Impervious cover is pavement, concrete, or any hard surface that water cannot penetrate. Stream quality suffers rapidly as impervious surface increases in its watershed. The graphic below from the Center for Watershed Protection is the result of dozens of studies from around the country. The graph shows that above 25% imperviousness, stream quality indicators rapidly become non-supporting. Even with as low as 10% imperviousness, stream quality is significantly impacted. Two-acre lot development gives 10% imperviousness (from houses, driveways, roads, etc).
This model suggest that we will get a better bang for our buck if we preserve those sensitive watersheds that have less than 25% imperviousness, and especially those that have less than 10% imperviousness. We get a much lower return from decreasing imperviousness in already significantly impacted watersheds.