Benthic Discontinuity between an Unregulated Tributary and the Dam-Controlled Colorado River, Grand Canyon, Arizona, USA

Comparative analysis of factors influencing benthic macro invertebrate (BMI) assemblages in unregulated tributaries can inform management about tributary to mainstream discontinuities (TMDs) with adjacent regulated mainstream rivers. Tapeats Creek is a cool-water tributary of the highly regulated Colorado River ecosystem in Grand Canyon downstream from Glen Canyon Dam, having similar geochemistry, water temperature, and annual relative flow variability compared to the mainstream. The creek supports a diverse BMI assemblage, including Ephemeroptera, Plecoptera, and Trichoptera (EPT) that are absent in the mainstream. We used field measurements and experiments in six microhabitats around the creek mouth to test the impacts of water quality (temperature, geochemistry), flow variability, and substrate embeddedness impacts on BMI distribution, particularly during stepped hydropower-related mainstream stage shifts. The Tapeats Creek TMD was not attributable to temperature, geochemistry, or contemporary low-level hydropower flow fluctuations, but rather to natural embeddedness in mainstream substrata. Tap eats Creek is floored with gravel and cobble, with much interstitial space, whereas the mainstream benthos was composed of cobble/boulders embedded in fine sand, with marginal to suboptimal EPT habitat. The TMD was likely more pronounced in pre-dam time and from 1964-1990 due to larger seasonal and daily flow fluctuations and more prolonged confluence inundation, respectively. The Colorado River in Grand Canyon is adaptively managed to balance hydropower production, fine sediment mass balance for recreational camping and shoreline habitat, as well as native and recreational fisheries. However, strategies to promote both fine sediment storage and an optimal mainstream food base for fish are not be mutually compatible. Thus, not all desired river ecosystem conditions may be simultaneously achieved.

Keywords: Benthic macro invertebrate ecology, discontinuity, Colorado River, embeddedness, EPT, Glen Canyon Dam, Grand Canyon, tributary