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

Abstract

Abstract: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 7 Be (half-life 53Ð4 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 a 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 tributaries 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 7 Be activity of transitional bed load sediment. Moreover, the observed reduction in competence and capacity of the mainstem following flood control suggests that these spatial discontinuities may be a consequence of impoundment.

Temporal and spatial scales of geomorphic adjustments to reduced competency following flow regulation in bedload-dominated systems

“…Evacuation of fine sediment associated with incision may also lead to sandbar erosion (Grams et al, 2007;Wright et al, 2008), decreased embeddedness (Salant et al, 2006), and decreased ecological diversity and health of native benthic species (Svendsen et al, 2009). Farther downstream, the impact of the dam is modified as the supply of newly derived sediment from downstream tributaries, in addition to sediment removed via evacuation upstream, ultimately exceeds the reduced sediment transport capacity of the regulated mainstem, leading to channel aggradation (Graf, 1980;Andrews, 1986;Everitt, 1993;Grant et al, 2003;Salant et al, 2006;Svendsen et al, 2009).…”

“…Tributary junctions represent critical locations where the addition of water and sediment into the main channel results in significant changes to channel morphology (Best, 1988;Benda et al, 2004a,b) and creates fundamentally important ecological hotspots (Rice et al, 2006;Svendsen et al, 2009). While previous research has determined the general evolution of a main channel's response to regulation (Everitt, 1993), there is limited knowledge specifically about the temporal period of readjustment at tributary junctions and how these effects may differ where sediment transport competency effects exceed sediment transport capacity effects.…”

Early recolonization of a dredged lowland river by dragonflies (Insecta: Odonata)

“…The consequences of these changes, and possibilities for mitigating or reversing the changes, have received great attention (Kiene, 1996;Kamada et al, 2004;Lüderitz et al, 2004;Svendsen et al, 2009). These consequences are nearly always assessed as negative.…”

Effect of Wetland Enhancement on Parasites of Juvenile Yellow Perch

“…This favors the establishment of few large perennial taxa and the development of a substrate essentially composed of rhizomes anchored in a mixture of decomposed organic matter, silt and clay (Foucrier et al 2007). In addition, sediments transported into enhanced wetlands in spring during high flood may be trapped behind the dams (Tockner et al 1999;Svendsen et al 2009). These factors most likely had a determining effect on the quality of the substrate and on the prevailing environmental conditions.…”