Basal Resources in Backwaters of the Colorado River Below Glen Canyon Dam-Effects of Discharge Regimes and Comparison with Mainstem Depositional Environments

Abstract

Eight species of fish were native to the Colorado River before the closure of Glen Canyon Dam, but only four of these native species are currently present. A variety of factors are responsible for the loss of native fish species and the limited distribution and abundance of those that remain. These factors include cold and constant water temperatures, predation and competition with nonnative fish species, and food limitation. Backwaters are areas of stagnant flow in a return-current channel and are thought to be critical rearing habitat for juvenile native fish. Backwaters can be warmer than the main channel and may support higher rates of food production. Glen Canyon Dam is a peaking hydropower facility and, as a result, has subdaily variation in discharge because of changes in demand for power. Stable daily discharges may improve the quality of nearshore rearing habitats such as backwaters by increasing warming, stabilizing the substrate, and increasing food production. To evaluate whether backwaters have greater available food resources than main-channel habitats, and how resource availability in backwaters is affected by stable flow regimes, we quantified water-column and benthic food resources in backwaters seasonally for 1 year using both standing (organic matter concentration/density; chlorophyll a concentration/density; zooplankton concentration; benthic invertebrate density and biomass) and process measurements (chamber estimates of ecosystem metabolism). We compared backwater resource measurements with comparable data from main-channel habitats, and compared backwater data collected during stable discharge with data collected when there was subdaily variation in discharge. Rates of primary production in backwaters (mean gross primary production of 1.7 g O2/m2/d) and the main channel (mean gross primary production of 2.0 g O2/m2/d) were similar. Benthic organic matter standing stock (presented as ash-free dry mass-AFDM) was seven times higher in backwaters relative to main-channel habitats (median value of 210 g AFDM/m2 versus 27 g AFDM/m2); this likely reflects greater retention of tributary-derived organic matter in backwaters relative to main-channel habitats. Water-column and benthic organic matter were higher during periods of steady discharge relative to periods of fluctuating discharge. However, our steady-discharge data collection was confounded by tributary activity. Flooding tributaries contribute substantial quantities of sediment and organic matter to the Colorado River; there were two large tributary floods during our steady-discharge data collection but none during our fluctuating-discharge data collections. Although only preliminary data on invertebrate biomass are available at this time, invertebrate biomass in backwaters (range 2-27 mg AFDM/m2) appears low relative to previously published data from main-channel habitats (~100 mg AFDM/m2). The rate of water turnover in backwaters may be a master variable that affects both physical (for example, warming) and biological (for example, primary production) processes in backwaters. We used dye tracer studies to estimate turnover rates in backwaters across flow regimes. Turnover took considerably longer when discharge was stable compared to when there was subdaily variation in discharge (613 minutes versus 220 minutes). Our results indicate that backwaters may represent a sink for organic matter that enters from the main channel and that stable discharge, by lengthening water turnover times, will likely increase organic matter retention.