People have come to regard floods as disasters. Humans have altered the flow of natural waterways to meet their needs, with sometimes disastrous consequences. Though floods can be devastating to population centres, they have always been an integral part of nature's renewal process, providing many long-term positive effects.
Renewal of Groundwater and 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. Flooding also 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 civilisations 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 prevented the Nile from flooding major population centres 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 floodwaters 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/Replenish Ground Water
Many population centres depend upon groundwater and underground aquifers for fresh water. Floodwaters 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.