Flows take place much more slowly than do slides, and usually involve great amounts of water. After a heavy rainstorm the ground can become too wet to absorb any additional water. The result is that the water is forced to run off on the surface, gathering dust, dirt, rocks, and in some cases even boulders as it builds up. The leading edge of a flow gathers the most debris, causing it to be thicker and slower moving. This acts as a slow moving dam. Eventually, such as in a wide area on a slope, the more liquid mud from behind breaks through the dam and rushes outward creating a muddy plain.