| Carson River, NV Low Flow (411 gallons/second) |
Carson River, NV High Flow (205,700 gallons/second) |
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The January 1997 floods in the West were big water events -- floods of that magnitude would be expected probably less than once per 100 years. Of course the floods occurred because it rained so much. But the big problem was that it kept on raining! Normally, a lot of rainfall is absorbed into the ground, but when the ground gets saturated, additional rain stays on the surface and starts moving downhill (as runoff) towards a creek, river, or lake. Consider a sponge that is already full of water -- add any more water to it and it will run off. Take a look at these two pictures of the Carson River in Nevada. Same place -- very different circumstances.
The first picture shows the river during a normal low-flow period. About 55 cubic feet (411 gallons) of water are flowing past the bridge every second. The next picture shows the same "tranquil" location during the flood when the flow was about 27,500 cubic feet per second (205,700 gallons per second)! That's 500 times more water going by than at low flow.
Maybe you're thinking, "A couple days of rain couldn't possibly produce this much water!" The answer to this is that a river is a collection point (really more like a collection "line") for what might be many square miles of land. The area the river collects water from is called the watershed or drainage basin. Think of the drainage basin as a swimming pool in winter that has one of those big plastic covers on it. The cover always sags in the middle. The middle is the lowest point (where the river or lake is), and the rest of the cover is the drainage basin. When it rains, all of the water that falls on the cover drains toward the middle, the low point, resulting in a bunch of water right there.
The USGS California office has a Web page about
floods and flood plains.
Information on this page is from the U.S. Geological Survey's California Water Science Center Web page.
Measuring water
Human's influence
Water use
Water Science home page
U.S. Department of the Interior | U.S. Geological Survey
URL: http://ga.water.usgs.gov/edu/cafloods.html
Page Contact Information: Howard Perlman
Page Last Modified: Friday, 07-Nov-2008 15:42:27 EST
