Hydraulic design and environmental permitting are heavily dependent upon Ordinary High Water Marks (OHWM) because they define the active river channel. The United States Army Corps of Engineers (USACE) use OHWM for regulation of the “Waters of the United States” as well as for flood and drought management. Current methods to determine OHWM are based on detailed on-site surveys to identify physical characteristics like scouring, deposition around the banks, absence of vegetation and water staining. These characteristics are site specific so there are fluctuations in measurements based on the water body, weather conditions, channel morphology, slope, fluvial patterns and size of the channel. A more reliable way to estimate this variable for hydraulic design is required that is based on storm return periods. This study uses hydrologic and hydraulic modeling to relate OHWM to storm return periods by analyzing hydraulic and hydrologic parameters corresponding to design streamflow events for 26 watersheds in Indiana. The results show that the OHWM corresponds to discharges that have return periods ranging from 0.7 – 1.1 years. These results also suggest that that OHWM correspond to channel discharges much smaller than bank-full discharge, which typically has a return period of 1.5 – 2 years. The OHWM discharges are then related to 100-year discharges to enable the use of this relationship in approximately estimating the OHWM discharge when the 100-year discharge is known. For Indiana, it is found that ratio of OHWM discharge and 100-year discharge has an average value of 4.99% for the northern part, 3.60% for the central part, and 5.49% for the southern part.
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