The influence of riparian zone grazing on channel stability in an arroyo

The introduction of large herds of domesticated grazing animals by European immigrants in the late 19th century is one primary hypothesis used to explain the initiation of arroyo cutting in the American West. But studies designed to quantify the effects of grazing on watershed hydrology and sedimentation have had conflicting results that are often scale dependent. We used a before-and-after study design to examine effects of changes in cattle grazing practices on channel stability in Muddy Creek, an arroyo in the Colorado River headwaters. We digitized 620 meander loop cutoff geometries in the alluvial bottom of Muddy Creek and used meander loop cutoff rate as a broad measure of channel stability. Poisson regression modeling indicated that the change in grazing practices caused an order-of-magnitude decrease in meander loop cutoff rate that was independent of other forcings. Repeat measurements of channel geometry showed a decline in bankfull channel width of about 40% after the changes were implemented. Increases in riparian vegetation density are deduced to be the primary mechanism for the increase in channel stability. Prior to the changes in grazing practices, at least one meander loop cutoff occurred each year, some of which were large enough to create headcuts that scale with the depth of downcutting in this arroyo system. Our analysis demonstrates the profound effects that different grazing practices have on channel stability in headwater arroyo systems, and the potential for grazing animals to initiate a cascade of sediment yields to receiving stream networks. • Order-of magnitude decrease in meander loop cut-off rate corresponds to improvement in riparian grazing management practices. • Decline in cutoff rate was independent of other hydroclimatic and man-made factors. • Increase in riparian vegetation density is likely primary cause of increase in channel stability.