A consequence of river regulation is riparian vegetation expansion. But the absence of flooding produces multi-year accumulation of leaf litter in the riparian zone that is unavailable to aquatic systems. High flow releases from a dam may capture this accumulated leaf litter and provide a pulse of organic matter to the aquatic ecosystem. A planned high release from Glen Canyon Dam of 1175 m³/s (approximately twice the annual peak discharge of 560 m³/s) in March 2008 provided an opportunity to quantify litter biomass before and after the flood, and to estimate system-wide litter inputs during the flood. We also compared input estimates from 2008 (preceded by four years without flooding) to 2004 results (preceded by eight years without flooding) to see how flood frequency affects litter accumulation and export. To determine ash-free dry mass for litter subject to flooding, we collected litter in January and April 2008 from three 1 m2 quadrats that were haphazardly located between discharges of 560 m³/s and 1175 m³/s at three sites within each of 10 geomorphic reaches resulting in a total of 180 samples. We expanded site estimates using a GIS-based vegetation map to produce system-wide and geomorphic reach litter estimates. Data from vegetation mapping provided the extent and an estimate of cover for each of the dominant species contributing leaf litter to the Colorado River.
Results/Conclusions
Using this new methodology we estimated that total annual litter input values for discharges below 560 m³/s averaged 14.04 g AFDM/yr/m2 of river surface, values comparable to other large river systems. Previous studies evaluating litter and controlled flooding along the Colorado River below Glen Canyon Dam have yielded variable results. A 1996 study measuring litter depths associated with a high release (1275 m³/s) from Glen Canyon Dam concluded the controlled flood had little effect on litter evacuation. In contrast, a drift study conducted in fall 2004 estimated that terrestrial vegetation comprised 92% of all CPOM (>1 mm) and estimated riparian inputs averaged 205 g AFDM m2 for 100 km of river. Though the pre- and post-flood litter estimates remain to be determined, we predict a decrease in standing litter and comparable input values to those reported in 2004. These new methods and results provide critical tools and increased understanding of the dynamics between floods and litter accumulation/evacuation to riparian system resource managers in adaptive management settings.
