SURFACE-WATER AND NATURAL GROUND-WATER DIVIDES

The boundaries of natural ground-water systems are defined by GROUND-WATER DIVIDES that separate areas where the water flows in one direction from areas where it flows in another. For the shallow part of the ground-water system, the locations of divides are determined largely by the distribution of surface-water bodies. Some of the ground water might flow toward a stream located, say, on the west side of the basin, while the rest flows toward a lake located on the east side of the basin:

Schematic section of shallow ground-water flow systems separated by natural ground-water divides (source: USGS Water-Resources Investigations Report 00-4008)

SURFACE-WATER DIVIDES (also called "watershed divides") are defined by land topography and separate areas where water flows overland as runoff toward different surface-water bodies. In the schematic figure shown above, the ground-water and surface-water divides coincide and are located along a topographic high such as a ridgeline. However, in many real cases the two types of divides do not coincide. In fact, as we will see in the case of southeastern Wisconsin, under some circumstances they can fall many miles apart - even in the absence of pumping.

A single major surface-water divide separates areas that slope down toward the Great Lakes from areas that slope in other directions. This boundary is called a SUBCONTINENTAL DIVIDE.

The waters of the Great Lakes include the rivers that empty into them. Each of these rivers and their tributaries drain rain and snow that falls on its basin. The most landward boundary of these basins generally coincides with an identifiable ridge or other high point. On the other side of the basin boundary, runoff from rain and snow drains into streams and rivers that ultimately connect to other large bodies of water such as the Mississippi River or Caribbean Sea. It is possible, using topographic maps, to trace the basin boundaries that divide rivers that drain to the Great Lakes from rivers that drain elsewhere. Stitched together these basin boundaries define the surface-water subcontinental divide of the Great Lakes Basin.

Rivers and streams are not the only way to convey rain and snowmelt to the Great Lakes. The ground-water system also moves water in that direction. Some of the ground-water belonging to the Great Lake system discharges directly into one of the lakes, but most discharges to the Lakes indirectly by discharging to a river or stream within the watershed.

It is not as easy to determine the boundary enclosing Great Lakes ground water as it is to trace the surface-water divide. An important factor to consider is that the location of the ground-water divide depends on depth from the land surface. At the water-table surface or at shallow depths below the water table, the ground-water divide is likely to largely coincide with the surface-water divide because both boundaries are controlled by the rise and fall of the land surface. Shallow ground water typically (although not always!) follows short paths from upland recharge areas to local valleys cut by streams.

A local ground-water divide encloses each local flow system. Stitched together these local boundaries define a single regional boundary that separates ground water that discharges to rivers and lakes within the Great Lakes basin (or to a Lake itself) from ground water that discharges to rivers and lakes outside the Great Lakes basin.

Deeper into the subsurface, it is often the case that ground water follows longer paths and passes below local streams. In a deep aquifer separated from the shallow part of the system by a low-permeability formation, it is possible for the ground water to move tens of miles from a starting point in a basin that is outside the Great Lakes surface-water divide to an endpoint inside the Great Lakes basin or, more commonly, to a Great Lake itself. The deep ground-water divide can be unrelated to the surface-water divide and the region contributing ground water to the Great Lakes can be much larger than the surface-water basin.

Schematic block diagram of shallow and deep ground-water divides (source: D.T. Feinstein, U.S. Geological Survey)

A second factor that controls the location of the ground-water divides is pumping from wells. Pumping can shift ground-water divides from their natural locations and even cause ground water that previously discharged to a local stream to move across surface-water divides within the basin to a regional pumping center outside the basin. Increases in pumping gradually cause the ground water divides to shift, but they have no effect on the surface-water divides.