Fig. 53.—Diagram to illustrate the intermittency of streams due to fluctuations of the ground-water level. The water level aa would be depressed next the valley 2–2 by the flow of the water into the valley. The profile of the ground-water surface would therefore be aca rather than aa.
In general a permanent stream at one point in a valley means a continuous stream from that point to the sea or lake which the valley joins; but to this rule there are many exceptions. They are likely to arise where a stream heads in a region of abundant precipitation, and flows thence through an arid tract where the ground-water level is low, and evaporation great. In such cases, evaporation and absorption may dissipate the water gathered above, and the stream disappears ([Fig. 2, Pl. III], near Paradise, Nev.).
PLATE III.
U. S. Geol. Surv.
Scale, 2+ mile per inch.
Fig. 1. KANSAS.
U. S. Geol. Surv.
Scale, 4+ mile per inch.