Ground-water circulation does not extend into Precambrian crystalline rocks that generally have minimal permeability and function as a barrier to water movement. The surficial aquifer system is hydraulically connected to streams because of its shallow depth, ease of recharge by precipitation, and short ground-water flow systems. The degree of this connection, however, is affected by the permeability of the deposits comprising the aquifer. The similar concentrations of each constituent indicate that the low flow of the stream is composed almost entirely of ground-water discharge. Thus, in Segment 9, ground-water discharge from the surficial aquifer system sustains the low flow of streams, and the greatest low flows occur in areas underlain by permeable, thick, glacial deposits.
Low flows are less in areas underlain by less permeable glacial deposits.
An important aspect of ground-water movement in the surficial aquifer system is the relation of ground water to lakes. About 25,000 lakes that formed during glaciation are present just in Minnesota and Wisconsin. The lakes generally are an extension of the water table in the surrounding surficial aquifer system. This relation is typical of ground-water movement around lakes in the glaciated part of Segment 9. Potential well yields of 500 to 1,000 gallons per minute can be expected from outwash deposits in the northern and the western parts of the Fox-Wolf River basin, as shown in figure 38 .