People have come to regard floods as disasters in terms of lives lost and property damaged. Humans have altered the flow of natural waterways to meet their needs but with sometimes disastrous consequences. Though floods can be devastating to population centers, they have always been an integral part of nature's renewal process, providing many long-term positive effects.
Renewal of Wetlands
Floods contribute to the health of ecologically important wetland areas. Healthy wetlands promote healthy water supplies and even affect air quality. Floods inundate wetlands with fresh waste. They also carry and deposit nutrient-rich sediments that support both plant and animal life in wetlands. In addition, flooding adds nutrients to lakes and streams that help support healthy fisheries.
Returning Nutrients to Soil
Floods distribute and deposit river sediments over large areas of land. These river sediments replenish nutrients in topsoil and make agricultural lands more fertile. The populations of many ancient civilizations concentrated along the floodplains of rivers such as the Nile, the Tigris and the Yellow because periodic flooding resulted in fertile, productive farmland. The construction of the Aswan High Dam in Egypt prevented the Nile from flooding major population centers downriver, but it also depleted once fertile agricultural lands along the banks of the river.
Preventing Erosion and Maintaining Land Mass Elevation
Soil deposited by flood waters prevents erosion and helps maintain the elevation of land masses above sea level. The rapidly receding land of the Mississippi River Delta is a direct result of man-made flood controls and levees that prevent topsoil-replenishing sediments from being deposited in the delta.
Recharge and Replenish Ground Water
Many population centers depend upon ground water and underground aquifers for fresh water. Flood waters absorb into the ground and percolate down through the rock to recharge these underground aquifers, which supply natural springs, wells, rivers and lakes with fresh water.