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While many wetlands form along floodplains of rivers, streams, lakes, and estuaries, others have developed in depressions far removed from such waters. Depressional wetlands completely surrounded by upland have traditionally been called "isolated wetlands." Isolated wetlands are not confined to basins, as some occur on broad flats and others form on slopes. The term "geographically isolated wetlands" better describes these wetlands, since many are hydrologically connected to other wetlands and waterbodies through ground-water flows or by intermittent overflows (spillovers). Numerous types of geographically isolated wetlands occur throughout the United States. They may be naturally formed or the result of human action. Naturally formed types include prairie pothole wetlands, playas, Nebraska’s Rainwater Basin and Sandhills wetlands, West Coast vernal pools, sinkhole wetlands, Carolina bays, interdunal and intradunal wetlands, desert springs, terminal basins in the Great Basin, and kettle-hole bogs in glaciated regions. Human-caused isolated types may be intentionally built, such as ponds designed for various purposes and wetlands built on mined lands, or they may be accidentially created (e.g., natural wetlands that were once connected to rivers and streams but are now isolated by roads, railroads, and other development or isolated by altered river hydrology). Many of the functions and benefits attributed to non-isolated wetlands are present in isolated wetlands.