Floods. The Reasons for their Appearance

Floods result whenever the banks of a stream or lake can no longer hold the water available to it. When this happens, water flows over the banks onto the surrounding floodplain, a lowland that is often a valley floor. In those regions of the world where snow falls, floods most likely occur in the early spring as snowmelt and spring rain combine to increase the volume of water flowing into freshwater bodies.

Elsewhere, monsoons, typhoons, hurricanes, and other violent storms can contribute to flooding. Heavy downpours can cause savage flash flooding through canyons, gullies, and other locations where the floodplain is naturally narrow.

Stream channels cannot accommodate all possible sizes of flows, and it is entirely natural that flows substantially exceeding the average must overflow onto the floodplain. Significant benefits result. The water brings nutrients that fertilize the floodplain, among other things enabling agricultural development. It recharges groundwater, and it replenishes water supplies for fish and wildlife. In short, floods are part of nature's rhythm.

Flood warning sign in Colorado

But floods also threaten life and property, and humans who live within a floodplain assume some risk. Flood damage can truly be awesome. Whole communities eliminated, thousands killed, economic devastation, famine, and disease may result. Historical evidence suggests that floods have far exceeded other natural disasters, including fires and earthquakes, in their impact on life and property, especially in industrialized nations.

The power of unleashed water has been extensively recorded in documents that reach back into antiquity. Indeed, archeological evidence pushes our knowledge of floods back into prehistory. The best-documented example is the Bonneville Flood, which occurred about 15,000 BCE in the western United States. Prehistoric Lake Bonneville occupied 51,800 hectares of what is now western Utah and parts of Nevada and southern Idaho.

At its largest, it was about 217 kilometers wide and over 300 meters deep. Nearly seventeen thousand years ago the lake elevation rose, possibly because a tributary river above the lake captured another river and water from both rivers soon increased the lake water level above capacity. Lake water began escaping northward toward the present Snake River. Within a year, the lake elevation was lowered by 114 meters.

The flood carved out much of western Washington state and left behind a variety of geological relics, including Utah Lake, Sevier Lake, the Great Salt Lake, and Bonneville Salt Flats.

Within the confines of human history, floods can be myth, reality, or somewhere in between, depending on the sources. The biblical story of Noah and the flood finds its parallels in the Babylonian epic of Gilgamesh, Norse mythology, and flood legends among the indigenous people of North and South America. No doubt, some of these stories at least partly emanate from fact.

For example, archeologists have discovered a 2.4-meter layer of clay left by a flood of the Euphrates River around 3200 BCF. SO devastating was this flood that it is easy to understand that ancient civilizations may have thought it signaled the end of the world. Many of these flood stories are didactic: They may reveal the floods as divine retribution for human wrongdoing or, at the very least, as an act of god(s) beyond human power and understanding.

It makes no difference whether basins are large or small, deep or shallow; floods come. In small basins, agricultural activities and urbanization affect peak discharges of water by decreasing infiltration into the ground, increasing surface runoff, and, via storm water outlets, speeding surface water downstream. In large basins, the effect of these developments is less dramatic. Instead, floods usually result from some combination of continuous and extraordinary rainfall, frozen ground, and snowmelt over a large area.

To protect themselves against floods, humans have devised dams, dikes (levees), floodwalls, and jetties. They dig deeper channels, stabilize banks, and occasionally provide outlets for excess water. All of this is both ancient technology and current practice. Changes have been only in the details. Today engineering theory, new kinds of construction material, and vastly more powerful construction technology combine to allow for stronger, safer, and more durable structures, but the basic approach remains the same.

Neither better science nor improved technology ensures the success of flood projects or guarantees complete protection. In fact, history shows that success is generally only transitory and that eventual failure is more the rule than the exception.