BRIDGING SCIENCE AND MANAGEMENT: INVESTIGATING THE ORDINARY HIGH WATER MARK IN NORTHEAST STREAMS

The ordinary high water mark (OHWM) is a line impressed on the banks of a river that is represented by changes in channel shape, destruction of terrestrial vegetation, and/or changes in soil characteristics. It represents the domain of our waterways and the division between public and private land. Regulators have to use this simplistic definition of the boundary and apply it to streams of all shapes and sizes throughout the country, which are diverse dynamic systems. The need by regulators to identify this boundary has led to a national-scale research project on developing techniques to consistently delineate the OHWM. Detailed cross-section surveys were conducted in streams in the northeast and the rest of the country, including streams in New Hampshire and Vermont, with drainage areas between 0.05 to 3465 km². HEC-RAS flow models, along with statistical boundary detection techniques, are applied together to evaluate how the three most significant indicators vary with flow: 1) break in slope, 2) bank sediment changes, and 3) vegetation transitions. The location of these transitions are assessed against both flow data and lidar, to better evaluate which tools will assist regulators in delineating the OHWM. Although the OHWM is a regulatory boundary, it is relying on persistent channel features to recognize this boundary, therefore the OHWM is similar to the level of bankfull flows in northeast channels. Linking the scientific concept of bankfull stage with the regulatory concept of OHWM is essential for helping regulators and managers understand how to consistently delineate this boundary. These results are compared to streams around the country to discuss differences and difficulties in determining the location of OHWM in comparison to other climatic regions.