Photos in the news recently showed the dramatic flow of sediment into Lake Superior from flooding in Duluth, where torrential rains caused extensive damage.
In a similar fashion, heavy rains can wash soil and other pollutants into rivers in southern and western Minnesota, says the Minnesota Pollution Control Agency.
Only a few major storms can account for the majority of sediment – soil particles and other matter – in a river in a season, according to Pat Baskfield, MPCA hydrologist.
In other words, 20 minutes of hard rain can lead to weeks of murky water. That sediment can settle out anywhere along the river system, including pools, streambanks and lakes.
In May this year, Baskfield videotaped a 20-minute rain storm in the Watonwan River area near Mankato. Some of his footage has been incorporated in an MPCA video on the Internet. As the video shows, rain water immediately started running across a bare field. The water pooled at the field's lowest spot and formed a gully along the edge of the field. The muddy water ran through a culvert under the road, then it cascaded into a ditch that empties into the river. The video shows black water cascading from the culvert into the ditch.