The impacts to riparian areas are manifested in the quality of adjacent waterbodies throughout the United States. Only about two percent of the nation’s streams and rivers are classified as having high water quality (Benke, 1990). A 1998 summary of polluted waters for all 50 states indicates there are more than 300,000 miles of rivers and streams and more than 5 million acres of lakes that do not meet state water-quality standards (EPA, 2000).
Throughout history, societies have sought to regulate water resources. Today, over three-fourths of the 139 largest river ecosystems in the northern third of the earth are strongly or moderately fragmented by dams, interbasin diversions, and irrigation (Dynesius and Nilsson, 1994). In the contiguous 48 states, all large rivers greater than 1,000 km in length, except the Yellowstone River of Montana, have been severely altered for hydropower and/or navigation, and only 42 free-flowing river segments greater than 200 km in length remain (Benke, 1990). Disconnection of river systems from their historical floodplains is a severe problem worldwide about which there is limited but growing understanding (Naiman and Décamps, 1990).
Changes in natural hydrologic disturbance regimes and patterns of sediment transport include alteration of the timing of downstream flow, attenuation of peak flows, and other effects. Such alterations can result from dam construction, from transbasin diversions, or by water removal from rivers for irrigation or other consumptive uses, often in combination. For example, along the mainstem Columbia River in the Pacific Northwest, snowmelt peak flows have been suppressed by upriver storage facilities and the management of the river system for both power generation and flood control (NRC, 1996). Similarly, the Willamette River in Oregon has a reduced frequency of overbank flows, disconnected side channels, and greatly reduced potential for maintaining riparian and floodplain forests because of extensive bank stabilization and dam construction (Figure 3-1). Box 3-1 gives an example of the effects of various hydrologic manipulations on riparian plant communities and ecosystem processes in the arid Southwest.
The following sections discuss the specific effects of dams, bank-stabilizing structures, levees, and groundwater withdrawal on riparian structure and functioning. The extent to which downstream riparian areas are affected by these changes depends upon the degree of flow and sediment alteration plus the capability of the riparian plant communities to respond to these changing environmental conditions.
The vast majority of dam building and associated water resources development in the contiguous United States occurred during the middle portion of the