Semiarid Watershed Response to Low-Tech Porous Rock Check Dams.

Rock check dams are used throughout the world to mitigate erosion problems on degraded lands. Many restoration efforts on rangelands in the southwestern United States incorporate such structures; however, their impact on watershed response and channel morphology is not well quantified. In 2008, 37 porous rock structures were built on two small (4.0 and 3.1 ha) instrumented watersheds on an alluvial fan at the base of the Santa Rita Mountains in southern Arizona. Thirty-five years of historical rainfall, runoff, and sediment data are available to compare with 7 years of data collected after check dam construction. In addition, postconstruction measurements of channel geometry and longitudinal channel profiles were compared with preconstruction measurements. The primary impact of the check dams was retention of channel sediment and reduction in channel gradient; however, response varied between the proximal watersheds, with 80% of the check dams on one of the watersheds filled to 100% of their capacity after seven runoff seasons. Precipitation runoff ratios changed after construction, but the change was not persistent after check dams filled to capacity. The contrasting watershed experiences lower sediment yields, and only 20% of the check dams on this watershed was filled to capacity. Within the watersheds, the mean gradient of the channel reach immediately upstream of the structures has been reduced by 35% (from 0.061 to 0.039) and 34% (from 0.071 to 0.047). [ABSTRACT FROM AUTHOR]