Why are the drainage basin lines plotted on the map?
The horizontal direction of ground-water flow is generally away from drainage divides and toward surface-water bodies. This is important with respect to ground-water contamination. Aquifers in surficial materials are especially sensitive to contamination from spills of hazardous chemicals. Contamination plumes in sand and gravel aquifers will follow the flow of ground water away from the divides, towards the nearest surface water. Wells located in the path of the plume would become contaminated.
