Slide 17 of 75
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Generally, most urban streams begin to enlarge as impervious cover exceeds 10% in the watershed. This finding has been discovered in various places like Puget Sound, Washington; Austin, Texas; and Vermont. The enlargement process may take up to 50 years to fully occur, but urban streams with more than 10% impervious cover are characterized by various degrees of stream enlargement and widening, erosion, downcutting, decreased channel stability, and embeddedness.
The Maryland Piedmont presents numerous examples of the different ranges of the enlargement process. An undeveloped watershed with less than 5% impervious cover is characterized by excellent stream conditions� good riparian cover, high quality substrate, and wetted perimeter during low flow conditions.