When rain or snowmelt water accumulates on a field faster than it can flow into the soil, or infiltrate, it will begin to fill up all the depressions in the soil surface. If there is enough water that all the indentations fill up and spill over, runoff will begin. Initially, runoff can flow down hill as very thin sheets of water. It does not have to travel very far, sometimes just a few feet, before it starts to form very small but visible channels, called rills. As the water continues to flow down slope with more water flowing into them, the channels become bigger until they flow into gullies or intermittent streams.