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6 Glacial to Modern Changes in Global River Fluxes VICTOR R. BAKER University of Arizona ABSTRACT Continental records of fluvial paleohydrology indicate major changes in global fluxes of water and sediment since the last full-glacial maximum. Spectacular cataclysmic floods occurred during deglaciation of the temperate zone, while more equatorial areas had reduced average streamflow at full glaciation, about 18,000 yr ago (18 ka). Southern Asia, central Africa, and northern Australia experienced enhanced monsoonal activity and large floods in the earliest Holocene. Relatively small-scale cyclic fluctuations of fluvial activity characterized the Holocene of the temperate zone. New research methods, such as the study of flood slackwater deposits and paleostage indicators, offer the potential for very accurate paleoflow reconstructions. A global program of fluvial paleohydrology could be combined with ongoing work in paleoclimatology to greatly improve the understanding of possible future changes in the Earth system. INTRODUCTION Rivers form the dynamic link that operates on the Earth's land surface between the oceanic storage and the atmospheric transfer of global water. Along with a flux of water from the hydrosphere, rivers transfer a flux of sediment that modifies the lithosphere. Chemical components of the other great global environmental cycles move with these two great river fluxes of water and sediment. This chapter discusses ancient aspects of change in river fluxes of water and sediment. In adopting a paleohydrological approach to rivers, emphasis is placed on the time since the last glaciation. There are several important reasons for this approach. (1) River fluctuations in this period encompass the range of fluvial adjustment to Quaternary climatic change for which the future will be an extension. (2) Examples of cataclysmic change during this period illustrate the extremes of what is possible in river systems. (3) Very detailed late Quaternary records of fluvial hydrological change are preserved in certain ideal settings. (4) Records of late Quaternary paleohydrology can be compared to other very detailed Quaternary paleoclimatological data and to model simulations of the atmospheric-hydrologic system.
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FLUVIAL PALEOHYDROLOGICAL PRINCIPLES Records of fluvial changes since the last glaciation have been most extensively studied by the analysis of sediments in terraces, floodplains, and valley fills. Where possible, such studies are combined with the results of recent advances in understanding the relationship of channel morphology to climatic and hydrological controls (Schumm and Brakenridge, 1987). However, preserved paleochannels are limited to certain special alluvial settings such as those studied on the riverine plain of eastern Australia (Schumm, 1968), the north Polish plain (Kozarski and Rotnicki, 1977), and the Gulf Coastal Plain of the United States (Baker and Penteado-Orellana, 1977). Paleochannel preservation tends to be best for meandering river environments and poorer for environments characterized by braided or straight channels. Most often the sediments must be analyzed without the morphological clues. Pitty (1971, p. 16) aptly summarizes the situation: ''Lending confidence to the geomorphologist during his hypothetical leaps between form and process is the flimsy safety net provided by the study of sediments." Regime Changes and Sediment Transport Mechanics Two broad classes of paleoflow estimation derive from the two classical divisions of fluvial hydraulics (Leliavsky, 1955): regime theory and sediment transport mechanics. The regime approach to fluvial paleohydrology involves the use of various empirical relationships that relate the driving variables of (1) relatively high-probability flow discharge and (2) sediment characteristics to various dependent variables, including paleochannel dimensions, river patterns, and gradients. The relationships apply only to alluvial rivers with beds and banks composed of the same types of sediment as in transport by the channel-forming flows. Paleohydrological work in this area was pioneered by Dury (1954, 1965) and by Schumm (1965, 1968). Because of the interplay of sediment and water discharge in fluvial response, alluvial rivers may display a degree of complex response to changes in their drainage basins (Schumm, 1977). Lag times also occur between causative agents and responses. For example, the effect of glaciation on sediment yields continues as long as the unstable drift in proglacial or postglacial environments remains easily accessible to fluvial erosion and transport (Church and Ryder, 1972). The high sediment yields associated with glacier-related deposits may persist with long lag times from the emplacing processes. Church and Slaymaker (1989) show that Holocene sediment yields in British Columbia are dominated by these effects, and the influence of relict glacial sediments continues to the present day. Most of the equations used in alluvial regime paleohydrology are listed by Williams (1984). The approach is subject to many limitations and provides relatively low accuracy of paleoflow retrodiction (Ethridge and Schumm, 1978; Rotniki, 1983; Dury, 1985). However, in a semiquantitative sense, the regime approach combined with detailed studies of floodplain sedimentology can show the pattern of fluvial responses to changing environmental conditions (Baker and Penteado-Orellana, 1977). A variety of procedures from sediment transport theory have been used to relate sediment characteristics to shear stress, flow velocity, or stream power. Combined with information on paleochannel dimensions, these procedures can yield paleoflow estimates (Baker, 1974; Costa, 1983; Williams, 1983). Unfortunately, numerous problems may contribute to a relatively low accuracy level for these procedures (Church, 1978; Maizels, 1983). Despite the problems with both the regime theory and the sediment transport mechanical approaches to fluvial paleohydrology, these methods have the most universal range of applicability with regard to ancient river deposits. Most of the literature on late Quaternary fluvial change is based on the study of alluvial valleys interpreted by classical stratigraphy combined with some regime or sediment transport theoretical analysis. SWD-PSI Analysis A relatively new development in Quaternary paleohydrology is the recognition that certain stable-boundary fluvial reaches may, under ideal circumstances, preserve remarkably complete and accurate records of river flood stages. This technique was used for cataclysmic Pleistocene glacial floods (Baker, 1973; Patton et al., 1979) and found also to apply to arid-region Holocene floods (Baker et al., 1979, 1983; Kochel and Baker, 1982). Paleodischarges are calculated using hydraulic flow models (O'Connor and Webb, 1988) that relate slackwater deposits and paleostage indicators (SWD-PSI) to paleowater-surface profiles. Modern SWD-PSI paleoflood hydrology results in remarkably complete catalogues of the number, timing, and magnitudes of the largest floods occurring over periods of centuries or millennia (Baker, 1987a). The data can be used directly in magnitude-frequency analysis (Stedinger and Baker, 1987; Baker, 1989), understanding regional patterns (Enzel et al., 1993) or in interpreting the effects of environmental change on flood time series (Baker, 1987b; Jarrett, 1991). Although most SWD-PSI paleoflood hydrology studies have been done on relatively small rivers, the methodology is not limited in scale. If appropriate study reaches can be found, then large rivers can also be analyzed. Chatters and Hoover (1986) used the methodology to analyze a 1800-yr record of late Holocene floods on the Columbia
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River. The paleofloods of the Huang He (Yellow River) in China were studied by Shi Fucheng et al. (1987). The Narmada River in central India, which has historical floods as great as 60,000 m3/s, displays exceptional slackwater depositional sites in its middle reaches (Baker, 1988a; Kale et al., 1992). Excellent sites for SWD-PSI paleoflood analysis have recently been discovered on the Colorado River in Arizona (Ely et al., 1992). The SWD-PSI paleoflood studies require certain ideal combinations of conditions that allow rivers to chronicle their own cataclysms (Baker and Pickup, 1987). The method has been found to be especially applicable in arid, tropical, and savanna environments of relatively high flow variability. Because these environments have generally proven difficult to characterize with other fluvial paleohydrological tools, SWD-PSI paleoflood hydrology seems to have a high potential for increasing our understanding of flow changes in the global context (Figure 6.1). Some localities where excellent paleoflood records have recently been discovered include the following: northern Australia (Gillieson et al., 1991; Patton et al., 1993; Wohl, 1992a), South Africa (Smith, 1991, 1992), China (Ding Xianrong and Yan Yuanling, written communication, 1992), India (Vishwas Kael, written communication, 1993), Israel (Wohl et al., in press), Greece (Lewing et al., 1991), and Spain (G. Benito, written communication, 1993). FLUVIAL CATACLYSMIC PROCESSES The emphasis in global change studies by many of the emerging scientific initiatives has been on progressively acting processes. Atmospheric increases in various trace gases, tropical deforestation, and water pollution all proceed progressively. Moreover, responses to change are generally considered in terms of mean variables. The predictions of general circulation models, for example, are expressed in terms of changes in mean values for temperature and precipitation. The geological record has taught us, however, that the Earth also displays cataclysmic processes that lead to immense short-term responses. For river flows, our knowledge of cataclysmic processes is limited, and that limited knowledge has sometimes led to limited appreciation of these phenomena by modern geomorphologists (Baker, 1988b). Modern measurements of cataclysmic floods are inadequate because of the very small chance of cataclysm occurrence at a specific observational site and because of physical problems in actually measuring a powerful cataclysmic process. In nearly all the hydrological literature on extreme floods, such events are not treated directly. Rather, extrapolations are made according to statistical assumptions about an observational record of small flows, or a computer simulation is performed according to an ideal contemplation of flood behavior. The scientific inadequacy of such procedures, which were developed for purposes of engineering design, is only now becoming clear to the hydrological community (Klemeš, 1986, 1987, 1989). In relation to fluxes of water and sediment associated with rivers, if we are to understand changes, we must not limit that understanding to adjustments of mean conditions. It is also critical to understand variances in the extreme events that dictate local, intense fluxes. Are there global patterns of these extremes through time and space? In a general sense, most floods are the products of global climatic systems (Hayden, 1988). Moreover, extreme floods seem to result from important anomalous patterns of atmospheric circulation, including persistence of pattern, rare configurations, and unusual combinations or locations of circulation types (Hirschboeck, 1987, 1991). Even rather small-scale climatic changes, if persistent, can produce rather dramatic responses in flood magnitudes and frequencies, as demonstrated for the upper Mississippi River (Knox, 1984, 1993). To improve understanding of cataclysmic floods one must expand the horizons of time and space, reconstructing ancient flood occurrences on a global scale. A possible example of global adjustment to extreme river flux changes is provided by the long-recognized Younger Dryas cooling of northwestern Europe that took place approximately 11 to 10 ka. This change seems to have been FIGURE 6.1 Relationship of SWD-PSI paleoflood hydrological investigation sites to sources of tropical storms (dot pattern), typical tropical storm tracks (small arrows), and major cold-water ocean currents (large arrows). Locations of recently completed SWD-PSI studies are indicated by large dots. Locations of potential study sites with appropriate geomorphological settings are indicated by triangles. From Baker (1988a).
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abruptly imposed on the long-term deglacial warming trend. Broecker et al. (1988) attribute the Younger Dryas intense reversal of that trend to a sharp drop in sea surface temperature as North American meltwater was diverted from the Mississippi River to the St. Lawrence. Shaw (1989) posed the more provocative hypothesis that immense volumes of subglacially stored water may suddenly be released in floods exceeding 106 m3/s around the margins of the Laurentide Ice Sheet. Such floods would have immense consequences in rapid sea-level rise and the climatic effects of a cold meltwater lid on warmer ocean water. Considerable skepticism remains (Mueller and Pair, 1992), however, especially concerning the drumlin topography ascribed to cataclysmic flood origin (Shaw et al., 1989). A complex system of proglacial lakes developed during deglaciation along the southern margin of the Laurentide Ice Sheet in North America. As reviewed by Teller (1987, 1990), much of the proglacial discharge passed through the Mississippi River to the Gulf of Mexico until about 11 ka. Then the immense Lake Agassiz Basin was integrated into the St. Lawrence drainage to the Atlantic Ocean. At about 10 ka the Lake Agassiz discharge briefly returned to the Mississippi system. There still remains some controversy as to the correlations between land meltwater shifts and the paleoceanographic interpretation of marine sediments. Moreover, the mechanism of climatic cooling induced by the meltwater influx to the Atlantic remains a problem. However, the clear evidence of drastic changes in river fluxes and associated oceanic influences points to an important need to understand rapid changes in river processes. The usual model of proglacial drainage is one in which outwash streams gradually aggrade their beds, producing a wedge of sediment thickening toward the glacial front. The meltwater from the late Pleistocene Laurentide Ice Sheet initially produced such a pattern. However, after the ice sheet has retreated behind either major morainal fea- FIGURE 6.2 Map showing regions of the northwestern United States affected by cataclysmic flooding in the late Pleistocene (arrows) in relation to glacial Lake Missoula and Lake Bonneville. Areas of glaciation are indicated in black. From Baker et al. (1987).
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tures or landform divides, meltwater became mainly ponded as proglacial lakes. As lake levels rose, the water was released as glacial-lake outburst floods (Kehew, 1982; Kehew and Lord, 1986, 1987; Lord and Kehew, 1987). These floods eroded enormous volumes of sediment when they carved various glacial-lake spillways. The floods were rapidly conveyed to the ocean, particularly through the Mississippi River system to the Gulf of Mexico. Grosswald (1980) envisioned a somewhat similar set of circumstances for a late Pleistocene ice sheet of of northern Eurasia, which dammed north-flowing rivers in a radical pattern from Yenisei in the east (Siberia) to the Polish rivers in the west. A proglacial system of lakes and spillways involved water transfers to the Aral, Caspian, Black, and Mediterranian Seas. Velichko et al. (1984) reconstructed more limited extents to late Pleistocene Eurasian glaciers and ice-dammed lakes. Studies of the great late-glacial floods provide knowledge of possible extreme effects of short-term changes in river fluxes. In addition to the Laurentide Ice Sheet floods, glacial cataclysmic floods have recently been recognized for the Fennoscandian Ice Sheet in Swedish Lapland (Elfstrom, 1987). Rudoy (1988, 1990) and Ruday and Baker (1993) document cataclysmic late Pleistocene flood features, including huge gravel bars and giant current ripples, for glacial floods in the Altay Mountain region of south-central Siberia. However, to date the best studied cataclysmic flood features are those in the northwestern United States associated with the Cordilleran Ice Sheet and with Pleistocene Lake Bonneville (Figure 6.2). The Bonneville flood occurred about 14.5 ka (Currey, 1990), releasing a peak flow of about 9 x 105 m3/s (Jarrett and Malde, 1987; O'Connor, 1993). This compares to the largest known recent flood, the 3.85 x 105 m3/s discharged by the Amazon River in 1953 (Oltman, 1968). Lake Bonneville released 4700 km3 stored volume, sustaining flow for perhaps six weeks (Malde, 1968). The cataclysmic outbursts of glacial Lake Missoula (Figure 6.2) occurred repeatedly during two late-glacial phases: (1) approximately 35 to 40 ka (Baker et al., 1991), and (2) approximately 12 to 17 ka (Baker and Bunker, 1985). The relative number, timing, and sizes of individual floods in these phases remain controversial (Baker and Bunker, 1985; Waitt, 1985; Smith, 1993). However, it is clear that the largest discharges were among the greatest known freshwater flows on the planet (O'Connor and Baker, 1992). Baker (1973) originally estimated the peak flow at approximately 2.1 x 107 m3/s. However, the released discharge peak to the Pacific Ocean was probably about 1.0 x 107 m3/s because of downstream hydraulic ponding (O'Connor and Baker, 1992; Baker et al., 1993). Total water released from glacial Lake Missoula was on the order of 2000 km3, allowing large flows to persist from a TABLE 6.1 Flow Dynamics of Some Major River Floods (Baker and Costa, 1987; Baker et al., 1993). Flood Age Peak Discharge (m3/s) Power/Unit Area (W/m2) Altay Late Pleistocene 1.8 x 107 1,000,000 Missoula 12-17 ka 1 x 107 300,000 Bonneville 14.5 ka 9 x 105 75,000 Amazon Modern 3 x 105 12 Mississippi Modern 3 x 104 12 few days to a week (Baker, 1973; Clarke et al., 1984; Craig, 1987). The influence of various late-glacial cataclysmic floods on sediment fluxes remains largely underappreciated. Griggs et al. (1970) noted the influence of the Missoula floods in transporting sediment to the abyssal seafloor off the mouth of the Columbia River. However, few other studies have considered this mechanism of sediment transport for sporadic and rapid transfers of sediment through fluvial systems. Some measure of sediment transport capacity for cataclysmic floods can be estimated by using Bagnold's (1966) concept of stream power. The rate of energy dissipation ω, or power, per unit area of streambed m can be expressed as where W is bed width, Q is discharge, S is energy slope, γ is the specific weight of the transporting fluid, is the mean flow velocity, and τ is the bed shear stress. For short time periods cataclysmic floods generated tremendous fluxes. The peak Lake Missoula flows were an order of magnitude larger than the average total global discharge of all rivers to the ocean, approximately 1 x 106 m3/s estimated by Milliman and Meade (1983). Moreover, at least the potential sediment loads, expressed in terms of stream power, were also immense (Table 6.1). REGIONAL PATTERNS Global fluvial paleohydrology is a subject in its scientific infancy. Under the auspices of the International Union of Quaternary Research (INQUA) Holocene Commission, a working group on Global Paleohydrology was organized in 1987. On August 7, 1991, the INQUA Council initiated a new Commission on Global Continental Paleohydrology (GLOCOPH) led by Professor Leszek Starkel. The emphasis of this new commission will be to study changes in water fluxes and storages over the past 20,000 yr (Starkel,
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1993). The program will include (1) the compilation of computer data bases on individual elements of the hydrological cycle; (2) the establishment of interrelationships, including storages, transfers, and energetics, among elements of the hydrological cycle; and (3) the comparison of paleodata to simulations with a Global Hydrological Model (GHM). There is at present no functioning GHM, but several modeling groups are working on the goal of achieving one. A GLOCOPH data base was established at the Geodata Institute, University of Southampton, directed by Professor K.J. Gregory. This chapter can only highlight selected aspects of global fluvial paleohydrology, because the whole subject will be expanding greatly over the next several years. Europe and North America Considerable fluvial paleohydrological work was performed from 1977 to 1988 under the auspices of the International Geological Correlation Programme (IGCP) Project 158 "Paleohydrology of the Temperate Zone During the Last 15,000 Years." Subproject A under the leadership of Laszek Starkel organized 17 national efforts in the reconstruction of fluvial changes between 35 and 70°N latitude, concentrating on 20 major river valleys. Researchers employed the traditional methodologies of floodplain studies and regime-based paleoflow estimates (Starkel and Thornes, 1981). Most of the project studies were in Europe, with some related work in North America. As summarized in various IGCP Project 158 volumes (Gregory, 1983; Korzarski, 1983; Gregory et al., 1987; Starkel et al., 1991), there is a general pattern of late-glacial to modern fluvial changes. Glaciofluvial and post-glacial rivers were dominated by bed load transport until about 10 ka. A shift then occurred to meandering rivers dominated by suspended load. In Poland the change from braiding to large-scale meandering occurred at 13 to 12 ka, and from large to small paleomeanders at approximately 10 ka (Korzarski and Rotnicki, 1977). Smaller fluctuations in the Holocene reflect the influence of forest vegetation until about 5 to 4 ka, when anthropogenic influences began to increasingly influence fluvial systems. One of the most interesting results of IGCP Project 158 was the recognition of synchroneity in episodes of Holocene fluvial activity. These episodes are well documented in central Europe (Korzarski, 1983; Starkel, 1983) and the north-central United States (Brakenridge, 1980, 1981; Knox, 1983, 1985). The episodes may be of variable duration and magnitude, and their characteristics may be out of phase from region to region even though they are responding to the same large-scale dynamics of the global atmosphere-ocean system. Although the alluvial phases do not correlate precisely in time between sites, there is a FIGURE 6.3 Correlation of flood activity on major northern hemisphere rivers. Data are plotted for ages (years A.D.). Bars in the W Sequence show ages of major alluvial discontinuities in the United States associated with flooding episodes (Knox, 1983). Heights of the dashed lines in C illustrate relative levels of flood frequency on the Columbia River, Washington (Chatters and Hoover, 1986). Similarly, curve V shows interpreted levels of fluvial activity in the upper Vistula River basin of Poland (Starkel, 1983). The lower curve N shows Nile River activity in terms of the cumulative sum of discharge departures from the average divided by average discharge (Riehl and Meitin, 1979). Where this curve is increasing with time (from right to left), discharge is above the long-term average; where the curve is decreasing with time, discharge is below the long-term average. FIGURE 6.4 Exceptionally large paleofloods on rivers in the south-western United States plotted against age (years A.D.). Vertical bars show flood ages on the Pecos River, P, from Kochel and Baker (1982); the upper Salt River, S, from Partridge and Baker (1987); and the Verde River, V, from Ely and Baker (1985). Concentrations of large floods, indicated by the numbers, are shown for the Escalante River, E, from Webb (1985); and the lower Salt River, L, from Fuller (1987).
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consistent overall pattern: (1) 1000- to 2000-yr aggradational episodes coinciding with phases of lower flood frequency, and (2) 300- to 500-yr phases of higher flood frequency involving river entrenchment or avulsion. The overall period of 2000 to 2500 yr coincides with other detailed proxy records of Holocene climate such as Denton and Karlen's (1973) analysis of mountain glaciation and the oxygen isotope variations in the Camp Century, Greenland, ice core (Dansgaard et al., 1986). Paleoflood hydrological investigations are just beginning to provide a perspective on extreme river flows over the past 2000 yr (Figure 6.3). The most interesting patterns are appearing in the southwestern United States, where approximate 1000- and 500-yr periods are observed (Figure 6.4). A recent compilation of SWD-PSI studies (Ely et al., in press) has established the regional coherence of these patterns of flooding. South America Baker (1983) suggested that the effects of Quaternary climatic change on South American rivers might be used as a surrogate to explore potential effects of man-induced environmental change in the region. During the last full glacial, most of South America north of the Tropic of Capricorn was dominated by climates drier than present (Tricart, 1985). The Amazon Basin rivers have a remarkable diversity of patterns, reflecting the influence of Andean source region and the relative abilities of lowland rivers to rework relict alluvium deposited during the drier full-glacial periods (Baker, 1978). Although little fluvial paleohydrological work has been done in the region, the global importance of Amazon and Orinoco Basin hydrology would seem to warrant further attention. Central-western South America is strongly influenced by the El Niño-Southern Oscillation (ENSO) phenomenon. This coupled oceanic-atmospheric oscillation is associated with anomalous periods of flood and drought in diverse parts of the Earth (Yarnal, 1985). The ENSO is of considerable interest for understanding the clusterings of floods and droughts observed in paleohydrological records. Preliminary work from northern Australia suggests that patterns of tropical storm activity and related floods may reflect ENSO variations (Wohl, 1988). Western South America would appear to be very important for establishing the pulse of ENSO variation from studies of Holocene flood sediments (Wells, 1987). Africa, Australia, Southern Asia Fluvial responses in much of equatorial Africa, northern Australia, and southern Asia are dominated by the annual monsoonal weather cycle (Figure 6.5). Monsoons FIGURE 6.5 Maps showing wind patterns (arrows) and convergence zones (dashed lines) at 70 mbar (about 3000 m) for Eastern Hemisphere monsoons. Data from Nieuwolt (1977). (A) Pattern for the Asian summer monsoon (June to September). (B) Pattern for the Asian winter monsoon and northern Australia summer monsoon (December to March). Active (dot) and potential (triangle) SWD-PSI paleoflood investigation sites should document variations in monsoonal activity through the Holocene. are extremely complex meteorological phenomena characterized by dramatic variability from year to year, as documented by the historical record (Mooley and Parthasarathy, 1984). Both droughts and wet anomalies occur in runs, suggesting clumping or persistence of wet and dry anomaly values (Kutzbach, 1986). Paleohydrological data from tropical Africa indicate relatively dry conditions, reduced annual discharge in the Nile, Niger, and Senegal rivers, but with high flood peaks between 17 and 25 ka (Williams, 1985). The latest Pleistocene and early Holocene is a markedly wetter period (COHMAP, 1988). Exceptionally high Nile floods occurred around 12 to 11 ka, and fluctuations in flow levels occurred later into the Holocene, but at reduced levels from the late-glacial extremes.
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FIGURE 6.6 Exceptionally large paleofloods on rivers in Asia and Australia plotted against age (years A.D.). Vertical bars show flood ages for the Chang Jiang (Yangtze River), Y, in China (Luo Cheng-Zheng, 1987); the Finke River, F, in central Australia (Pickup et al., 1988); and the Herbert River, H, in northeastern Australia (Wohl, 1988, 1992b). The number 7 refers to the number of exceptional floods in the indicated time interval. Northern Australia shows a generally similar pattern to that in Africa. During full-glacial time, about 18 ka, monsoonal activity seems to have been weaker than at present (COHMAP, 1988). Drier, windier conditions prevailed until a period of enhanced monsoonal activity in the latest Pleistocene and early Holocene (COHMAP, 1988). Recent paleoflood work in central and northern Australia (Figure 6.6) is just beginning to reveal the occurrences of late Holocene floods. The clustering of large floods in recent years has been attributed to the influence of global warming on the generation of storm systems responsible for the floods (Pickup et al., 1988). The northwestern Deccan upland region of India has an excellent fluvial record of long-term variations in monsoonal hydrology (Rajaguru and Kale, 1985). Between about 17 to 10 ka the aggrading rivers in this area changed from coarse bed load streams to suspended load streams. Kale and Rajaguru (1987) attribute reduced streamflow in this period to weakened monsoonal activity, as also shown in Prell's (1984) paleoceanographic work from the Indian Ocean. A period of markedly increased runoff marks the early Holocene, resulting in regional incision of rivers (Kale and Rajaguru, 1985). This energetic early Holocene fluvial activity persisted until approximately 4.5 ka, at which time a phase of overbank sedimentation generated a lower terrace inset in relation to the extensive late Pleistocene aggradational fill. After approximately 3 ka the scale of fluvial change lessened with streams characterized by general incision and minor phases of deposition. FUTURE WORK Ultimately fluvial paleohydrological data will be compared to model simulations of hydrological change. This general approach is illustrated by COHMAP (Cooperative Holocene Mapping Project), which compared a global assay of well-dated paleoclimatic data to general-circulation model (GCM) simulations of climatic change (COHMAP, 1988). Such data-model comparisons are used to test model results with the view that well-tested models will best serve to predict the future global change critical to the habitability of the planet. This procedure is an attempted compromise between two partially conflicting approaches to the scientific study of global change. Reconstructions of past environmental change show how complex, interactive global systems actually functioned. However, the documentation of change does not explain the physical mechanisms inducing that change. Models, such as GCMs, on the other hand, provide physical mechanistic explanations, but only for the idealized assumptions on which the models are based. The GCMs are particularly deficient at incorporating the many complex feedback elements that occur within elements of the hydrological cycle. Data-based paleohydrological reconstructions have a more fundamental significance than the testing of various idealized models. By reconstructing the workings of the past hydrological processes of the planet one discovers those patterns of operation, anomalous behavior, and puzzling phenomena that are inherent in nature (Baker, 1991). These phenomena cannot be revealed by models that are but artificial simplifications of nature. When the patterns and anomalies are recognized, analyzed as to cause, and understood, the new understanding can be incorporated into the formulation of more realistic models. Thus, in attempting to understand a natural hydrological system dominated by complex feedbacks, there must be continual feedback between paleohydrological reconstruction of the real operation of that system and idealized model-based explanations of system operation. Maintaining the appropriate balance between paleohydrological approaches and predictive model building will be one of the challenges for modern scientific research on global change. A related problem of overreliance on idealized models for the prediction of long-term hydrological change is that most current models provide information on mean conditions, such as the GCM predictions of mean temperature and precipitation. Aside from issues of the spatial and temporal accuracy of these predictions, there is the question of extreme values and variability of parameters. Variability is of critical importance in river fluxes. As briefly reviewed in this chapter, global fluvial systems have responded to environmental change since the last glaciation with immense variations in water discharge, sediment yield,
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and resulting river patterns and sedimentary sequences. In addition to the progressive changes in these parameters, there has also been a clustering of river transfer energy into episodes of intense flooding. At present there are insufficient data with which to assess the role of such flood clustering relative to the usual assumption of the maximization of river work (sediment movement and erosion) by flow events of moderate frequency (Wolman and Miller, 1960). There is also a need to relate changes in atmospheric circulation patterns to the spatial and temporal variability of extreme floods. The approach to fluvial paleohydrology of IGCP Project 158 and of the INQUA Holocene Commission Working Group on Global Paleohydrology has been to reconstruct river fluxes on a regional scale in drainage basins selected to represent major climatic and hydrological zones. A problem with that strategy is that some rivers may act as better recorders of their past hydrological change than others. For example, the United Kingdom basin chosen for IGCP Project 158 work, the Severn, yielded meager information on its late-glacial and early Holocene paleoflow (Gregory et al., 1987). This contrasts with the abundant paleoflow data generated on the Polish Plain (Korzarski, 1983). The SWD-PSI paleoflood hydrology approach is especially sensitive to the choice of study area. Because some river reaches act as ideal recorders of SWD-PSI data, a major effort should be made to locate the appropriate study sites on the global scale for an optimization of paleoflow information. With increased societal needs for water there have been major changes in the patterns of water and sediment delivery from rivers. Most spectacular are various large-scale engineering plans that have been proposed for river diversions to supply water for arid regions. Such hypothetical changes as the man-made diversion of Siberian rivers from their Arctic outflow points to arid central Asia could have immense potential for changes in the Earth system. Late Quaternary changes in river systems can serve as natural experiments with which to assess the effects of proposed river diversions or other river mega-engineering projects. Fluvial paleohydrological research should contribute to the basis of understanding long-term river behavior in relation to the global environment so that the consequences of river adjustments will be fully anticipated. CONCLUSIONS To understand the complexity of the global earth system, geologists have traditionally sought to reconstruct past earth processes in a manner consistent with the best available physical theory. This is done not only to develop a unique history, but also to characterize the complex Earth systems that cycle over scales of time and space, and defy conventional measurement. Without an understanding of these systems, theoretical modeling is unverified. More important, however, is the recognition of anomalous behavior in the real, model-prototype system that would not have been anticipated in its hypothesized theoretical representation. Such anomalies provide the driving inspiration for scientific discovery. We are only beginning to formulate a global understanding of river fluxes since the last glaciation. Much of the existing knowledge base has developed from the study of changes in mean conditions of water and sediment discharge interpreted from records in alluvial river valleys. By using regime and sediment transport theories, it has been possible to approximate the magnitudes of change for local examples. A new approach to fluvial paleohydrology has developed over the past decade through studies of flood slackwater deposits and paleostage indicators (SWD-PSI) at ideal sites; SWD-PSI paleoflood hydrology is especially applicable to arid, tropical, and savanna environments of high variability. The methodology can also be used to achieve remarkably accurate reconstructions of cataclysmic late-glacial floods. By concentrating on extreme flow events. SWD-PSI paleoflood hydrology can provide a new perspective on fluvial changes. Changes in river flow may be related to the extremes of global circulation, including variations in monsoons, tropical cyclones, and temperate-region storm complexes. Advancement of knowledge on changes in global river fluxes will require an aggressive program of international collaboration. Methods of paleohydrological reconstruction must be used in ways that complement related paleoclimatological work, with both paleoenvironmental data and model-based studies. The importance of this effort is underscored by the key role of river runoff for the future habitability of the Earth. ACKNOWLEDGMENTS This chapter was originally prepared while the author was on sabbatical at the University of Adelaide, South Australia. It was presented at the 1988 Earth System Science Summer Workshop "The Global Water Cycle—Past, Present, and Future," attendance at which was facilitated by Eric Barron, Earth System Science Center, Pennsylvania State University. J.C. Knox and S.A. Schumm provided useful reviews of the manuscript. This chapter is Contribution No. 1 of the Arizona Laboratory of Paleohydrological and Hydroclimatological Analysis (ALPHA), University of Arizona.
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Representative terms from entire chapter: