Impacts of flow regulation on geomorphic adjustment and riparian vegetation succession along an anabranching reach of the Upper Yellow River

Abstract

Closure of Longyangxia Dam in 1986, the uppermost large dam along the trunk stream of the Upper Yellow River in western China, permanently altered the flow regime in Guide reach, an anabranching section of river located 41 km downstream of the dam. Two smaller hydropower structures, Nina and Laxiwa Dams, were constructed between Longyangxia Dam and Guide in 2003 and 2009 respectively, but they had a minor influence on the flow regime. This study uses the Indicators of Hydrologic Alteration model, satellite imagery and field analyses to assess how the magnitude and duration of changes to the flow regime impacted upon geomorphic adjustments and riparian vegetation succession from 1987 to 2015. Flow impoundment resulted in near instantaneous reduction in the width of the active channel zone. Human activities have manipulated the additional floodplain areas created by these changes. Aggregation of mid-channel bars into larger compound features decreased channel multiplicity. Negligible bed incision likely reflects local supply of sediment from tributary sources downstream of the dam. Significant correlations are shown between changes to the balance between channel depositional and erosional processes induced by flow regulation and the functional composition of vegetation succession patterns. Maintenance of a higher water table for longer periods of time is inferred to have altered mutual interactions between hydrogeomorphic processes and riparian vegetation in this semi-arid setting.