How much water does the Rio Grande River need? The ecological consequences of an altered flow regime on an arid river

Authors: Demitra Blythe, Phaedra Budy Dr.

Year: 2016 (xlviii)

Abstract

The flow regimes and associated ecosystem function of many arid desert rivers in southwestern North America have been significantly altered from their historic state due to drought, anthropogenic alteration and urban use. Since the 1800s the construction of dams, human use and consumption, and declining yearly precipitation has significantly decreased flows, degrad ed water quality, and changed the frequency and magnitude of large flood events throughout the Rio Grande River. In this study, we examine the impacts of an altered hydrologic regime for the Rio Grande River native fish community. We use a multi -faceted approach to 1) examine the fish community and structure, 2) compare algal samples across levels of habitat complexity to track food resources spatially, and 3) examine discharge patterns around known hypoxic events. We found fish diversity (evenness ba sed on Shannon’s Diversity Index) was greatest and lowest in reaches we a priori classified as high and low (‘simple’) complexity, respectively, at the microhabitat scale. At the macrohabitat scale, where we compared diversity between alluvial valleys and canyons, fish diversity was greater in canyon sites than at alluvial sites. The proportion of native minnows (as potential indicators of endangered Rio Grande Silvery Minnow habitat) was greatest within ‘moderate’ or ‘simple’ habitat reaches. Further, seston algae and soft sediment periphyton abundance (i.e., food resources) were significantly lower within ‘complex’ reaches compared to ‘moderate’ or ‘simple’ reaches, which occur primarily in alluvial sections. Conversely, periphyton abundance on hard substrat es was greater and more variable for ‘complex’ reaches, which occur primarily in canyon sections. Finally, there were more frequent hypoxic events following steady, low magnitude (<100 cfs) flows lasting longer than one week. Collectively, these results sug gest reaches within the alluvial valleys or those considered more ‘simple’, have less diversity and are potentially more susceptible to changes in flow magnitude than sites considered highly ‘complex’ or located within a canyon. Further, habitat availability, particularly in alluvial reaches, may be a potential limiting factor for native fish maintenance and recovery in the Rio Grande River.