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On a developed landscape, less rainfall soaks into the ground and more flows off of buildings, parking lots, roads, and other paved surfaces, collectively known as “impervious surfaces,” directly into storm drains and creeks. Research suggests that streams retain good water quality and stable channels when impervious surfaces cover less than 10% of the total watershed area. When this percentage rises above 25%, severe physical and ecological damage to streams is manifest in a variety of ways.
Nonpoint source pollution: Rainwater that washes over roads, parking lots, and lawns picks up metals, oils, fertilizers, and pesticides. This urban runoff is collected in storm drains and discharged through outfalls up and down the creek. The greater the amount of runoff, the more pollutants end up in the creek. Because this pollution is not generated from a single location, it is called nonpoint source pollution.
More severe erosion and flooding: Rainwater moves across roads and through storm drain pipes much more quickly than it flows across a natural landscape. This, combined with the fact that less rainfall has been absorbed into the ground, means more water arrives at the creek more quickly and in more concentrated surges than it would naturally. Increased creek flow volumes and velocities exacerbate erosion and flooding problems.
Reduced year-round flow in creeks: Because paved surfaces deliver rainfall straight into storm drains, less water is absorbed into the ground. Water in the ground helps keep plants healthy, provides water for wells, and is the source of creek flow during the summer. In the San Francisquito Creek watershed, groundwater springs keep at least a trickle of water flowing into the upper watershed tributaries year-round, even though the main creek dries up downstream as it gets closer to the Bay. |
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