National Ordinary High Water Mark Field Delineation Manual for Rivers and Streams : Interim Version

Abstract

The ordinary high water mark (OHWM) defines the lateral extent of nontidal aquatic features in the absence of adjacent wetlands in the United States. The federal regulatory definition of the OHWM, 33 CFR 328.3(c)(7), states the OHWM is “that line on the shore established by the fluctuations of water and indicated by physical characteristics such as [a] clear, natural line impressed on the bank, shelving, changes in the character of soil, destruction of terrestrial vegetation, the presence of litter and debris, or other appropriate means that consider the characteristics of the surrounding areas.” This is the first manual to present a methodology for nationwide identification and delineation of the OHWM. A two-page data sheet and field procedure outline a weight-of-evidence (WoE) methodology to organize and evaluate observations at stream sites. This manual presents a consistent, science-based method for delineating the OHWM in streams. It also describes regional differences and challenges in identifying the OHWM at sites disturbed by human-induced or natural changes and illustrates how to use remote data to structure field inquiries and interpret field evidence using the principles of fluvial science. The manual demonstrates that, in many landscape settings, the OHWM may be located near the bankfull elevation.

Remote sensing tools to support ordinary high water mark delineation

“…From cross section 121, remote geomorphic channel indicators of the potential OHWM are marked by breaks in slope and/or berm formation as indicated by red circles and various dashed lines (Figure 7). Visible breaks in slope are noted by dashed lines in Figure 7 Using the weight-of-evidence technique described in David et al (2022), the visual interpretation of this ArcGIS Pro-derived cross section could be used as a supporting line of evidence in the OHWM determination for this specific site. The identification of geomorphic indicators for OHWM determination using ArcGIS Pro is suitable for this site based on high-resolution DEM's (1 m); however, depending on the level of detail required more detailed analysis may be appropriate.…”

“…This section provides an example of FG applications that can be used to support OHWM delineations. The location of this case study was also utilized as a case study in David et al (2022) and Hamill and David (2021). As part of those case studies, field surveys of Antelope Creek were taken at cross-section locations 121 on 27 June 2017 (Figure 10).…”

“…tools available through ESRI's ArcGIS Pro, and recently developed tools like Fluvial Geomorph (FG) are capable of using lidar data and products to derive elevation profiles from lidar data and products, which can aid in identifying and delineating the OHWM through the identification and interpretation of breaks in slopes and shelving observed in those elevation profiles. The interpretation of what these geomorphic characteristics represent in relation to the OHWM can be understood by referencing the OHWM national manual (David et al 2022). This TN provides four options for using these tools to assist in OHWM determinations:…”

“…For this report, Antelope Creek, North Dakota, was chosen to illustrate the processes for identifying OHWMs using Google Earth Pro, ArcGIS PRO and FG. Antelope Creek is a tributary to the Missouri River in North Dakota and is also used as a case study in Hamill and David (2021) and in David et al (2022). For the FG analysis, the fieldsurveyed cross-section locations (x,y,z) were compared to the existing 2014 lidar.…”

“…The site is located at latitude 46°32'43", longitude: ˗101°38'42", at the 60th Street SW bridge crossing Antelope Creek in Grant County, North Dakota. Priority was placed on identification of physical channel characteristics (Figure 2) such as breaks in slope and shelving by deriving channel cross sections to identify geomorphic indicators, which may be useful as OHWM indicators or bankfull/channel forming discharge markers (Lawlor 2004;Hamill and David 2021;David et al 2022). Once the study reach was defined, the four options were applied to determine their utility in defining OHWM at the Antelope Creek site.…”