River basin development projects, including the construction of dams and irrigation diversions, were led by state and federal government partnerships during the 1950's and 1960's. Human demands for flood protection, water for irrigation, hydroelectric power, navigation, and bank stabilization resulted in large public works undertaken by the U.S. Army Corps of Engineers and the U.S. Bureau of Reclamation. These projects pose pervasive threats to midcontinent aquatic ecosystems because they have significantly altered the physical characteristics of most of the region's major river systems, which has resulted in numerous adverse effects, including the loss or decline of many native plant and animal species, especially fishes.

In fact, The Nature Conservancy's Troubled Waters: Aquatic Ecosystems at Risk (Flack and Chipley 1996) states that nationally 67% of freshwater mussels, 64% of crayfish, 37% of freshwater fishes, and 29% of amphibians are at risk. Alterations of river habitats have been instrumental in causing the decline in abundance and diversity of many species (Figure).

a. Preimpoundment

b. Postimpoundment

The large-river systems of the midcontinent are characterized by native riverine species of fish, mussels, and crayfish. These systems course through an arid to semiarid environment where often the only forested habitat is the extensive riparian corridor that lines these streams. This riparian zone provides important habitat for wetland species and serves as a migratory corridor for waterfowl, shorebirds, mammals, and other animals. The native river fishes require flowing water habitats for either all of their life stages (for example, the darters) or for only a portion of their life history (such as paddlefish, sturgeons, and other migratory species).

Melting snowpack in the distant mountains and high-intensity summer rainstorms are the dominant forces that shape the river channels as they cut through alluvial materials. The rivers are composed of shallow, often braided channel habitats, with warm, turbid water interspersed with deep pools along the outside of meander bends. The annual scour and fill cycle deposits sediments on the floodplain and replenishes the bars with the fresh sand and soil necessary for cottonwood regeneration.

The impacts of damming and flow regulation can be classified as immediate or delayed (Holden 1979) or as first-, second-, and third-order effects (Becker and Gorton 1995). The immediate or first-order effects are obvious blockage of upstream and downstream migration of fishes and alteration of the downstream habitat by dewatering and releasing cool (or cold) and clear (free of suspended fine sediments) water through the low-flow portion of the channel. Many kilometers of upstream river corridors are inundated and converted to lakelike habitats. The cold, clear water tends to pick up sediment from the riverbed or banks, which causes the bed to gradually lower or widen. Water releases from midcontinent reservoirs generally produce lower water temperatures in summer and higher water temperatures in winter. Native riverine fishes adapted to the natural temperature regime are displaced downstream and may be unsuccessful in reproduction because of changes in timing of physiological processes keyed to temperature cues. Consequently, these native riverine species may be (and often are) gradually replaced by generalist species, which are often nonindigenous species introduced by humans and adapted to cool or cold water. Basses, sunfish, and northern pike often escape from reservoir stocking programs into the river below, where these predatory nonindigenous fishes not only compete for habitat but also prey on the young native fishes.

The longer-term effects of dams include a degraded and widened channel that can carry a higher volume of flow. This, along with a decrease in the magnitude of peak flows and the trapping of sediments by reservoirs, results in a much lower frequency of overbank flooding. The lack of overbank floods, which deposit sediments, and the erosion of the bed and banks by the sediment-starved reservoir releases result in the loss of sandbars and cause bankwater habitats to be replaced by steep, raw banks along the channel.

Over a period of decades, the cessation of the annual scour and fill cycle that replenishes the bars with fresh sand and soil causes riparian cottonwood stands to gradually become open; these stands are eventually replaced by nonindigenous plant species. Thus, biological productivity is reduced as the number and diversity of wetland and riparian communities along the river corridors decline. In addition, the reproductive success of the native riverine fishes is greatly reduced due to temperature changes and loss of spawning areas. Those fish that do spawn have fewer of the remaining backwater habitats they need to feed and grow. As previously noted, after nonindigenous species such as pike and centrarchids invade, they compete with native species for habitat and they prey on the young native fishes.

Restoration of these declining large-river ecosystems is being discussed widely among resource agencies, conservation groups, and the public. Restoration of these areas to their natural predevelopment condition is almost impossible, however; most restoration efforts are simply attempts to rehabilitate selected segments of river to some predetermined structure and function (Gore and Shields 1995). Dramatic examples include the Kissimmee River in Florida, which biologists are trying to reroute to its original channel, and the March 1996 "test flooding" by the Department of Interior in the Grand Canyon, Arizona. Federal listing of several native large-river species as threatened or endangered and declining biodiversity of aquatic and riparian communities throughout the midcontinent have prompted a reevaluation of how the U.S. Army Corps of Engineers and the U.S. Bureau of Reclamation operate many large federal reservoirs.

Two major studies are ongoing in the Colorado River basin: the Recovery Implementation Program for Endangered Fish Species in the Upper Colorado River basin and the Grand Canyon studies. Extensive monitoring studies have recently begun in the Missouri and Yellowstone rivers. The U.S. Army Corps of Engineers has funded wetland, riparian, and fisheries studies as part of the Missouri River Master Manual Review and Update Study, and the U.S. Bureau of Reclamation is funding fisheries and geomorphological studies in the upper Missouri and Yellowstone rivers. The objective of these studies is to build an analysis and decision support system to allow water managers to better understand the trade-offs associated with various operating scenarios. Biological monitoring is being designed and coordinated through the Missouri River Natural Resources Committee, a group of scientists from each of the basin states, the U.S. Army Corps of Engineers, U.S. Fish and Wildlife Service, U.S. Geological Survey, U.S. Bureau of Reclamation, and the Western Area Power Authority.

Much remains to be learned about the ecology and life-history requirements of the biological communities of the midcontinent river corridor before rehabilitation schemes can be designed by ecologists and natural resource scientists. The opportunities for designing and carrying out scientific studies on the physical and biological processes of large warmwater environments appear good for the next decade. By perfecting large-river sampling techniques and intensive monitoring in large-scale experiments (such as the Grand Canyon), we will have more science-based information about how to manage these systems, which in turn will allow us to more accurately assess the status of these biological communities.