Results of long-term field studies of wetlands in four different hydrogeologic and climatic settings in the United States indicate that each has considerably different sources of water, which affects their response to climate variability and land-use practices. A fen wetland in New Hampshire is supplied almost entirely by ground water that originates as seepage from Mirror Lake; therefore, stream discharge from the fen closely follows the pattern of Mirror Lake stage fluctuations. A fen wetland in northern Minnesota is supplied largely by discharge from a regional ground-water flow system that has its recharge area 1 to 2 km to the east. Because of the size of this wetland's ground-water watershed, stream discharge from the fen has little variability. A prairie-pothole wetland in North Dakota receives more than 90 percent of its water from precipitation and loses more than 90 percent of its water to evapotranspiration, resulting in highly variable seasonal and annual water levels. A wetland in the sandhills of Nebraska lies in a regional ground-water flow field that extends for tens of kilometers and that contains numerous lakes and wetlands. The wetland receives water that moves through the ground-water system from the upgradient lakes and from ground water in local flow systems that are recharged between the lakes. The difference in sources of water to these wetlands implies that they would require different techniques to protect their water supply and water quality.