Which statement best describes the relationship between imperviousness and stream degradation risk?

Which statement best describes the relationship between imperviousness and stream degradation risk?

• A. The risk increases with imperviousness, with a threshold around 10%
• B. The risk decreases as imperviousness increases
• C. The risk is unrelated to imperviousness
• D. The risk only increases with vegetation

Answer: (A)

Rising impervious surfaces change how a watershed handles rainfall. When more pavement and rooftops exist, rain quickly becomes surface runoff instead of infiltrating the soil. That produces higher, faster peak flows, shorter response times, and more erosive energy in streams, which carries sediments and pollutants and disturbs habitat. As imperviousness increases, these effects accumulate, making stream degradation more likely. There’s also a recognized threshold around 10% impervious cover, where the likelihood and severity of degradation begin to rise more noticeably. So the best description is that degradation risk increases with imperviousness, with a threshold near 10%. The other statements don’t fit because they imply the opposite or ignore the link between runoff and stream health. It’s not that risk decreases with imperviousness, and it’s not unrelated to imperviousness. The idea that risk only increases with vegetation doesn’t reflect how impervious surfaces drive runoff and degradation in streams.

Rising impervious surfaces change how a watershed handles rainfall. When more pavement and rooftops exist, rain quickly becomes surface runoff instead of infiltrating the soil. That produces higher, faster peak flows, shorter response times, and more erosive energy in streams, which carries sediments and pollutants and disturbs habitat. As imperviousness increases, these effects accumulate, making stream degradation more likely. There’s also a recognized threshold around 10% impervious cover, where the likelihood and severity of degradation begin to rise more noticeably. So the best description is that degradation risk increases with imperviousness, with a threshold near 10%.

The other statements don’t fit because they imply the opposite or ignore the link between runoff and stream health. It’s not that risk decreases with imperviousness, and it’s not unrelated to imperviousness. The idea that risk only increases with vegetation doesn’t reflect how impervious surfaces drive runoff and degradation in streams.