Little Calumet River
The best way to track the river level is to check the website for the Little Calumet River Gage at Cottage Grove Avenue. The graph at this site gives river height in "stage." A stage of zero equals 575 feet above sea level. If the graph reads 5 feet, the river is 580 feet above sea level at Cottage Grove.
For comparison, at 18.7 feet, 170th Street on the West side of Thorn Creek is covered with water. The June 1981, November 1990 and July 1996 floods all reached a stage of between 20 and 21 feet. A stage of 23 feet would cover the area shown as Zone A on the floodplain map.