After heavy, prolonged rain or rapid melting of thick snow, streams swell and overflow their banks. Coastal lands can be inundated by 'tsunamis', giants waves that race across the ocean after submarine earthquakes. Failure of natural or artificial dams can also cause floods.
Roll the cursor over the buttons from left to right to learn about floods.
The top left diagram is a profile across a stream valley under a night sky. Clear water flows in the channel over sediment of different sizes. Below, the length of each yellow line indicates the velocity of the water directly above it: fastest at the center of the stream; slowest at the sides. The bottom diagram shows the maximum size sediment particle that can be transported.
Somewhere in the area that provides water to the stream, a storm is in progress. The water in the stream begins to rise and its velocity increases to the point where it can pick up and begin to transport the small, yellow grains. The larger, purple, red and green particles remain stationary.
The water continues to rise, but the stream still flows completely within its channel. The velocity of the water increases. The larger, purple particles begin to move along with the smaller, yellow grains.
Now the water has risen and spilled out over the stream banks onto the surrounding part of the valley floor. The stream is now 'in flood'. The water on the flooded valley floor moves much more slowly than the water in the channel, but fast enough to carry the small, yellow grains.
The water level rises further and the flood spreads across the valley floor. In the channel, the water is moving very fast and also begins to move some of the large, red rocks. Where the water leaves the channel and flows onto the valley floor, it slows down, depositing purple particles.
The flood has reached its maximum height and extent. In the channel, the water is so rapid that even some of the giant, green boulders begin to shift. The flood waters on the part of the valley floor adjacent to the channel are moving fast enough to transport red as well as purple and yellow particles. At edges of the flood, the slower water moves only yellow and purple particles.
The flood begins to recede. As the level drops, the velocity decreases. At the edges of the flood, yellow grains are stranded. On the flooded part of the valley near the channel, the water can no longer transport the red and purple sediment, which sinks to the floor. Within the channel, all sizes of sediment except the green can still be transported.
As the water slows, the larger particles are always the first to be deposited. As more ground emerges from the flood, it is covered by a layer of purple particles covered on top by yellow particles. That is, the particles are sorted according to size.
The water retreats to the stream channel. The larger particles settle to the channel floor. Only the yellow and purple particles continue to be transported. On the valley floor on each side of the stream channel is a thick deposit of sediment, grading up from large particles (red) at the bottom to small (yellow) at the top.
The flood is over. The stream only occupies the lower part of the channel. Only the small, yellow grains continue to be transported. The valley floor on each side of the channel that received thick deposits of sediment, ends up being slightly higher in elevation than adjacent parts of the floor.
The effects of the storm on the stream are now over. The water level is low. The velocity has dropped so much that no sediment, not even the yellow, is being transported. The water is once again clear. On each side of the channel where the valley floor received thick deposits of sediment, the elevation of the land is now higher than adjacent areas.
When another flood takes place, the same processes are repeated. Over time, the valley floor becomes buried by successive layers of sediment to form a gently sloping plain (called a 'floodplain'). The raised parts of the floodplain adjacent to the stream channel, underlain by thicker, coarser deposits of sediment, are called 'levees'. Each individual layers of sediment has coarser grains at the bottom and finer grains at the top. The sediment in such a layer is said to be "graded". Graded sediments are typical of all stream deposits.
David J. Leveson