Why do some farmers create contour strips in their fields?

The most common pollutant in the Brandywine Creek is sediment. Sediments are released through the process of erosion. Erosion happens naturally and with human assistance. For example, after a farmer tills a field located on a hill, the loosened soil can move downhill (erode) and end up in the closest stream, especially following a rainfall. Contour strips are used on hilly terrain; rather than planting an entire hill in one crop, thereby increasing the potential for large amounts of soil running down hill, farmers cultivate at right angles to the natural slope. This almost eliminates the runoff of water by allowing it to soak back into an area that is not cultivated and can reduce soil erosion by more than 50%!