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.

To support the determination of hydrophytic vegetation in wetland delineations in Alaska, USA, a series of tests were conducted to develop a group of "test positive" species to be used in a "cryptogam indicator." in 2004, non-vascular cryptogam species (bryophytes, lichens, and fungi) from interior and South-Central Alaska in the vicinities of Fairbanks and Anchorage were collected at a series of ten 50 × 50 cm plots along two 30 m transects in each of six upland and five wetland sites. Nineteen moss and liverwort species were selected from 86 species surveyed to test for wetland fidelity. In 2005, a plot-based analysis of frequency and cover data yielded a revised list of 17 bryophyte species that were specific to wetland communities dominated by black spruce, Picea mariana (p. Mill.) b.S.p. Fungi and lichens were found to be inadequate wetland indicators in the sampled locations because the lichen species were sparsely distributed and the fungi were too ephemeral. The cryptogam indicator was thus restricted to bryophytes. Also in 2005, bryophytes were analyzed for their presence on microtopographic positions within the landscape, including tops of hummocks and hollows at the bases of hummocks. Upland bryophyte species were found on hummock tops inside the wetland boundary, but were not abundant in the hollows (p < 0.05). The fidelity of the species selected for use in the cryptogam indicator was tested. it was determined that if more than 50% of all bryophyte cover present in hollows is composed of one or more of the 17 wetland bryophytes tested in 2005, then vascular vegetation can be considered to be hydrophytic (p < 0.001).

Three methods for making vegetation determinations-the Prevalence Index (PI), the Dominance Ratio (DR), and the Hydrophytic Cover Index (HCI)-were compared using national wetland delineation data. The PI and the DR produced significantly fewer hydrophytic determinations (69%, p < 0.01, and 76%, p = 0.04) than the HCI (80%). The three methods disagreed 16% of the time. The HCI produced hydrophytic determina-tions only in plots where hydrophyte (Facultative, Facultative Wetland, and Obligate species) cover was greater than 50% of the total cover and never produced hydrophytic determinations in plots where hydrophyte cover was 50% or less. The PI disagreed with the HCI 12% of the time, producing nonhydrophytic determinations in plots where hydrophyte cover was greater than 50%. The DR disagreed with the HCI 9% of the time. Disagreements included nonhydrophytic determinations in plots where hydrophyte cover was greater than 50%, hydrophytic determinations in plots where hydrophyte cover was 50% or less, and a nonhydrophytic bias in plots dominated by even numbers of plant species. These results demonstrate that HCI determinations are more accurate and consistent than those of the PI and the DR. The HCI method is recommended for making vegetation determinations during wetland delineations in future revisions of the Corps delineation manual and its supplements.

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