Interactions between vegetation, flow dynamics, and channel change on the Colorado River, Colorado

Sustained drought in the Upper Colorado River Basin allowed native and nonnative plant species to colonize low-lying bar surfaces within the last five years. The effects of this vegetation at bar and reach scales were assessed by combining a 2-dimensional hydraulic model with models for rigid and flexible vegetation developed by Smith (2001; 2006). Modeling results showed that the vegetation characterized in 2005 produced the greatest reduction in velocity and shear stress (15% and 35% respectively) at flows equal to 28% of bankfull discharge. At the highest flow modeled (76% of bankfull discharge) the vegetation produced almost no change in velocity, but reduced the bed shear stress by 11%. While local effects on the bar were pronounced, overall flow patterns were not greatly changed in the reach. Additional model results simulating future growth of vegetation indicate significant increases in shear stress in the main channel, which could cause channel change.