Degradation of Colorado River native fish habitat as a result of the complex interactions between vegetation, hydrologic changes, and geomorphology

Authors: Rebecca Manners, John C. Schmidt

Year: 2010 (xlii)

Abstract

Many of the streams of the Colorado River basin have narrowed and their planforms have become more simple. The result of these geomorphic changes is less available habitat for the native endemic fishes. Photographs taken in the early part of the 20th century depict wide and shallow channels. Today, the same channels are narrower with less complex habitat. The geomorphic changes have occurred, because sediment has been deposited within the formerly active channel as inset floodplains; sediment has also filled side channels and thereby transformed multi-thread reaches to single thread channels. The controlling variables, processes, and time duration over which these channels have changed are complex and are related to decadal changes in runoff, dam construction, transbasin diversions, and the invasion of non-native woody plants. In the Colorado River basin, large dams have significantly altered the hydrology and sediment loads many rivers. These changes occurred concurrently with natural climatic cycles and the spread of the invasive non-native riparian shrub tamarisk (Tamarix spp), that has served to increase hydraulic roughness and stabilize sediment. Feedbacks exist among water, sediment, and vegetation. As such, the interactions among these profound environmental changes have led to non-linear trajectories of channel change in the basin. We are exploring the interactions and feedbacks among tamarisk, natural climatic cycles, and dam operations in shaping the modern riverine and riparian environment along the large river’s of the Colorado River basin. We are measuring channel changes and the spatial and temporal encroachment of tamarisk in three canyons in the eastern Uinta Mountains in Dinosaur National Monument. These canyons vary in their degree of tamarisk cover and hydrologic alteration: 1) the relatively unregulated Yampa River has the least tamarisk cover, 2) the highly regulated Green River upstream from the confluence with the Yampa (Lodore), and 3) a hybrid between the two, the Green River downstream from Echo Park. The channel has been progressively narrowing since at least the early 1960’s in all study reaches. Channel changes are not as profound on the Yampa River as they are on the Green River, but are still considerable in certain geomorphic settings, such as areas ponded at high flows. The temporal signature and dominant flow regime at the time of tamarisk establishment along the Yampa River is specific to the geomorphic surface. For example cohorts that developed during droughts persist on gravel bars, while cohorts that developed during floods persist on high, sandy channel margin bars. In contrast, on the Green River, similar geomorphic surfaces have a wide range of tamarisk establishment ages. Spatial homogeneity of tamarisk is likely a result of the reduced flow variability. However, in all settings, cohort ages corresponded either to flood years (by-pass flows on the Green River) or drought years. These findings indicate that there are hydrologic and geomorphic controls on tamarisk establishment. These factors, along with the dynamics of tamarisk invasion, define the contemporary riverine and riparian environment.