Why is there such a long time lag between heavy rains like the kind that we recently experienced and river cresting?
Hydrologist Bill Morris of the Chicago National Weather Service Office explains, “Many factors affect the lag time between the end of the heaviest precipitation and the crest on a river: the size of the watershed, rainfall distribution over the watershed, rainfall intensity, land use, and antecedent conditions before the rain began, such as soil moisture. For widespread rainfall events, the size of the watershed is a major factor. Because the terrain here is relatively flat, it takes considerable time for water to move through larger watersheds like the Fox River in northeast Illinois. Smaller creeks will generally respond much more quickly to heavy rainfall.”