Flood Inundation Models using Airborne Laser Altimetry (LIDAR) to Establish Significance of Spatial and Temporal Distribution of Indicators for Ordinary High Water Marks in Arid Southwestern Stream Systems

"Waters of the United States" (WoUS) are identified by use of physical features in the field to determine the location of the "Ordinary High Water Mark" (OHWM) and the limits of the Federal jurisdiction under the Clean Water Act (33 CFR 328.3). Arid fluvial systems that dominate the western landscape are critically important environments that provide valuable ecological benefits to the nation. These arid streams and rivers recognized as WoUS convey floodwaters and help ameliorate flood damage; maintain water quality and quantity; provide habitat for plants, aquatic organisms and wildlife, which determine the physical characteristics and biological productivity of downstream environments. Recent research efforts have identified a variety of field OWHM indicators useful for delineation purposes in the arid west. As part of that same ongoing effort, testing the reliability of the field OHWM indicators in identifying WoUS boundaries has provided insight to the distribution patterns of physical and biological features in response to a suite of discharges of varying magnitude and frequency. The objective of this research is to utilize 1-D HEC-RAS flood inundation models (2, 5, 10, 25, 50 and 100 year return intervals) for two intermittent stream channel reaches at different landscape positions in the Mojave Desert, California using highly accurate lidar topographic data and detailed field mapping results to test the reliability of the field OHWM features in identifying the WoUS boundary and to better understand the frequency of inundation necessary for creation and maintenance of the various field OHWM features. Our preliminary results show that 80-90 percent of the OHWM indicators are located within or near the active floodplain, and some are sorting along the bankfull channel. Surficial textural differences and vegetative cover patterns are sorting along the active floodplain boundaries while the distribution of other vegetative indicators within the stream channel reach are associated with distinct flood inundation levels. It has been previously noted that relationships between flood frequency and physical and biological responses, vegetation structure and species diversity of riparian communities exist. Although, current wetland delineation and evaluation methods for the arid west do not connect OHWM or other physical and biological features to flood frequency which are the over riding factor in assessing the WoUS functional levels of a watershed.