However, because the water has to move through soil, downhill movement is much slower than flow over the land. Water that penetrates or infiltrates soil is very important because it continues to supply water to plants and streams long after the rainstorm stops. Scientists and engineers study watersheds of different sizes to see how much water reaches streams after rainfall events. The amount and speed of water flows from watersheds change when trees are cut and when roads are built. All kinds of material are carried by water.
Many substances dissolve in the water itself and are carried directly with water flows.
Other materials may stay suspended in the water or in particles carried by the water to move downstream to streams, rivers and lakes. Pollutants like oil and pesticides that are dumped on the soil will eventually be carried into rivulets, streams and rivers. Contaminants carried by water can be concentrated in the fish and other organisms. Contaminants can also be deposited in mud at the bottom of lakes and rivers. Some of the contaminants alter the kinds of organisms that live in the water.