sediments and dredging

The problem of a lake or pond is that the sediment will build up and eventually the lake will become too shallow and it has to be dredged to maintain its water holding capacity.  An artificial lake/pond formed by daming a creek needs dredged every 15-20 years. 

To reduce sediments, storm runoffs are first stored in the ditch (called swale) around the lake.  In a major storm, only the overflow water (with less sediments) would run into the lake.  This way sediments are reduced and lake needs longer time until next dredging.