Sediment erosion and deposition in two sets of paired (treated and untreated) upland drainages in the Torreon Wash watershed, upper Rio Puerco Basin, New Mexico, were examined over a 3 1/2-year period from spring 2009 through fall 2012. The objective was to evaluate the effectiveness of shallow, loose-stone check dams, or “one-rock dams,” as a hillslope gully erosion stabilization and mitigation method, and its potential for retaining upland eroded soils and decreasing delivery of sediment to lower ephemeral stream channels. Two high-resolution topographic surveys, completed at the beginning and end of the study period, were used to assess the effects of the mitigation measures at paired-drainage sites in both Penistaja Arroyo and Papers Wash watersheds, and at six main-stem-channel cross-section clusters along Penistaja Arroyo and Torreon Wash in the Torreon Wash watershed.

For both drainage pairs, the treated drainage had greater sediment aggradation near the channel than the untreated drainage. Erosion was the dominant geomorphic process in the untreated Penistaja Arroyo drainage, whereas aggradation was the dominant process in the other three drainages. For the Penistaja Arroyo paired drainages, the treated site showed a 51-percent increase in area aggraded and 67-percent increase in volume aggraded per area analyzed over the untreated site. Both Papers Wash drainages showed net aggradation, but with similar treatment effect, with the treated site showing a 29-percent increase in area aggraded and 60-percent increase in volume aggraded per area analyzed over the untreated site. In the untreated Penistaja Arroyo drainage, the calculated minimum erosion rate was 0.0055 inches per year (in/yr; 0.14 millimeters per year [mm/yr]), whereas the calculated aggradation rates for the three drainages for which aggradation was the dominant geomorphic process were 0.0063 in/yr (0.16 mm/yr) for the Penistaja Arroyo treated drainage, 0.012 in/yr (0.31 mm/yr) for the Papers Wash untreated drainage, and 0.988 in/yr (2.51 mm/yr) for the Papers Wash treated drainage.

Changes in the channel cross section along the main-stem Penistaja Arroyo and Torreon Wash were also examined. Channel-bank slumping and erosion of previously deposited bed material were apparent sources for sediment suspended in ephemeral streamflow. Cross-sectional channel surveys indicated examples of both erosion and deposition along each channel over the study period. Because the drainage area of the treated drainages is small compared to that of the Torreon Wash watershed, the upland mitigation measures would not be expected to measurably affect short-term concentrations of suspended sediment in main-stem channels.

One-rock-dam mitigation structures in the upland drainages appear to have resulted in a decrease in sediment delivery to the main-stem channel. One-rock-dam mitigation structures may affect streamflow through their influence on runoff volume (via infiltration) and runoff rate (via detention), both of which may vary with time after structure installation.