Is the ordinary high water mark ordinarily at bankfull? Applying a weight-of-evidence approach to stream delineation

The ordinary high water mark (OHWM) is a regulatory boundary essential to identifying the lateral jurisdictional limits of rivers and streams in the United States (U.S.). Bankfull is a scientific concept that has been defined and identified in a multitude of ways by scientists. Geomorphologist and hydrologist have long recognized that there can be variability in the identification of bankfull depending on how bankfull is defined. Furthermore, this variability is only increased by the inherent variability in stream characteristics that occurs along a reach of channel. Because of the overlap in the regulatory definition of OHWM and the scientific definitions of bankfull, one of the primary purposes of the study is to apply the definition of OHWM and compare it to bankfull in a variety of channel types in different climatic, hydrologic, and geologic settings. Our results show that there is a clear overlap between the identification of the OHWM and bankfull elevations. Regulatory practitioners are generally not specialized in fluvial geomorphology and yet are tasked with consistently and accurately identifying the OHWM in a variety of stream types throughout the U.S. Therefore, we also present how to apply a weight-of-evidence approach through a clear step-by-step process to potentially improve consistency and accuracy in identification of OHWM and bankfull by both scientists and non-scientists.