The impacts of porous rock check dams on a semiarid alluvial fan

Authors: Nichols, Mary; Polyakov, Viktor

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/31/2019
Publication Date: 2/1/2019
Publication URL: http://handle.nal.usda.gov/10113/6302782
Citation: Nichols, M.H., Polyakov, V.O. 2019. The impacts of porous rock check dams on a semiarid alluvial fan. Science of the Total Environment. 664:576-582. https://doi.org/10.1016/j.scitotenv.2019.01.429.
DOI: https://doi.org/10.1016/j.scitotenv.2019.01.429

Interpretive Summary: Check dams are a popular tool for restoring degraded rangeland watersheds. They are small structures that are relatively easy to construct using local materials, and the way they work is conceptually simple. The dams back up water and sediment is deposited as runoff velocities slow. The deposits store moisture that supports vegetation. They are also described as reducing watershed runoff and sediment yields. Because actual measurements of these variable are rare, in 2008 we constructed 37 rock check dams in two small (4.0 and 3.1 ha) instrumented watersheds located 300 m apart on an alluvial fan on the Santa Rita Experimental Range in southern Arizona. After 10 years, we found that we could not detect changes in the outlet runoff volume, but we did find changes to the ratio of runoff to precipitation. However, on one watershed the change was persistent and not on the other. We also found that measured sediment concentration was reduced on one watershed but not on the other. In addition, the check dams back filled to capacity on one watershed while the other can store additional sediment. These differences in response are likely due to varying soils, vegetative cover, and the degree of initial channel network development. The dominant impact of the check dams is not on watershed outlet runoff and sediment, but on internal watershed channel morphology and increased channel vegetation. With monitoring and maintenance, check dams can be an effective tool for channel grade stabilization, but watershed restoration requires additional practices to address degraded areas between channels.

Technical Abstract: Low-tech rock check dams are widely used to address land degradation; however, assessments of their impacts on runoff and sediment are lacking and are usually limited to the first few years after construction. In 2008, two small (4.0 and 3.1 ha) instrumented watersheds located 300 m apart on an alluvial fan on the Santa Rita Experimental Range in southern Arizona were treated with 37 porous, loose rock check dams. Ten years after construction, the watersheds are experiencing contrasting responses to treatment. The ratio of runoff to precipitation was higher after check dam construction on one watershed and lower on the other, but not significantly in either case. Statistically significant changes in peak runoff rate are not detectable 10 years after construction at either watershed. However, a statistically significant reduction in sediment concertation was found on one watershed and no change was found at the other. The check dams have altered channel grades. However, backfilling of the dams is nearly complete on one watershed and the other has remaining capacity. The alluvial fan setting poses a complex restoration environment due to high sediment loads that deposit in response to vegetated areas of accumulated sediment creating conditions for channel avulsion and new incising concentrated flow paths. Check dams have a lesser impact on watershed outlet runoff and sediment than on internal watershed channel morphology and vegetation establishment. With monitoring and maintenance, check dams can be an effective tool for grade stabilization, but watershed restoration requires additional practices to address degraded interfluve areas.