Slide 36 of 49
Notes:
Both anecdotal evidence and field research support the notion that the larger and more frequent discharges that accompany watershed development cause downstream channels to enlarge, whether by widening, downcutting, or a combination of both.
This graph compares past and current stream channel cross-sections in Watts Branch, an urban stream in the Maryland Piedmont that has been the subject of considerable development and study for more than 40 years. The historic cross-section is from 1953 (approximately 5% imperviousness) and the current cross-section is from 1999 when the impervious cover in the watershed had reached about 30% impervious cover. It is interesting to note that this channel enlargement occurred despite the fact that nearly half of the watershed development over this time period was built with two-year peak discharge controls.