A Field Guide to the Identification of the Ordinary High Water Mark (OHWM) in the Arid West Region of the Western United States: A Delineation Manual

Abstract

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.

Vegetation and Channel Morphology Responses to Ordinary High Water Discharge Events in Arid West Stream Channels

“…However, in the Arid West, studies suggest that intermittent and ephemeral streams do not have separate bankfull channels and active floodplains; instead the bankfull and active floodplain combine to make one active floodplain where the majority of work occurs (Lichvar and McColley 2008). While this study does support this theory, for the purpose of the study methodology, the bankfull channel and the active floodplain were analyzed as separate units so that differences in response features could be observed.…”

“…• Do the results of evaluating the channel morphology and vegetation responses for various discharge events support the published OHW methodology (Lichvar and McColley 2008)?…”

“…By analyzing aerial photos from before and after discharge events, we tried to identify how low to moderate discharges typically associated with delineating the OHW (Lichvar and McColley 2008) impact the vegetation within a variety of watersheds.…”

The Benefits and Limitations of Hydraulic Modeling for Ordinary High Water Mark Delineation

“…For OHWM delineation in arid and semi-arid areas, HEC-GeoRAS can be especially valuable because the OHWM tends to have a visual signature based on a characteristic look of active channels (Lichvar and McColley 2008;Mersel and Lichvar 2014). Furthermore, individual field indicators may be visible in the imagery, especially changes in sediment texture and changes in vegetation.…”

“…The OHWM is not associated with a specific streamflow recurrence interval; however, it is generally associated with streamflow levels well above mean discharge but less than extreme and infrequent flood events (Lichvar and McColley 2008;Mersel and Lichvar 2014). In non-perennial arid stream systems, for instance, the OHWM signature has been associated with flows generally ranging from about the 1-to 15-year flood event (Curtis et al 2011).…”