Distribution of Ordinary High Water Mark (OHWM) Indicators and Their Reliability in Identifying the Limits of Waters of the United States" in Arid Southwestern Channels"

Abstract

Abstract : The U.S. Army Corps of Engineers (COE) is actively involved in flood plain management and regulation of dry wash flood plains 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 flood plain 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.