Unintended consequences of rangeland conservation structures

Authors: Nichols, Mary; BRANDAU, W.K. - University Of Arizona; SHAW, F.R. - Colorado State University

Submitted to: International Soil and Water Conservation Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/21/2020
Publication Date: 3/1/2021
Citation: Nichols, M.H., Brandau, W., Shaw, F. 2021. Unintended consequences of rangeland conservation structures. International Soil and Water Conservation Research. 9(1):158-165. https://doi.org/10.1016/j.iswcr.2020.11.006.
DOI: https://doi.org/10.1016/j.iswcr.2020.11.006

Interpretive Summary: By the 1930s vast areas of the western US were severely degraded by overgrazing and erosion. During the 1930s and '40s, thousands of structures such as check dams, water spreaders, and contour berms were built across the western US to control water and sediment in an attempt to restore degraded rangelands. However, many of the structures proved ineffective and were subsequently abandoned. Even in cases where structures were effective, many never received maintenance after their initial construction. But they remain on the landscape and are currently controlling runoff and erosion in ways that are counter to their original intention. We evaluated structures that were built to control runoff and erosion in Arizona, USA. In the absence of periodic maintenance during the last half century, 100% of the contour berms evaluated (n=67) have breaks through which water flows, and runoff has scoured either around or through 96% of the water spreader berms evaluated (n=26). Localized failures in these structures created concentrated flow paths that reorganized routing of runoff and sediment thus altering large landscapes and initiating a geomorphic feedback loop that drives the expansion of gully networks. This study emphasizes the unintended legacy impacts of soil and water conservation structures and highlights their role as a potential constraint on contemporary resource management.

Technical Abstract: By the 1930s vast areas of the western US were severely degraded by overgrazing and erosion. In response, Congress authorized conservation work including the construction of erosion control and water storage, distribution, and conveyance structures. Thousands of structures such as check dams, water spreaders, and contour berms were built during the 1930s and '40s to control water and sediment in an attempt to restore degraded rangelands. However, across most of the western US, these soil and water conservation measures were implemented without the benefit of local hydrologic data or technical design guidance. Many of the established conservation practices had been developed for humid regions and were untested for use in semiarid areas that are characterized by highly variable rainfall and flash floods. As a result, many structures proved ineffective and were subsequently abandoned. Even in cases where structures were effective, many never received maintenance after their initial construction. Although structurally compromised, abandoned and unmaintained structures continue to alter surface runoff patterns and can greatly exacerbate erosion. Four sites in Arizona, USA where erosion control and water conservation structures have altered drainage and erosion patterns were selected to characterize the multi-decadal impacts to surface runoff and geomorphic processes caused by these structures. In the absence of periodic maintenance during the last half century, breaches formed in 100% of contour berms (n=67) while 96% of water spreader berms (n=26) were compromised by either breaches or flanks. Localized failures in these structures created concentrated flow paths that reorganized routing of runoff and sediment thus altering large landscapes and initiating a geomorphic feedback loop that drives the expansion of gully networks. This study emphasizes the unintended legacy impacts of soil and water conservation structures and highlights their role as a potential constraint on contemporary resource management.