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hill cranes (Gnus canadensis), which made the Platte River famous, have abandoned river segments that have narrowed the most (Krapu et al. 1984). Changes in the duration of flow conditions also have significant bio- logical consequences. Riparian plant species respond dramatically to chan- nel dewatering, which occurs fre- quently in and regions due to surface water diversion and groundwater pumping. These biological and eco- logical responses range from altered leaf morphology to total loss of ri- parian vegetation cover (Table 2). Changes in duration of inundation, independent of changes in annual volume of flow, can alter the abun- dance of plant cover types (Auble et al. 1994). For example, increased duration of inundation has contrib- uted to the conversion of grassland to forest along a regulated Austra- lian river (Bren 1992). For aquatic species, prolonged flows of particu- lar levels can also be damaging. In the regulated Pecos River of New Mexico, artificially prolonged high summer flows for irrigation displace the floating eggs of the threatened Pecos bluntnose shiner (Notropis sinius pecosensis) into unfavorable habitat, where none survive (Robertson in press). Modification of natural flow tim- ing, or predictability, can affect aquatic organisms both directly and indirectly. For example, some native fishes in Norway use seasonal flow peaks as a cue for egg hatching, and river regulation that eliminates these peaks can directly reduce local popu- lation sizes of these species (Nxsje et al. 1995). Furthermore, entire food webs, not just single species, may be modified by altered flow timing. In regulated rivers of northern Califor- nia, the seasonal shifting of scouring flows from winter to summer indi- rectly reduces the growth rate of juve- nile steelhead trout (Oncorhyncus mykiss) by increasing the relative abundance of predator-resistant in- vertebrates that divert energy away from the food chain leading to trout (Wootton et al. 1996). In unregu- lated rivers, high winter flows re- duce these predator-resistant insects and favor species that are more pal- atable to fish. Riparian plant species are also strongly affected by altered flow tim- 1750LPrior to 1776, widespread beaver dams naturally control streamflow: dams gradually disappear as beavers are hunted to near extinction; mill dams replace beaver dams as territory is settled. 1824 - Creation of Arty Corps of Engineers. with task of keeping rivers navigable: federal government begins support of commercial navigation on the Mississippi. 1825 - Completion of Erie Canal, creating transport route from the Hudson River to the Great Lakes. 1849. 1850, 1860 - Swamp Land Acts. transferring 65 million acres of wetlands in 15 states from federal to state administration for purpose of drainage:. 1850 Act gives Everglades to Florida. 1880'5 - ditching and draining of wetlands in tributaries to the Mississippi River begins. 1900 1901 - canal built from Colorado River to Salton Sink and the Imperial Valley is bom. Foods of 19W-1905 create Salton Sea, and the river is put back in its original channel. 1902 - Reclamation Project Act. establishing Reclamation Service to 'nationalize the works of irrigation'. 1920 - Federal Power Act authorizes licensing of non-federal hydropower dams. 1925 1927 - Mississippi River floods, proving existing levees inadequate and leading to 1928 Flood Control Act 1928 - Colorado River Compaq ratified. partitioning the river's water 1933 - Tennessee Valley Autlprily Act passed, and nation embarks on rust multipurpose project for controlling and using a river. 1935 - Hoover Dam dedicated by FOR. 7930.1940 - U.S. Amry Corps constructs 9-Foot Channel Project, turning upper Mississippi into an inbacondnental channel. 1940 - channel straightening of tributaries to the Mississippi River begins. 1944 - Flood Control Act auftnzes federal participation in flood control projects. and establishes recreation as a full purpose for flood control projects. 1950 1953 - building of flood control dams begins on the Mississippi River. 750 miles channelized upstream from mouth. 1954 - Watershed Protection and Flood Prevention Act, begins active Soil Conservation Service involvement in helping farmers to channelize streams. 1963 - Glen Canyon Dam completed: 1964 - U.S. and Canada ratify Columbia River Treaty, 1965 - California State Water Project approved. F1968 -Wild and Scenic Rivers Act passed to preserve certain rivers in "free4lomng condition'. 19751 1978 - PURPA passed. providing market for small-scale hydropower generation. 1986 - Electric Consumers Protection Act - amends Federal Power Act. requires FERC to give equal consideration to power generation potential and fish, wildlife, recreation, and other aspects of environmental quality during dam licensing/relicensing. 1992 - legislation approved for federal purchase and removal of 2 private dams on the Elwha River, to restore fish passage. 1993 - major food on Mississippi River causes extensive damage. 2000 1996 - Controlled flood of Colorado River at Grand Canyon: restoration of Everglades begins. Figure S. A brief history of flow alteration in the United States. ing (Table 2). A shift in timing of peak flows from spring to summer, as often occurs when reservoirs are managed to supply irrigation water, has prevented reestablishment of the Fremont cottonwood (Populus fremontii), the dominant plant spe- cies in Arizona, because flow peaks now occur after, rather than before, its germination period (Fenner et al. 1985). Non-native plant species with less specific germination require- ments may benefit from changes in flood timing. For example, salt cedar's (Tamarix sp.) long seed dis- persal period allows it to establish after floods occurring any time during the growing season, contributing to its abundance on floodplains of the west- ern United States (Horton 1977). Altering the rate of change in flow can negatively affect both aquatic and riparian species. As mentioned above, loss of natural flashiness threatens most of the native fish fauna of the American Southwest (Minckley and Deacon 1991), and artificially increased rates of change caused by peaking power hydroelectric dams on historically less flashy rivers cre- ates numerous ecological problems (Table 2; Petts 1984). A modified rate of change can devastate riparian species, such as cottonwoods, whose successful seedling growth depends on the rate of groundwater recession following floodplain inundation. In the St. Mary River in Alberta, Canada, for example, rapid draw- downs of river stage during spring have prevented the recruitment of young trees (Rood and Mahoney 1990). Such effects can be reversed, however. Restoration of the spring flood and its natural, slow recession in the Truckee River in California has allowed the successful establish- ment of a new generation of cotton- 778 BioScience Vol. 47 No. 11