Interactions Between Vegetation Expansion and Flow Dynamics on the Colorado River, CO, USA

Sustained drought in the Upper Colorado River Basin has severely limited peak flows, allowing native and non-native plant species to colonize low-lying bar surfaces within the last five years. This vegetation has the potential to locally shift velocity patterns, forcing adjustments in channel morphology. The role of vegetation in influencing velocity and altering shear stress in the Colorado River was assessed using measurements of vegetation density, channel geometry, water surface profiles, and hydraulic modeling over a low-lying gravel bar in the vicinity of the USGS gauge near the Colorado-Utah state line. Native sandbar willow ( Salix exigua) made up 87% of the vegetation on the bar, and averaged 0.54 m tall and 0.35 cm in diameter. The peak discharge of 850 m3/s in 2005 was sufficient to inundate the bar to a depth of 2 m, covering all plant stems. Flows in the study reach were above the threshold for gravel transport (500 m3/s) for 12 days, yet most of the plants survived. At specific plots, the number of stems measured prior to peak decreased by 27% on average afterwards. The number of remaining stems that produced leaves after the peak varied between 30% and 84%. Despite some losses, most of the vegetation remained and continued to grow rapidly throughout the summer of 2005. This suggests that vegetation density was sufficient to limit the intensity of bed load transport over the bar surface, allowing vegetation to survive.