USGS Water-Resources Investigations Report 96-4092
Use of Frequency-Volume Analyses to Estimate Regionalized Yields and Loads
of Sediments, Phosphorus, and Polychlorinated Biphenyls to Lakes Michigan and
Superior
By Dale M. Robertson
Abstract
In most rivers, transport of various constituents occurs largely during
short-term, high-intensity events. A method is described to make 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 polychlorinated biphenys (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
suspended sediment loads were primarily affected by river gradient and
secondarily affected by the texture of surficial deposits, whereas totoal
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 deposit and
agricultural areas. During high flow, inputs of phosphorus and suspended
sediment from rivers e ntering the southwestern part of Lake Superior become
very important to the total input of these constituents; these rivers have
steep gradients and rain 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.