What Is a Flood?
A flood occurs when a river or stream overflows its banks. Seasonal floods are the norm in many rivers, for example when spring rains or snowmelt increase the flow. During a flood, the channel is completely filled and water moves onto the floodplain and slows down. As it slows, it can carry less material. A lot of sand, silt, leaves, and other materials that were carried while the water was swirling along in the channel are dropped on the floodplain.
There are about 79 Great Rivers on our planet, and the flood pattern (or hydrograph) of each is unique, much like a signature. This signature reflects the region and climate. Some great rivers have two peaks each year, others have one. Before it was dammed, the Missouri River had a March “rise” caused by snowmelt in the Great Plains and by breakup of ice in the river and its tributaries. A second “rise,” in June, was caused by snow melting off the distant Rocky Mountains and traveling down the river to join runoff from rainfall in the Plains. Dams on the Missouri now store the spring floods and release water during the summer, so the flows downstream do not change as much as they once did.
Although seasonal floods may be predictable in the sense that they are generally expected to occur at certain times of the year, they never occur on a strict schedule. The duration of seasonal floods depends to a great extent on the weather — either very recent weather, or precipitation over the previous few months. Small streams and rivers may be very “flashy.” This means that a big rainstorm in the catchment will cause a flood fairly quickly, but the floodwaters also will recede quickly.
The larger the river, the more land it receives water from, and the less flashy its flood behavior. Great Rivers integrate the water events over a large area as well as a long time period. The term integrate is from the Latin integrare, “to make whole.” It means to bring parts together to make a whole. A Great River will integrate the condition of its catchment (i.e., forested, urban, clean, polluted, steep, flat), the precipitation events that have taken place there over some period of time, the effects of human-built structures such as dams or levees, and any other factors that influence runoff and flow. When a Great River floods, the flood usually builds relatively slowly and lasts over a long period of time.
Because seasonal floods are regular events, the living organisms of rivers and floodplains have adapted to them. They even take advantage of seasonal floods: river-floodplain ecosystems are said to be “re-set” by the flood. As you move down the river, at some point seasonal floods are not simply re-set mechanisms, they become essential to the ecosystem.