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Abstract:The Ordinary High Water Mark (OHWM) is an approach for identifying the lateral limits of non-wetland waters. However, determining whether any non-wetland water is a jurisdictional "Water of the United States" (WoUS) involves further assessment in accordance with the regulations, case law, and clarifying guidance. In the Arid West region of the U.S., the most problematic Ordinary High Water (OHW) delineations are associated with the ephemeral/intermittent channel forms that dominate the Arid West landscape. This report presents a method for delineating the lateral extent of the non-wetland waters in the Arid West using stream geomorphology and vegetation response to the dominant stream discharge.

Methods for wetland identification and delineation require the investigator to determine whether vegetation is hydrophytic. Two widely used techniques for making hydrophytic vegetation decisions involve dominance ratios (i.e., the percentage of dominant species that are rated obligate (OBL), facultative wetland (FACW), and facultative (FAC)) and prevalence indices (i.e., the weighted-average wetland indicator status of alt plants present). We sampled 338 vegetation plots on sites throughout the United States and calculated the dominance ratio and a pitt-based prevalence index for each plot. We found that hydrophytic vegetation decisions based on the two methods disagreed on 16% of field plots. Analysis of simulated plot data (n = 80,000) indicated that frequencies of disagreement increase as vegetation complexity (i.e., number of strata and number of species per stratum) increases. We conclude that the two methods for hydrophytic vegetation decisions disagree too often to be considered equivalent. Additional studies are needed in different biogeographic regions and plant cormnunity types to determine the conditions under which prevalence indices, dominance ratios, or some other treatment of vegetation data provide more reliable indicators of wetland vegetation.

The U.S. Army Corps of Engineers (COE) is actively involved in floodplain management and regulation of dry wash floodplains in the western United States. The COE also regulates "Waters of the United States" (WoUS) under Sec. 404 of the Clean Water Act by determining the extent of surface indicators related to "ordinary" flood discharges known as Ordinary High Water Marks (OHWM). The return interval for inundation to the WoUS boundary is not well understood in the arid West. COE hydrologic models require detailed site information for rainfall and stream flow characteristics, as well as on-site surveys to determine channel morphology, width, fluvial patterns, slope, and other physical attributes. The focus of this research is to quantitatively correlate field indicators to flood return inundation levels in support of identifying the events that best represent the extent of the "ordinary" high water using high-resolution topography acquired through Light Detection and Ranging (LiDAR). The NASA's Airborne Topographic Mapper (ATM) was the source of high-resolution topographic data for several stream reaches throughout the Mojave Desert, California. This research showed that field indicators commonly used to delineate the extent of the OHW are distributed across the entire floodplain and are not associated with a particular level of event. However, a consistent pattern of geomorphic signature and vegetation density is highly correlated to moderate flood events. A hypothesis and a working model were developed to explain the distribution pattern of various OHWM indicators and the repeating geomorphic signature.