Survey of OHWM indicator distribution patterns across Arid West landscapes

Arid West fluvial systems are regulated as “Waters of the United States” (WoUS) under Section 404 of the Clean Water Act. The jurisdictional limits in Arid West channels are defined by using the “ordinary high water mark” (OHWM). Hydrology is the driving factor in the development and distribution of OHWM indicators, but incorporating landscape position may provide further insight into their distribution and reliability. The US Army Corps of Engineers OHW delineation manual indicates a large variability in the occurrence and position of OHWM indicators across channels but fails to evaluate the distribution of indicators at a larger scale. Evaluating OHWM indicators at a larger scale requires an understanding of what indicators are present, their sorting and distribution within a watershed, and their relationship to landscape features. For this study, the frequency and occurrence of OHWM indicators were evaluated to understand their distribution patterns across the Arid West region in 14 mountain, 18 foothill, and 17 basin ephemeral and intermittent channels within multiple watersheds. We found no significant OHWM indicator distribution patterns in any specific landscape position, but there were several OHWM indicators that occur more frequently than others across the entire landscape. The lack of a significant relationship between OHWM indicators and types of channels and their landscape positions supports previously reported results that showed that indicators are randomly distributed in all locations across a channel and may best be described as flow indicators rather than OHWM indicators. DISCLAIMER: The contents of this report are not to be used for advertising, publication, or promotional purposes. Citation of trade names does not constitute an official endorsement or approval of the use of such commercial products. All product names and trademarks cited are the property of their respective owners. The findings of this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents. DESTROY THIS REPORT WHEN NO LONGER NEEDED. DO NOT RETURN IT TO THE ORIGINATOR. ERDC/CRREL TR-13-2 iii

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