Flow and sediment regimes at tributary junctions on a regulated river: impact on sediment residence time and benthic macroinvertebrate communities

Tributaries may either ameliorate or exacerbate the geomorphic and ecologic impacts of flow regulation by altering the flux Of water and sediment into the flow-regulated mainstem. To capture the effects of tributary influences on a flow regulated river. long-term discharge and cross-sectional data are used to assess the geomorphic and hydrologic impacts of impoundment. In addition. the use of the short-lived cosmogenic radioisotope Be-7 (half-life 53.4 days) to link sediment transport dynamics to benthic macroinvertebrate community structure is evaluated. It is found that the 7 Be activity of transitional bed load sediment is highly seasonal and reflects both variations in activity of sediment sources and limited sediment residence time within the junction. Benthic communities also exhibit it strong seasonal variability. In the spring, neither the 7 Be activity of the sediment, nor benthic communities exhibit clear relationships with sample site location. In contrast, during the late summer the ratio of Ephemeroptera (mayflies)/Trichoptera (caddisflies) decreased significantly below tributary junctions. This decrease in benthic community ratio was driven by increases in caddisfly abundance and was strongly correlated with the presence of recently 7 Be tagged transitional bedload sediment. These observations are probably associated with the presence of coarse, stable, and unembedded Substrate downstream of triburaries and the rapid turnover of sediment that may also be associated with a rapid flux in nutrients or seston. The results show that tributaries are impacting the flow-regulated mainstem and that these impacts are reflected in the benthic community structure and in the Be-7 activity of transitional bed load sediment. Moreover. the observed reduction ill Competence and capacity of the mainstem following flood control suggests that these spatial discontinuities may be a consequence Of impoundment. Copyright (c) 2008 John Wiley & Sons, Ltd.