Wetlands have important filtering capabilities. They intercept surface water runoff from higher dry land, before the runoff reaches open water. As the runoff water passes through, the wetlands retain the excess of nutrients and some pollutants. Next to his, they reduce the sediment that would clog waterways and affect fish and amphibian egg development. In performing this filtering function, wetlands save us a great deal of money. For example, a 1990 study showed that, without the Congaree Bottomland Hardwood Swamp in South Carolina, the area would need a $5 million waste water treatment plant. In addition to improving the water quality through filtering, some wetlands maintain a stream flow during dry periods, and many replenish groundwater. Many Americans depend on groundwater for drinking.