Contemporary and Historic Comparisons of Aquatic Macroinvertebrates in the Regulated Green River and Unregulated Yampa River within Dinosaur National Monument
Similar to other large, hypolimnetic dams, Flaming Gorge Dam (FGD) has dramatically altered the hydrologic, sediment and thermal regimes of the Green River. Extensive studies of aquatic macroinvertebrate response and recovery patterns have occurred within 50 km of the dam, while quantification of downstream recovery trajectories have not been extended into Dinosaur National Monument (DINONM) where the Yampa River, a large relatively unregulated tributary, joins the Green River. Given the growing number of water development proposals on the Yampa and Green Rivers, understanding the remediating effects of the Yampa River is imperative. The National Park Service in cooperation with the BLM/USU National Aquatic Monitoring Center has intensively sampled the mainstem of the Green and Yampa Rivers throughout DINONM for over nine years. Although individual reports have resulted from such efforts, no comprehensive synthesis of this work has been conducted. Therefore, we paired short-term monitoring of a recently established network of 21 sentinel sites with exhaustive contemporary and historical macroinvertebrate inventories to quantify the extent of species extirpations and alterations to assemblage composition imposed by the management of FGD.
Our results largely support the downstream recovery trajectories of macroinvertebrate assemblages outlined by the serial discontinuity concept. We observed considerable recovery of macroinvertebrate species richness in Lodore Canyon, 70 km downstream of FGD; however, both historical and contemporary analyses suggest the persistence of significant compositional differences until the confluence with the larger Yampa River. After the confluence with the Yampa River we failed to detect significant differences in macroinvertebrate richness or assemblage composition on the Green River relative to both the Yampa and historic conditions, highlighting the significant remediating effects of the Yampa River. In contrast to other tailwaters, invasive invertebrates, although present, were not found at densities likely to have significant effects on the structure or function of the lotic ecosystem. Rather, of greater management concern is the increased hydrogeomorphic stability and water clarity created by FGD, which appears to have created an alternative steady state where invertebrate composition significantly differs from both the Yampa and historic conditions. Further research is needed to understand the effects of compositional differences on food web structure and function and the subsequent ecological stability of the system to inform FGD release patterns and management of aquatic invasives and endangered fishes of the Upper Colorado River Basin.
