increasingly abundant rainfall (Glantz 1994). Central plains homestead entries climbed rapidly in the 1880s (see, e.g., Mock 2000). As available rainfall records became longer, however, they revealed alternating periods of relatively abundant moisture and drought. As Saarinen (1966) noted, “although Great Plains wheat farmers are aware of the drought hazard they appear to underestimate its frequency and to overestimate the number of very good years” in spite of being preoccupied with weather conditions and drought potential. Especially severe droughts in the 1930s Dust Bowl period and in the 1950s led to population declines, although many inhabitants held on through difficult periods. HYDROLOGICAL CHANGES Three of the most important hydrological changes that have occurred along the Platte River since the early 1880s are the development of reservoir storage in the Platte watershed, the development of extensive irrigated farming, and municipal and industrial water use in growing urban areas. Reservoir Storage Reservoir construction began in the Platte River Basin around the beginning of the 20th century and continued until the early 1980s (Table 2-1). The first relatively large dams built in the basin were constructed on the South Platte River: Jackson Lake (1900, 47,000 acre-ft), Lake Cheesman (1905, 87,227 acre-ft), and Antero Reservoir (1909, 115,000 acre-ft). Cheesman Dam was the world’s highest dam at the time of its construction. In 1909, a much larger dam, Pathfinder Dam, was constructed on the North Platte River; its 1,016,500 acre-ft of storage made it one of the biggest dams in the world at that time. Dam construction during the period
OCR for page 39
Endangered and Threatened Species of the Platte River TABLE 2-1 Dams in the Platte River Basin Dam River Height (ft) Storage (acre-ft) Year Completed Drainage Area (mi2) Owner Eleven Mile DBWC South Platte 150.5 128,000 1932 963 Canyon Prewitt South Platte (OS) 42.5 51,387 1912 105 LID Cheesman South Platte 221 87,227 1905 1,750 DBWC Antero South Fork South Platte 53 115,000 1909 185 DBWC Empire South Platte 38 52,280 1973 11.1 BIC Jackson Lake South Platte (TR) 38 47,000 1900 15.9 JLRC Milton Lake South Platte 55 39,660 1975 120 FRIC Julesberg #4 South Platte (TR) 76 38,600 1910 6.5 JID Spinney Mountain South Platte 95 83,300 1982 772 City of Aurora Chatfield Dam South Platte 148 355,000 1973 3,018 CENWO Alcova North Platte 265 184,300 1938 10,376 USBR Glendo North Platte 190 1,118,653 1958 1,500 USBR Guernsey North Platte 135 45,228 1927 2,145 USBR Pathfinder North Platte 214 1,016,500 1909 14,600 USBR Seminoe North Platte 295 1,017,279 1939 7,210 USBR Kingsley North Platte 162 1,900,600 1941 CNPPID Southerland North and South Platte 30 65,000 1935 NPPD Johnson Lake Platte Canal 24 59,000 1941 CNPPID Riverside Sanborn Draw 41 94,500 1904 89.7 RID Abbreviations: BIC, Bijou Irrigation Company; CENWO, Corps of Engineers Northwestern Division Omaha District; CNPPID, Central Nebraska Public Power and Irrigation District; DBWC, Denver Board of Water Commissioners; FRIC, Farmers Reservoir and Irrigation Company; JID, Julesburg Irrigation District; JLRC, Jackson Lake Reservoir Company; LID, Logan Irrigation District; NPPD, Nebraska Public Power District; OS, off stream; RID, Riverside Irrigation District; TR, tributary; USBR, U.S. Bureau of Reclamation. Source: USACE 1996. was primarily in response to the need for irrigation water and municipal water. From 1910 to 1939, a series of dams were constructed in the South Platte Basin (Julesburg, 1919, 38,600 acre-ft; Prewitt, 1912, 51,387 acre-ft; and Eleven Mile Canyon, 1932, 128,000 acre-ft) and in the North Platte Basin (Guernsey, 1927, 45,228 acre-ft; and Alcova, 1938, 184,300 acre-ft). From the late 1930s to the late 1950s, storage in the North Platte Basin increased dramatically with the construction of three dams that added over 4 million acre-ft of storage (Seminoe, 1939, 1,017,279 acre-ft; Kingsley, 1941, 1,900,600 acre-ft; and Glendo, 1958, 1,118,653 acre-ft). The largest dam in terms of storage quantity is Kingsley Dam. Its reservoir, Lake McConaughy, impounds a maximum of 1.743 million acre-ft (2.15 km3) at
OCR for page 40
Endangered and Threatened Species of the Platte River elevation 3,265 on the North Platte River (Figure 2-5). The South Platte Basin in Colorado added a series of off-stream storage reservoirs during the 1970s and 1980s, although the volume added was less than 600,000 acre-ft (Empire, 1973, 52,280 acre-ft; Chatfield, 1973, 355,000 acre-ft; Milton, 1975, 39,660 acre-ft; and Spinney Mountain, 1982, 83,300 acre-ft). Water supplies in the Platte River Basin are augmented by transbasin diversions. The Colorado-Big Thompson project diverts about 400,000 acre-ft of water annually from the Colorado River Basin to the South Platte River. Tunnels conduct the water under the Continental Divide and empty it into Big Thompson Canyon, a tributary of the South Platte River north of Denver. Water is also shifted from the North Platte to the South Platte by a diversion west of the town of North Platte, Nebraska. In sum, all the dams in the Platte River system are capable of storing more than 6 million acre-ft of water (Figure 2-6). They provide water for a distribution system of almost 90 canals on the Platte River in Nebraska that was essentially complete by 1930 (Eschner et al. 1983). Additional flows occasionally occur in FIGURE 2-5 Kingsley Dam on North Platte River, completed in 1941, directly controls flows downstream through central Platte River. Source: Photograph by W.L. Graf, May 2003.
OCR for page 41
Endangered and Threatened Species of the Platte River FIGURE 2-6 Cumulative storage in reservoirs, number of canals constructed, and selected stream gage periods in Platte River Basin. Cumulative storage in reservoirs increases over time in form of step functions at lower right, because as new reservoirs were added storage increased at date of closure. Extent of canals caused more gradual increase in total cumulative number of canals as shown at lower left. Canal construction was earlier than construction of large dams. Stream gage records, shown by lines at top, include most of postdam period but little of predam period. Source: Adapted from Eschner et al. 1983.
OCR for page 42
Endangered and Threatened Species of the Platte River the river as a result of runoff from local rainstorms and from drainage of irrigated areas. Early gage records (“gage” is the hydrological term for “gauge”) indicate that before 1930 the mean annual water yield of the Platte River at North Platte, essentially the upstream end of the portion of the river of interest in this report, was about 2.2 million acre-ft (2.7 km3). That diminished to about 590,000 acre-ft (0.7 km3) per year in the period 1930-1970 and was about 650,000 acre-ft (0.8 km3) per year after 1970 (Simons and Associates 2000). The dams, reservoirs, canals, and hydroelectric plants provide valuable benefits to the economy of the region. For example, on the north bank of the river in the Big Bend reach, the Nebraska Public Power District supplies irrigation water for 35,000 acres (14,000 ha) of productive farmland (Nebraska Public Power District 2003); the district generates enough electricity from its water-driven power stations to supply the needs of 111,000 homes. South of the river, the Central Nebraska Public Power and Irrigation District system provides additional power generation and supplies water to more than 112,000 acres (44,800 ha) of irrigated land. Irrigated Farming The development of storage water made water supplies for irrigation more reliable. Today, there are more than 1.1 million acres of farmland irrigated with surface water in Colorado, more than 600,000 acres in Nebraska, and nearly 300,000 acres in Wyoming. Of all this irrigated land (approximately 2 million acres), nearly 1 million acres in Colorado and Nebraska are supplied by the South Platte River, 750,000 acres by the North Platte River, and over 200,000 acres by the Platte River mainstem. Water storage and distribution systems have produced an artificial water network along the central Platte River (Figure 2-7). In the middle and lower Platte hydrological subregion, the general area of concern in this report, agriculture consumes 1,366,400 acres-ft (1.7 km3) of surface water annually (Nebraska Natural Resources Commission 1994); additional substantial (but unmeasured) quantities are pumped from groundwater. This subregion is the most intensely irrigated region in Nebraska (Figure 2-8). Agriculture uses 90% of the water consumed in Nebraska (Nebraska Natural Resources Commission 1994); thus, agriculture will be an integral part of any solutions related to water for threatened or endangered species. In addition to irrigation for agriculture, benefits of the water projects include flood suppression downstream and flatwater recreation opportunities on the reservoir surfaces behind the dams. Farming in the Platte River Basin has become highly mechanized and specialized, and farms are becoming larger and fewer. Shifts in agriculture occurred with increasing availability of irrigation water, changing technology, the growing importance of economies of scale, and social changes.
OCR for page 62
Endangered and Threatened Species of the Platte River overestimated by the early studies. Moreover, the blank space between banks on the maps may have led to the impression that the presettlement Platte River was rather featureless, comprised of large expanses of sand and water, and was generally devoid of trees and other woody vegetation. They may also have led to the conclusion that any riparian vegetation in the modern river is unnatural and a product of flow regulation (Johnson and Boettcher 2000a). A partial reconstruction of predevelopment vegetation with the plat maps and field notes (Figure 2-20) was produced by Johnson and Boettcher (1999). FIGURE 2-20 Reconstruction of predevelopment vegetation based on GLO plat maps and field notes (source of witness tree data) for two Platte River townships. Small unsurveyed riverine islands are not shown. Light green areas were considered as typified by grassland; two types of timbered areas are also noted, as well as some agricultural activity. Source: Johnson and Boettcher 1999. Reprinted with permission; copyright 1999, Land and Water, Inc.
OCR for page 63
Endangered and Threatened Species of the Platte River The concept of a largely nontimbered river was bolstered by photographs showing a mostly treeless river near wooden bridges during early settlement (Williams 1978). However, the photographs were taken many years after Fort Kearny was built, 350,000 pioneers had passed by the Platte River on the Oregon Trail, the Union Pacific Railroad had been completed, and many farms were built and fenced. Virtually no wood was available in the vast grassland bordering the Platte River, so the high wood use associated with those human activities led to deforestation wherever there were trees, including islands within the Platte River (Johnson and Boettcher 2000a). The earliest photographs were taken too late to record the natural extent of woodland in the presettlement Platte River Basin. The most recent assessments based on the survey notes and plat maps portray the presettlement Platte as a river that had scattered trees on its outer banks but also was well studded (“immense numbers” was also used by the surveyors) with wooded islands of all sizes throughout its length. Cottonwood and willow trees dominated both the presettlement and modern floodplain vegetation of the Platte River; the exotic Russian olive and native red cedar are now abundant but were not so historically (Currier 1982; Johnson and Boettcher 2000a). The open, savanna-like nature of presettlement woodlands on large, higher islands just above floodplain level may have been the product of prairie fires, competition with grasses, and wood use and horse pasturing by American Indians. Smaller, floodplain-level islands were probably more protected from fire and had well-developed middle-story and upper-story vegetation. Current woodlands along many parts of the central Platte River have high tree density and well-developed, shrubby understories. Those conditions are different from the complex of wooded islands and large expanses of open water, sandy beaches and islands, and low-growing ephemeral vegetation shown in the earliest aerial photography from the 1930s (Box 2-1). GENERAL SPECIES RESPONSES TO EUROPEAN SETTLEMENT Invertebrates The aquatic invertebrates of the Platte River in Nebraska include 18 species of unionid mollusks (Hoke 1995) and 63 taxa of insects (McBride 1995). The common taxa of insects include Ephemeroptera (Caenis, Tricorythodes, and Heptagenia), Plecoptera (Isoperla), Odonata (Argia and Gomphus), Hemiptera (Corrixidae and Gerridae), Coleoptera (Elmidae and Dytiscidae), Trichoptera (Hydropsyche and Cheumatopsyche), and Diptera (Dicrotendipes, Cladotanytarsus, Chironomids, and Rheotanytarsus). Most of the taxa belong to the collector-gatherer or collector-filterer functional feeding groups and occupy the shoreline habitats rather than the more abundant, shifting sandbar habitat. Analysis of macroinvertebrate densities
OCR for page 64
Endangered and Threatened Species of the Platte River BOX 2-1 The Question of Presettlement Woodland Along the Central Platte River What was the areal extent, composition, and structure of vegetation communities along the central Platte River before European settlement, and what controlled their dynamics? The significance of this question is that modern attempts to restore the river to greater functionality for wildlife using open riparian areas requires knowledge of its original condition. “Original” in this case refers to conditions before European settlement, the imposition of land-use changes, and the installation of river-control structures, such as dams and diversions. Present-day human activities in and near the river, and throughout its drainage area upstream, make it impossible to completely restore presettlement vegetation communities. A riverine environment less modified than the present one may be possible, however, and may be beneficial to the endangered species and to other species using the area. The nature of the presettlement vegetation communities of the central Platte River has become better known, but some details are the subject of debate. The most recent summary discussions in the refereed journal literature are by Johnson and Boettcher (2000a,b) and Currier and Davis (2000). These and other interpretations rely on historical descriptions of the river and its vegetation, including journal entries of early travelers, soldiers, and settlers. One source of historical conditions is photography of the river, but few images are available, and their coverage does not begin until the 1860s. The earliest systematic information is from General Land Office (GLO) surveys conducted along the central Platte River in 1859-1869. The general objective of the surveys was to lay out the township and range system before organized settlement. The survey records include plat maps and surveyors’ notes describing conditions along the survey lines. Two divergent views of the river emerge from the historical information: the Platte was a wooded river coursing through a prairie landscape and the Platte was a prairie river dominated by nonarboreal vegetation. The complex braided system of the historical Platte River provided a variety of physical environments for vegetation, each with a different set of conditions for vegetation. The most commonly recognized settings include the active channel, abandoned channels, banks, low islands, and high islands. The active channel of the river was generally without vegetation except during summer low-flow conditions, when annual plants colonized portions of the exposed bed. Although the stream was not normally more than a foot deep (except during floods), its current was swift, and the unstable sandy sediments were not a suitable substrate for vegetation. The river was so shallow that steamboats and keel boats were not used by the early fur trappers who began using the river as early as 1807, and they relied on bull boats, lightweight wooden-frame craft with stretched animal skins (Chittenden 1935). Before the construction of large storage reservoirs upstream, the active channel was much wider than the present channel, as shown by photographs and aerial photographs made before the complete effects of upstream controls became apparent. Abandoned channels are common in other modern braided systems, and they occurred in the presettlement Platte River. When channels were abandoned by active flows, they sometimes became the location of stable cottonwood forests. A military observer during the middle 1800s described just such a case on the
OCR for page 65
Endangered and Threatened Species of the Platte River Platte. Lieutenant Woodbury verbally described and mapped a heavily timbered half-mile-wide strip of trees occupying an abandoned channel near Grand Island. Such dense woodlands probably assumed a patch shape that mimicked the shape of the underlying abandoned channel, a geometry commonly observed in other modern rivers on the plains. During the presettlement period, the outer banks of the central Platte River marking the general limits of fluvial activity were apparently the locations of cottonwood-dominated woodlands, with trees growing in isolation from each other or in limited groves. Accounts of surveyors and travelers usually described the trees on the banks as scattered, sparse, or absent in some cases from the outer, high banks of the river. Cottonwoods along the banks were often in a broken line of trees. Levi Jackman, an 1847 Mormon pioneer, penned a typical description: “cottonwood skirting the river is all the timber to be found, and very scarce at that” (Jackman 1847). This general quote probably indicates the general nature of the banks along the Platte River west of Kearney along the Oregon Trail, which joined the river at that point. The most precise information regarding vegetation along the river is from GLO surveys, cited here and elsewhere in this report, and they specify the vegetation conditions a decade or more after this example representative statement. Willow, prairie shrub, and grasses occurred along with the stream-bank cottonwoods in a community probably affected by grazing and fire before the intervention of humans several thousand years ago. Bison and deer, for example, grazed the area before the introduction of domestic livestock. The heavy use of the Oregon Trail, which followed the length of the central Platte, introduced exceptionally heavy grazing by the animals of emigrants bound for California, Oregon, and Utah. Grazing was so intense in 1849 and 1850 that travelers who previously had crossed from one side to the other in search of forage found that both sides of the river had been so heavily grazed that no forage remained (Steed 1850). The role of fire, a common feature of the prairie ecosystem, was also likely to have been a control on the structure of vegetation communities on the outermost river banks, but less so on riverine islands. Numerous islands occurred between the high, outer banks and among the multiple channels of the river. They were of two general types: smaller, low islands rising a few inches to a few feet above the average flow of the stream, and larger, high islands that rose several feet above the flow. The small islands were apparently the locations of relatively dense woodlands with substantial understories. GLO surveyors described those islands as “innumerable,” “numerous,” and “scattered promiscuously.” They characterized the river as being “studded with” and “filled with” islands of all sizes. The surveyors used the descriptors “brush,” “brushy islands,” and “covered with timber and undergrowth” for the islands. Surveyors’ 1863 notes for the river just downstream of the town of Grand Island stated that “the greater part of the timber is on the small islands in the Platte River … there are many small islands in the river covered with timber and undergrowth” (cited by Johnson and Boettcher 2000a). Islands between the banks probably hosted relatively dense forest cover with substantial undergrowth because they were probably exposed to lower rates of fire and grazing than the prairies along the banks; present knowledge is insufficient for us to know how much lower. Grass fires can leap across wide unvegetated spaces (Wilson 1988), but the active channel of the central Platte River was a mile or more across, large enough to protect many midchannel islands. One surveyor noted that “there is no timber except that confined to the islands which is protected from the fall fires.”
OCR for page 66
Endangered and Threatened Species of the Platte River Fire frequency was probably not reduced as much on the very large, high islands, such as Grand Island, which is so large that fires might have been ignited on the island itself. The island was also separated from the banks of the main river by more narrow active zones than in the case of the smaller islands. The vegetation cover of the very large islands consisted of more scattered cottonwoods, probably with more grass in the understory. In summary, the ecological conditions of the central Platte River in presettlement times is not completely known, and the mechanisms that maintained the vegetation communities and their structure have not been completely investigated. Many details of the early vegetation communities and their structure are open to some debate. There is enough information from early accounts, however, to develop a first approximation: the wide active channels had little or no vegetation except for annual plants during low-flow periods, the banks had scattered trees and small groves with prairie-like vegetation, smaller islands were numerous and heavily wooded with dense undergrowth, and large islands had some woodland and some grassland. A successful restoration of the central Platte River ecosystem will include all those components. in the Platte River downstream of the mouth of the Loup (Peters et al. 1989) found that rock substrates supported the highest numbers of organisms per unit area, 65,245/m2; most were chironomids and trichopterans. Sand had the next highest density, 8,218/m2, followed by gravel, 7,576/m2. Invertebrate densities on silt substrates and on submerged wood totaled 6,610/m2 and 6,572/m2, respectively. The invertebrate community in the Platte River before European settlement is unknown. Woody debris is now an important substrate for aquatic invertebrates, but how important it was in determining invertebrate diversity and abundance before European settlement and water development is undeterminable. Vertebrates It is difficult to assess the change in relative abundance or species composition of the various fish, amphibian, reptile, bird, or mammal faunas in response to changes in the hydrology or vegetation of the central Platte ecosystem since European settlement. Data to quantify accurately the comparative abundance of any species then and now do not exist. Fishes The present fish fauna of the Platte River includes about 100 species (in 20 families), of which 76 are native to at least a portion of the basin
OCR for page 67
Endangered and Threatened Species of the Platte River (Schainost and Koneya 1999). Wide fluctuations in flows with flooding and high turbidity followed by low flows and high water temperatures impose special restrictions on the biota of the Platte River. Many fish species native to the mainstream Platte—including red shiner, sand shiner, river shiner, bigmouth shiner, western silvery minnow, plains minnow, speckled chub, flathead chub, river carpsucker, quillback, and channel catfish—are adapted to widely fluctuating conditions. Pre-1940 records also indicate that several species—such as shovelnose sturgeon, sturgeon chub, and sauger—were found in the Platte River drainage as far west as Wyoming. In addition, headwater, tributary, and spring-fed side-channel reaches support species that require clear or cool water; some, such as horny head chub, have been extirpated from the basin, but others—such as northern redbelly dace, finescale dace, plains topminnow, and Topeka shiner—are found in isolated populations in the drainage. In general, the number of native species declines in the western portion of the basin, where 30 native species have been recorded in the North Platte Basin in Wyoming and 26 in the South Platte Basin in Colorado. In addition, the proportion of nonnative fish species increases to almost 50% in the North Platte Basin in Wyoming and 41% in the South Platte Basin in Colorado (Schainost and Koneya 1999). Herpetofauna—Amphibians and Reptiles The herpetofauna of the Platte drainage includes salamanders (two species), frogs and toads (11), turtles (eight), lizards (11), and snakes (29). The most common representatives of these groups along the Platte River are the tiger salamander (Ambystoma tigrinum), the Rocky Mountain toad (Bufo woodhousii), the western striped chorus frog (Pseudacris triseriata), the painted turtle (Chrysemys picta), and the spiny softshell (Trionyx spiniferus). Birds A diverse assemblage of birds use the Platte River. The many life-history types include residents (such as blue jays, woodpeckers, and quail), migratory birds that are summer (breeding) residents (such as terns, plovers, warblers, orioles, and kingbirds), and migratory nonbreeding birds that use the river or floodplain only during spring and fall migration (such as cranes, ducks, geese, shorebirds, and songbirds). The Platte River’s woodlands are especially rich in species. Scharf (2003) netted 77 species on the Platte’s floodplain (grassland, woodland, and channel habitats), including 19 species of summer-nesting neotropical migrants that are in decline in other parts of their ranges (Dobkin 1994; Robinson et al. 1995). Three that nest in riparian woodlands—the
OCR for page 68
Endangered and Threatened Species of the Platte River yellow-billed cuckoo, Bell’s vireo, and western kingbird—are considered priority species for conservation in Nebraska because of their continental declines (Forsberg 1999). Colt (1997) captured 50 species of nesting birds in the Platte’s woodlands, of which 31% were neotropical migrants. He also found high fledging rates and low rates of cowbird parasitism. Thus, as the Platte River’s woodlands have expanded coincidentally with water development, songbird populations have increased accordingly, ironically off-setting some of the effects of the huge losses of riparian woodland caused by dam building and poor land management in much of the Great Plains (Knopf et al. 1988). Open (i.e., non-forested grassland or wetland) habitat is also important to bird species of conservation concern in Nebraska (Appendix B). Forty-one (57%) of 72 of these species occur on the central Platte and most (27 species, 66%) of the 41 species are associated with open habitat. Another eight species are characterized by using partially wooded (i.e., open woodland or savanna) habitats. Only six species in Appendix B (American woodcock, black-billed cuckoo, great-crested flycatcher, long-eared owl, ovenbird, Baltimore oriole) require more closed forest. Large numbers of migratory waterbirds migrate through Nebraska’s central Platte River Basin and adjacent Rainwater Basin area each year. Population surveys and management activities for waterfowl and cranes treat those two areas as one regional staging area because extensive interchange of birds between the two wetland systems occurs (Cox and Davis 2003). Variations in staging bird numbers from one year to another are large because migration patterns are determined by frequent, variable weather systems and by physiological needs of migrating birds. In years when spring advances slowly, larger numbers of birds will stage for longer periods in the central Platte Valley and the Rainwater Basin because conditions farther north are still frozen. In contrast, when spring develops rapidly and over a large area, migrating waterbirds may move through the Rainwater Basin area and central Platte River Basin more quickly (Cox and Davis 2003). The Rainwater Basin area serves as important habitat for migrating waterbirds but is also susceptible to periodic drought and disease outbreaks (Chapter 5). Waterbird habitats in or near the central Platte River, in contrast, are dominated by flowing water or extensive exchange between groundwater and surface water; these habitats are not as susceptible to the occurrence of drought or disease. The central Platte River, therefore, provides important habitat for spring-staging waterbirds at times when little habitat is available elsewhere and when low temperatures temporarily freeze shallow wetlands in the adjacent Rainwater Basin area. On the central Platte, the primary types of habitat used by staging waterbirds are open water, wetland, meadows, grassland, and cropland.
OCR for page 69
Endangered and Threatened Species of the Platte River Forest communities in the central Platte River do not provide staging habitat for waterbirds. Four populations of geese, one population of sandhill crane, and several species of ducks depend on current staging habitats (Table 2-2). Over 85% of the midcontinent sandhill crane, snow goose, Ross’s goose, and greater white-fronted goose populations migrate through the central Platte River region. The distribution of those waterbird populations, such as the midcontinent population of sandhill cranes, is now restricted to a narrow band of migration habitat in the central United States (Figure 2-21). The central and lower Platte River hosts a variety of species, and restoration of the river to benefit one species should not unduly impair other species. That issue arises because clearing woodland areas to benefit whooping cranes that prefer long, open sight lines may reduce the available habitat for songbirds. The lessons from this observation are that such decisions should take into account the general distribution of the species in question, the legal standing of the species, and the ecosystem perspective. First, most of the songbirds have a general distribution in the region (Davis 2001), so the loss of some woodland in and along the river should not adversely affect the general population. However, whooping cranes are strongly connected with the river and its habitats, and there are few alternatives. Wholesale removal of woodlands without regard to the effects on other species would be irresponsible; recognition and mitigation of effects wherever possible constitutes wise management. Second, The Nebraska Partnership for All-Bird Conservation identified 72 bird species of conservation concern in Nebraska (Appendix B). However, whooping cranes, piping plovers, and interior least terns have special legal standing in that they are federally listed as endangered or threatened species, whereas the songbirds and other bird species of concern are not. That does not mean that the interests of songbird and waterfowl populations may be safely disregarded, but special efforts must be taken on behalf of the listed species. Finally, an ecosystem perspective would logically suggest that the central and lower Platte River should be a diverse ecosystem when viewed as a whole, with adequate accommodation for both listed and nonlisted species. The diversity ought to be manifest over the entire length of the central and lower river, whereas it might not be present in some localities and areas of restricted spatial extent (a square mile or so, for example). In the final analysis, provision should be made to care for all species, not to see the elimination of one from the total ecosystem of many square miles in extent. Arriving at clear and achievable conservation goals in situations like these requires good science but also good policy and good process. How those elements are integrated in this report is discussed in Chapter 4.
OCR for page 70
Endangered and Threatened Species of the Platte River TABLE 2-2 Proportions of Waterfowl Populations That Use Nebraska’s Rainwater Basin Area and Central Platte River Valley Species Population Considered Total No. Birds Populationa Citation Number of Birds in Central Platte River Valley Citation Proportion of Population Using Central Platte Sandhill crane MCP 435,050 Sharp et al. 2003b 375,875c Solberg 2002b 86%b Mallard MCP 7,785,800 USFWS 2003 4,097,000 Gersib et al. 1990 50% Northern pintail MCP 2,547,970 USFWS 2003 756,000 Gersib et al. 1990 30% Total ducksd MCP 34,527,900 USFWS 2003 23,815,000 USFWS 2003 61% Snow goose, Ross, goose MCP 2,490,800 USFWS 2003 2,679,300 J. Drahota, pers. comm., U.S. Fish and Wildlife Service, 2003 90% Canada goose GPP/WPP 651,330 USFWS 2003 558,000 J. Drahota, pers. comm., U.S. Fish and Wildlife Service, 2003 75% Canada goose SGPP 160,570 USFWS 2003 110,000 J. Drahota, pers. comm., U.S. Fish and Wildlife Service, 2003 25%
OCR for page 71
Endangered and Threatened Species of the Platte River Canada goose TGPP 421,900 USFWS 2003 165,000 J. Drahota, pers. comm., U.S. Fish and Wildlife Service, 2003 50% Greater white-fronted goose MCP 802,200 USFWS 2003 950,000 Gersib et al. 1990 90% aTotal number of birds considered in population is most recent 3-year average of breeding, migration, or winter survey used by Cooperative Flyway Management Plans to depict population status. bThese estimates of MCP sandhill crane population are indices, represent most recent 3-year average (Dave Sharp, U.S. Fish and Wildlife Service, pers. comm., 2003), and probably underestimate total proportion that migrates through Platte River Valley, because of population turnover. Telemetry and other marked-bird data for sandhill cranes suggest that as much as 99% of MCP migrates through Platte River Valley (Gary Krapu, U.S. Geological Survey, pers. comm., 2003). cThis estimate includes birds in North and central Platte. About 15-20% of sandhills migrating through the Platte River Basin go through the North Platte region, and remainder go through central Platte (Gary Krapu, U.S. Geological Survey, pers. comm., 2003). dIncludes all Anatidae (it includes canvasback, redhead, ring-necked duck, lesser scaup, gadwall, northern shoveler, blue-winged teal, and American widgeon); does not include scoters, eiders, mergansers, long-tailed ducks, and wood ducks. Abbreviations: MCP, midcontinent population; GPP/WPP, Great Plains population/western prairie population; SGPP, short-grass prairie population; TGPP, tall-grass prairie population.
OCR for page 72
Endangered and Threatened Species of the Platte River FIGURE 2-21 Generalized annual migration from southern to northern latitudes. Source: Adapted from Sharp et al. 2003.
Representative terms from entire chapter: