Use of frequency-volume analyses to estimate regionalized yields and loads of sediment, phosphorus, and polychlorinated biphenyls to lakes Michigan and Superior

In most rivers, transport of various constituents occurs largely during short-term, high-intensity events. A method is described to make regionalized estimates of the long-term average loads of selected streamwater constituents, as well as loads occurring during high-flow events with specified recurrence intervals. This method is used to estimate the load of suspended sediment, total phosphorus, and sediment-borne constituents, such as poly chlorinated biphenyls (PCB's), from all the rivers in the United States that drain into Lake Michigan and Lake Superior whose drainage basins are greater than 325 square kilometers. Statistical comparisons of estimated loads and environmental factors indicate that suspendedsediment loads were primarily affected by river gradient and secondarily affected by the texture of surficial deposits, whereas total phosphorus loadings were primarily affected by the texture of surficial deposits and secondarily affected by river gradient. Total phosphorus loads were highest in rivers entering into the middle to southern part of Lake Michigan, especially rivers in areas draining clay surficial deposits and agricultural areas. During high flow, inputs of phosphorus and suspended sediment from rivers entering the southwestern part of Lake Superior become very important to the total input of these constituents; these rivers have steep gradients and drain surficial deposits with high clay content. The single largest contributor of PCB's during the entire period and during each type of high-flow event was the Fox River, which supplied 46 to 64 percent of the total PCB load to both lakes.