In Massachusetts, floodplain wetlands were deemed so effective for flood control by the U. S. Army Corps of Engineers that they purchased many wetlands rather than build expensive flood control structures. The Louisiana coastal marshes are being destroyed at an alarming rate due to subsidence and saltwater intrusion because the nutrient rich water and sediments no longer flow through the marshes, but out to sea. Some wetlands recharge groundwater, which is a common drinking water source. Surface water in these wetlands percolates down through the soil beneath the wetland and enters the groundwater or aquifer. Some studies have shown wetlands to be more important than uplands to groundwater recharge.
Wetlands also improve the quality of water.
Wetlands have been shown to remove organic and inorganic nutrients and toxic materials from the water that flows across them. When environmental conditions are optimal, waste organic compounds are rapidly decomposed, and the accumulated heavy metals and phosphorous are buried in deep sediments. In Arkansas and other states in the Mississippi River basin, the water quality of some rivers, streams, and bayous has been degraded due to the runoff from surrounding lands. This means that more nitrogen fertilizer is running off farmland directly into the river and into the Gulf of Mexico. Excess nutrients lead to increased algal production and increased availability of organic carbon within an ecosystem, a process known as eutrophication.