Surface-water watersheds vs. ground-water watersheds
Surface-water watersheds are generally delineated from topographic maps based on land elevations ("height-of-land" method). The Minnesota Department of Natural Resources (DNR) completed a standard delineation of minor watershed boundaries for Minnesota in 1979. Using U.S. Geological Survey quadrangle maps, the DNR defined minor watershed outlets and delineated height-of-land minor watershed boundaries for all watersheds greater than 5 square miles. However, actual boundaries may be different due to map interpretation assumptions or human-induced changes that have occurred since the map was made. Field inspection of areas in question is required to be certain of actual boundaries.
Ground-water watersheds are conceptually similar to surface-water watersheds because ground water flows from high points (divides) to low points (outlets, discharge areas). However, the boundaries of surface-water and ground-water watersheds do not always coincide. Ground-water movement occurs in below-ground aquifer systems and is subject to 1) hydraulic properties of the aquifer, 2) input to (recharge) and outflow from (discharge) the aquifer system, and 3) geological factors such as formations that block the flow of water and tilted formations that create a flow gradient. Surficial aquifers (the water table) generally mimic surface-water watersheds, and their flow usually does not cross surface boundaries. Deeper (confined) aquifers, on the other hand, are less likely to conform to surface features and exhibit watersheds (or basins) determined by geologic factors.