How does pavement or smooth rock runoff affect waterways

Run-off from pavement or smooth rock during rainfall is much greater and faster than run-off from soil or other surfaces as there is little obstruction (friction) to the water flow. These surfaces also absorb very little water, meaning that there is a larger amount of run-off.

This means that waterways can have to deal with large intakes of water in a very short amount of time, often leading to flash flooding, as the waterway can not handle such a large amount of water at once.