Effect of Tributary and Braid Confluences on Aquatic Macroinvertebrates in an Alpine River System (2013)

Undergraduate: Patrick Clay

Faculty Advisor: Martin Doyle
Department: Environmental Science

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River confluences are the intersections of two channels of water, most often a tributary and its main stem. Confluences foster increased levels of community density via the input of allochthonous detritus and coarse sediment, and by creating habitat heterogeneity. However, confluences of headwater streams and of streams that do not input coarse sediment and detritus are understudied. We sampled benthic communities within and surrounding confluence junctions. Our sampling sites included both headwater streams and braided confluences, the latter of which do not input exogenous sources of detritus and coarse sediment. Additionally, we measured environmental factors at all of these locations. We found that without an influx of coarse sediment and detritus, as is the case in braid confluences, junctions have depressed macroinvertebrate density and richness. We found that headwater stream confluences have minimal impact on confluence communities, and that in headwater streams, flow in the junction did not exhibit flow zonation. However, all confluences created small step changes in community composition, and so braided rivers may create new microhabitats every time a channel divides or joins. Most importantly, our results show that confluence junctions are not intrinsically locations of increased species richness and density for macroinvertebrates, but rather are areas where benthic communities will be negatively affected unless there is sufficient tributary flux of food sources.