Prescribing flood regimes to sustain riparian ecosystems along meandering rivers

The composition and structure of native riverine ecosystems are tightly lin ked to natural hydrologic variability. By managing river flows for water su pplies and power generation water management agencies have inadvertently ca used considerable degradation of riverine ecosystems and associated biodive rsity. New approaches for meeting human needs for water while conserving th e ecological integrity of riverine ecosystems are greatly needed We describ e an approach for identifying the natural flooding characteristics that mus t be protected or restored to maintain riparian (floodplain) ecosystems alo ng meandering rivers. We developed a computer model to simulate flood-drive n changes in the relative abundance of riparian patch types along the Yampa River in Colorado (U.S.A). The model is based on research suggesting that the duration of flooding at or above 209 m(3) per second (125% of bankfull discharge) is particularly important in driving lateral channel migration w hich is responsible for initiating ecological succession in the Yampa's rip arian forest. Other hydrologic variables, such as the magnitude of annual p eak flows, were not as strongly correlated with lateral channel migration r ates. Model simulations enabled us to tentatively identify a threshold of a lteration of flood duration that could lead to substantial changes in the a bundance of forest patch types over time should river flows be regulated by future water projects. Based on this analysis, we suggest an ecologically compatible water management approach that avoids crossing flood alteration thresholds and provides opportunity to use a portion of flood waters for hu man purposes. Recommended improvements to the Yampa model include obtaining additional low-elevation aerial photographs of the river corridor to enabl e better estimation of channel migration rates and vegetation changes. Thes e additional data should greatly improve the model's accuracy and predictiv e capabilities and therefore its management value.