Practice a College Board-style free response question on Earth Systems & Resources. Write your response, then reveal the model answer to see exactly what earns each point.
A 2,000-hectare watershed contains a mix of forest and farmland that drains into Clearwater Lake. Over the past 15 years, half of the watershed's forest has been cleared for row-crop agriculture using conventional tillage (plowing that leaves soil bare between rows). Researchers measured the lake's annual sediment input and water clarity (measured in cm of visibility) over this period.
| Year | Sediment input (metric tons/year) | Water clarity (cm) |
|---|---|---|
| 0 | 800 | 210 |
| 5 | 1,450 | 160 |
| 10 | 2,300 | 95 |
| 15 | 3,100 | 60 |
Sediment input increased steadily from 800 to 3,100 metric tons/year, while water clarity declined from 210 cm to 60 cm over the same period — an inverse relationship. This trend is most likely caused by converting half the watershed's forest to row-crop agriculture using conventional tillage, which leaves soil bare and exposed between rows. Without forest root systems and ground cover to hold soil in place, rainfall and runoff carry far more topsoil into the lake than the forested land did.
A watershed is defined as all the land area that drains into a single body of water. Because every part of the watershed's surface runoff and precipitation eventually flows toward Clearwater Lake, land-use changes anywhere within those 2,000 hectares — not just near the shoreline — can introduce sediment, nutrients, or pollutants that travel downstream and accumulate in the lake.
The farmers could switch to no-till farming, which avoids plowing and leaves the previous season's plant residue on the soil surface. This residue physically shields the soil from rainfall impact and wind, while the undisturbed root systems left in place help bind soil particles together — both factors that reduce the amount of sediment that washes into the lake during rain events. (Other valid answers: planting cover crops, contour plowing, or establishing a vegetative buffer strip along the lake's edge.)