What is the lower limit of imperviousness that can result in stream degradation?

What is the lower limit of imperviousness that can result in stream degradation?

• A. 10%
• B. 5%
• C. 15%
• D. 20%

Answer: (A)

Increasing impervious cover reduces infiltration and boosts runoff, changing the stream’s flow regime. When imperviousness in a watershed reaches about ten percent, these hydrologic changes become enough to start degrading stream conditions—more frequent high flows, greater erosion, sediment delivery, and warmer water reduce habitat quality. So, ten percent is the earliest point at which degradation can occur. Below this threshold, streams are generally better protected by natural infiltration and storage, while higher levels (fifteen or twenty percent) will certainly worsen degradation, but the question asks for the lower limit. In practice, aim to keep impervious cover under this level or implement controls that reduce effective imperviousness and manage runoff.

Increasing impervious cover reduces infiltration and boosts runoff, changing the stream’s flow regime. When imperviousness in a watershed reaches about ten percent, these hydrologic changes become enough to start degrading stream conditions—more frequent high flows, greater erosion, sediment delivery, and warmer water reduce habitat quality. So, ten percent is the earliest point at which degradation can occur. Below this threshold, streams are generally better protected by natural infiltration and storage, while higher levels (fifteen or twenty percent) will certainly worsen degradation, but the question asks for the lower limit. In practice, aim to keep impervious cover under this level or implement controls that reduce effective imperviousness and manage runoff.