The dynamic feedbacks between channel changes in the Colorado River Basin and the rapid invasion of Tamarisk

The resiliency and sensitivity of western rivers to future climate change may be partly anticipated by the response of these rivers to past perturbations in stream flow and sediment supply. Predictions of earlier spring runoff and reduced peak flows of snowmelt-dominated streams mimic hydrologic changes caused by the closure and operation of large dams built within the past century. In the Colorado River Basin, channels have narrowed between 5 and 26% following large dam construction, but the correlation between flow reduction and channel narrowing is confounded by changes in bank strength caused by the rapid spread of the non-native riparian shrub, tamarisk (Tamarix spp.). Thus, predictions of future changes in channel form and analysis of past changes related to dams must distinguish between channel narrowing caused by direct changes in flow, and caused by the indirect effects wherein changes in flow regime allow expansion of non-native riparian vegetation that in turn leads to accelerated channel narrowing. Our research evaluates the geomorphic controls on tamarisk colonization, the role of tamarisk in accelerating the narrowing process, and the dynamic feedbacks between channel changes on western rivers and the invasion of non-native riparian species. The transformation of formerly active bars and channel margins into stable inset floodplain surfaces is the dominant process by which these channels have narrowed, as determined by detailed alluvial stratigraphy and dendrogeomorphology. We recreated the 3-dimensional bar surface present at the time of tamarisk establishment by excavating an extensive network of trenches. In doing this, we evaluated the hydraulic environment within which tamarisk established. We also characterized the hydrodynamic roughness of aging tamarisk stands from ground-based LiDAR scans to evaluate the role of tamarisk in the promotion of floodplain formation. Our study sites are representative of the predominant geomorphic organization of the trunk streams of the Colorado River basin, where significant narrowing within the past century has occurred. The most upstream study sites are on the Yampa River, the largest relatively natural tributary of the Colorado River. Results to date indicate that the floodplain surfaces in the Yampa Canyon and along the Green River in Whirlpool Canyon may be categorized as either hydrologically and geomorphically active or disconnected. Disconnected floodplain surfaces were last inundated in 1984 during the flood of record, yet this flood was not energetic enough to rewiden the channel significantly. Instead, this large flood contributed to further floodplain disconnection by contributing to vertical floodplain aggradation. The position of tamarisk cohorts and site history indicate that within a given hydrogeomorphic setting, tamarisk is capable of controlling the narrowing process. Site-specific findings will be matched to future work on trends in tamarisk abundance and the degree of narrowing at the reach scale evaluated using both 1- and 2-dimensional hydraulic indicators. Results from our work will help identify geomorphic settings which are most sensitive to further channel change with predicted changes to the climate and proposed hydrologic development projects.