This map shows the saturated thickness of the Columbia Formation. The Columbia Formation covers most of the Coastal Plain of Delaware. Because it consists primarily of coarse sand, it is important to the hydrology of the area. It is an important groundwater reservoir and in most places water must pass through it to reach deeper units. The water budget of the Columbia Formation also influences runoff and baseflow components of streamflow. The saturated thickness was determined through interpretation of data in publications and files of the Delaware Geological Survey, United States Geological Survey, and the Water Resources Center of the University of Delaware. The thicknesses shown on the map represent the best judgment of the authors based on available data. Detailed investigations of specific sites will require additional data.