Effect of Storms and Gage Location on Tributary Load Estimate

Abstract

Monthly and annual pollution loads delivered to Great Lakes receiving waters are made from data often collected far upstream of the mixing zone at the tributary receiving water confluence. The reliability of confluence loading estimates as determined from these upstream gages has not been established. Using data collected during June 1981 in Sandusky Bay/Lake Erie, a combined hydrodynamic and chloride transport model is used to synthesize the load at the confluence. This load is compared to that obtained with data collected during the same time period from the USGS Fremont, Ohio gage which is 29 km upstream of the confluence. The load at the confluence was 2.5 times greater than that estimated with the USGS data. This difference is, for the conservative substance sampled here, attributed to a timing or phasing error. However, the implication for nonconservative substance load estimates is that this difference would be more severe than that resulting from a timing problem.