Flooding occurs when a stream’s normal capacity to carry is exceeded by the quantity of water present. To understand flooding, one must first understand what “normal” is, and thus one must understand how rivers and normally work. The equilibrium profile of a stream, from source to mouth, is up with a steep gradient upstream and a gentler gradient downstream. Rapid changes to a profile--for example, construction of a dam, an increase in sediment supply, draining of a lake-- result in changes in erosive power, channel pattern, or , all of which eventually act to return the stream to an equilibrium profile. Several styles of flooding can occur. Intense precipitation in a limited area, or dam failure, can cause flash flooding. Steady, prolonged over days or weeks can result in widespread regional flooding that persists for weeks or even months.