Given the less persistent streamflow regimes characteristic of non- perennial streams, particularly ephemeral systems, the characterization of ordinary high water flows is perhaps more challenging than in perennially flowing systems. Moreover, depending on climate, vegetation, and other related factors, the appearance of some OHWM indicators may vary greatly between wet and dry seasons or between relatively infrequent flow events, more so than in many perennial streams. Mountainous terrain can present additional challenges to OHWM delineation. For instance, the relatively steep and confined valleys in which mountain streams commonly flow can restrict the development of some alluvial features (e.g., flood-plains, bankfull benches, etc.) that are typical of low-gradient systems and that may help to identify the OHWM. Thus, in non-perennial mountain streams, it is often difficult to determine what constitutes ordinary high water and to interpret the physical and biological indicators established and maintained by ordinary high water flows.

Challenges and inconsistencies pertaining to OHWM delineation practices are becoming increasingly relevant in mountainous parts of the western U.S. in light of expanding development. This increased pressure on fluvial systems highlights the need for accurate, consistent, and repeatable OHWM delineation practices in this region. These factors, combined with the particular challenges of OHWM delineation in non-perennial mountain streams, provided the impetus for developing this delineation guide.

This guide presents the concepts, field indicators, and methods for assessing, delineating, and documenting the OHWM in non-perennial streams in the Western Mountains, Valleys, and Coast (WMVC) Region of the United States (Figure 1). The information presented here is based on the findings of Mersel et al. (2014) (discussed in Section 1.5) and on years of field observations and data gathering in the WMVC Region and in other regions of the U.S. by the authors and other contributing experts. The remainder of Section 1 provides background information regarding the concept of the OHWM and pertaining to stream hydrology and geomorphology in general. Section 2 discusses and provides examples of the specific field indicators used to identify the OHWM in non-perennial streams in the WMVC Region. Section 3 discusses field methods for delineating the OHWM and addresses additional techniques and lines of evidence that may help in problematic delineation scenarios.