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A controlled flood in the Grand Canyon ince the Glen Canyon dam first began to store water in 1963, creating S Lake Powell, some 430 km (270 miles) of the Colorado River, including Grand Canyon National Park, have been virtually bereft of seasonal floods. Before 1963, melting snow in the upper basin produced an average peak discharge exceeding 2400 m3/s; after the dam was constructed, releases were generally maintained at less than 500 m'/s. The building of the dam also trapped more than 95% of the sediment moving down the Colorado River in Lake Powell (Collier et al. 1996). This dramatic change in flow regime produced drastic alterations in the dynamic nature of the historically sediment-laden Colorado River. The annual cycle of scour and fill had maintained large sandbars along the river banks, prevented encroachment of vegetation onto these bars, and limited bouldery debris deposits from constricting the river at the mouths of tributaries (Collier et al. 1997). When flows were reduced, the limited amount of sand accumulated in the channel rather, than in bars farther up the river banks, and shallow low-velocity habitat in eddies used by juvenile fishes declined. Flow regulation allowed for increased cover of wetland and riparian vegetation, which expanded into sitesthat were regularly scoured by floods in the constrained fluvial canyon of the Colorado River; however, much of the woody vegetation that established after the dam's construction is composed of an exotic tree, salt cedar (Tamarix sp.; Stevens et al. 1995). Restoration of flood flows clearly would `help to steer the aquatic and riparian ecosystem toward its former state and decrease the area of wetland.- andriparianvegetation, but precisely how the system would respond to an artificial` flood could not be predicted. In an example of adaptive management (i.e., a planned experiment to guide further actions), ?a controlled, seven-day; flood of X274. in A was released through the Glen Canyon dam in late MV rch 1996 This flow, roughly 35% of the pre-dam average fora spring flood-(and ar1ess?than ' 14, s the maxiuiumIow ;tliat,could bass some large historical floods), 'wa through the-power plant turbines plus four steepdrainpipes, and it cost approximately$2million inlost hydropower revenues'(Collie et'a1:1997_) The immediate result was significant beach' building: Ov?r?S3°/a of the beaches increased in size, and just 10%? decreased in size. Full docuriienta- tion of the effects will continue to be monitored- by measiiring channel cross-sections and fussing riparian vegetation and fish populations instead, route it quickly downstream, increasing the size and frequency of floods and reducing baseflow levels during dry periods (Figure 3b; Leo- pold 1968). Over time, these prac- tices degrade in-channel habitat for aquatic species. They may also iso- late the floodplain from overbank flows, thereby degrading habitat for riparian species. Similarly, urban- ization and suburbanization associ- ated with human population expan- sion across the landscape create impermeable surfaces that direct water away from subsurface path- ways to overland flow (and often into storm drains). Consequently, floods increase in frequency and in- tensity (Beven 1986), banks erode, and channels widen (Hammer 1972), 774 and baseflow declines during dry pe- riods (Figure 3c). Whereas dams and diversions af- fect rivers of virtually all sizes, and land-use impacts are particularly evi- dent in headwaters, lowland rivers are greatly influenced by efforts to sever channel-floodplain linkages. Flood control projects have short- ened, narrowed, straightened, and leveed many river systems and cut the main channels off from their flood- plains (NRC 1992). For example, channelization of the Kissimmee River above Lake Okeechobee, Florida, by the US Army Corps of Engineers transformed a historical 166 km meandering river with a 1.5 to 3 km wide floodplain into a 90 km long canal flowing through a series of five impoundments, resulting in great loa of river channel habitat and Adjacent.`-, floodplain wetlands (Toth 1995), Because levees are designed to pre. vent increases in the width of flows rivers respond by cutting deeper : channels, reaching higher velocities,e or both. Channelization and wetland, drainage can actually increase the magnitude of extreme floods,-be.:] cause reduction in upstream storage capacity results in accelerated water. delivery downstream. Much of the damage caused by the extensive,,;; flooding along the Mississippi River; in 1993 resulted from levee failure as the river reestablished historic con. -_ nections to the floodplain. Thus, al- though elaborate storage dam and levee systems can "reclaim" the floodplain for agriculture and hu- man settlement in most years, the occasional but inevitable large floods J will impose increasingly high disas-, F ter costs to society (Faber 1996). The severing of floodplains from rivers also stops the processes of sediment; erosion and deposition that regulate the topographic diversity of flood- plains. This diversity is essential for *M maintaining species diversity on '16 floodplains, where relatively small r differences in land elevation result in large differences in annual inunda tion and soil moisture regimes, which regulate plant distribution and abun dance (Sparks 1992). Ecological functions of the natural flow regime. Naturally variable flows create and maintain the dynamics of in-channel" and floodplain conditions and hagI-? tats that are essential to aquatic aQdl riparian species, as shown schetnad rally in Figure 4. For purposes W, illustration, we treat the component) of a flow regime individually, oll? though in reality they interact to complex ways to regulate geomo,, phic and ecological processes. In di scribing the ecological functions as sociated with the compctrules,, at- flow regime, we pay pa tention to high- and low-flow even i because they often serve as ecolo id ? cal "bottlenecks" that present ct cal stresses and opportunities fota` wide array of riverine species (p2 and Ward 1989). ;:: BioScience Vol. 47 Aro.