GRAZING PRACTICES CONTROL CHANNEL STABILITY IN AN ARROYO

Arroyo cutting in the dry regions of the western United States has been an enigmatic subject of study in the geosciences for more than a century. 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. But there are numerous locations where arroyo cutting was observed before grazing began or where grazing had not occurred, and studies designed to quantify the effects of grazing on watershed hydrology and sedimentation have had mixed results that are often difficult to interpret with increasing spatial scale. We quantified the effect of grazing management and sediment control practices on channel stability in Muddy Creek, an arroyo draining 2,471 km² of cold desert in the Colorado River headwaters of southern Wyoming. The Muddy Creek Basin is mostly public land managed by the Bureau of Land Management for grazing, and was the focus of an abrupt, intense, and well-documented coordinated resource management (CRM) program implemented from 1992 to 2005. The CRM focused on changes in grazing management including off-channel watering troughs, fenced stream exclosures, use of herders to move animals, and rotation grazing. The CRM program also used spreader dikes and low-head dams to expand an existing wetland complex along the mainstem near the center of the basin to trap sediment and attenuate peak flows. We used aerial images to identify, digitize, and age 620 oxbows in the bottomland of Muddy Creek over its entire 150 km range. These data were used to quantify changes in meander loop cutoff rate since 1938. Trend analysis on peak flow and climate data was used to identify any coincident changes in basin flood hydrology and precipitation. Poisson (count) modeling indicates that, prior to the CRM program, meander loop cutoff rate was at least 1 per year, and declined to about 0.1 per year after the CRM program. A decline in peak flows was only detected downstream from the wetland complex, and no downward shift in precipitation was detected in the region, but the increase in channel stability was detected both upstream and downstream from the complex. Our models indicate grazing practices were the primary driver of the change in channel stability in an arroyo setting common throughout the arid western US.