Soil erosion caused by water can be controlled by afforestation and erection of bunds. Afforestation involves planting trees in areas where there is soil erosion, especially on steep slopes. Trees help to hold the soil together with their roots and reduce the force of raindrops on the soil surface. Erection of bunds involves building a barrier made of soil, stones or other materials across the slope to slow down the flow of water and trap sediments, thus reducing soil erosion. This technique is often used in combination with other erosion control measures such as grass strips and contour ploughing.