The Ordinary High Water Mark in New England Rivers: Relationships Between Field Indicators and Hydrology

The Ordinary High Water Mark (OHWM) defines the lateral extent of Federal jurisdiction in rivers and streams of the United States for the purposes of enforcing the Clean Water Act (Section 404) and Rivers and Harbors Act of 1899 (Sections 9 and 10). Substantial intra- and inter-regional variability with respect to hydrology, fluvial dynamics, and the physical and biological indicators of 'ordinary' streamflow conditions makes accurate and consistent identification of the OHWM challenging. These challenges are compounded by inconsistent interpretations of what the OHWM represents geomorphically (in terms of where the OHWM occurs in the fluvial landscape and the physical processes that shape and maintain it) and hydrologically (in terms of the streamflow conditions associated with the OHWM). While scientifically-based and ecologically-conscious concepts and methods for determining the location of the OHWM are essential for protection of fluvial systems, they must be constrained by the practical need for rapid, repeatable, and legally-defensible OHWM identification and delineation practices. This study explores the relationships between OHWM field indicators and the associated hydrologic conditions that shape and maintain them in New England rivers and streams. Based on field sampling in a variety of fluvial settings throughout New Hampshire and Vermont, the existence of hydrologically and/or geomorphically consistent OHWM indicators is assessed. The spatial and temporal stability of various physical and biological indicators of ordinary high water and how these vary in different fluvial settings is explored. This study provides insight into both the conceptual understanding of the OHWM and the field methods used to identify and delineate it in New England. Additionally, this work allows for inter-regional comparison of OHWM indicator occurrence and stability and the hydrologic and geomorphic concepts associated with the OHWM based on similar work in other regions.