4.4: The Water Balance of Lakes

Lakes have many sources of both inflow and outflow. The main sources of inflow are:

• streams
• precipitation onto the lake surface
• ground water

The main sources of out flow from a lake are:

• streams
• evaporation from the lake surface
• groundwater

Various configurations of streams are possible:

• Stream or streams flow in, stream flows out (Figure 4-3A). Usually only one stream flows out. In this kind of configuration the stream can be viewed as simply a fat place along a river. This is the most common kind of lake.
• Stream or streams flow in, no stream flows out (Figure 4-3B). The loss is made up by groundwater outflow and/or net evaporation. This is common in arid and semiarid regions; the streams are usually ephemeral. Lakes like this tend to be playa lakes. The Great Salt Lake is a good (and large) example.
• Stream flows out, no stream flows in (Figure 4-3C). The lake is fed by groundwater, usually not just by precipitation.
• No stream flows in, no stream flows out (Figure 4-3D). The lake is both fed and drained by groundwater movement. Lakes like this are common in glaciated areas underlain by a thick mantle of highly permeable glacial sediment, as on Cape Cod.

Figure 4-3. Configurations of streams flowing into and out of lakes.