The Nebraska Sandhills overlay the High Plains Aquifer - commonly referred to as the Ogallala Aquifer. This groundwater resource creates an interspersion of shallow lakes, semi-permanent, and temporary wetlands in the lower elevations and valleys where the groundwater level is exposed. Water resources are the driving force supporting the ecological diversity and integrity of the Nebraska Sandhills. There are 37 major wetland complexes totaling approximately 13,000 acres on the Refuge. These wetlands are a mix of shallow lakes, marshes, seasonal wetlands, wet meadows, fens, and small streams that run during high water periods. Many wetlands, a most valuable wildlife habitat, have been drained to make way for agriculture and development.