Fig. 51.—Diagram illustrating the further development of [Fig. 49]. The land here has been reduced greatly, though not yet to base-level.
The permanent stream.—From the foregoing discussion, it is seen that a valley may be developed by the run-off of successive showers. If supplied only from this source surface streams would cease to flow soon after the rain ceased to fall, and a valley might attain considerable size without possessing a permanent stream. How does the valley developed by the run-off of successive showers come to have a permanent stream? The answer to this question involves a brief consideration of that part of the rainfall which sinks beneath the surface.
If wells be sunk in a flat region of uniform structure and composition the water in them is generally found to stand at a nearly common level. The meaning of this fact is not far to seek. If a hole 60 feet deep fills with water up to a point 20 feet from the surface, it is because the material in which the well is sunk is full of water up to that level. When the well is dug the water leaks into it, filling the hole up to the level to which the rock (or subsoil) is itself full. This level, below which the rock and subsoil (down to unknown depths) are full of water, is known as the ground-water level, ground-water surface, or water-table.
The ground-water level fluctuates. In a wet season it rises, because more water has fallen and sunk beneath the soil; but several processes at once conspire to bring it down again. Where there is growing vegetation its roots draw up water from beneath, and evaporation also goes on independently of vegetation. The water is drawn out through wells and runs out through openings. It may also flow underground from one region to another where the ground-water surface is lower. All these processes depress the ground-water surface.
Fig. 52.—Diagram illustrating the fluctuation of a ground-water surface. a = wet-weather ground-water level; b = ground-water level during drought. Well No. 1 will contain water during the wet season, but will go dry in times of drought. Well No. 2 will be permanent.
A well sunk to such a level as to be supplied with abundant water in a wet season may go dry during a period of drought because the ground-water level is depressed below its bottom. Thus either well shown in [Fig. 52] will have water during a wet season when the water-level is at a; but well No. 1 will go dry when the water surface is depressed to b.
The principles applicable to wells are applicable to valleys. When a valley has been deepened until its bottom reaches below the ground-water level, water seeps or flows into it from the sides. The valley is then no longer dependent on the run-off of showers for a stream. It will be readily seen that at some stage in its development, the bottom of a valley may be below the ground-water level of a wet season without being below that of a dry one. Thus the valley represented in cross-section by the line 2–2, in [Fig. 53], will have a stream when the ground-water level is at aa, but none when this level is depressed to bb. If the rainfall of the year were concentrated in a single wet season, the intermittent stream would flow not only during that season, but for so long a time afterward as the ground-water level remained well above the valley bottom. In regions subject to frequent and short periods of heavy precipitation, alternating with droughts, the periods of intermittent flow may be many and short. Since the precipitation of many regions varies greatly from year to year, it follows that a stream may flow continuously one year and be intermittent the next. Many valleys in various parts of the earth are now in the stage of development where their streams are intermittent.
As a valley containing an intermittent stream becomes deeper, the periods when it is dry become shorter, and when it has been sunk below the lowest ground-water level, it will have a permanent stream (3, [Fig. 53]). Since a valley normally develops headward, its lower and older portion is likely to acquire a permanent stream, while its upper and younger part has only an intermittent one ([Fig. 47] and [Fig. 1], [Pl. III], near Anthony, Kan. The intermittent part of the stream is indicated by the dotted blue line). For the same reason the head of a stream is likely to be farther up the valley in wet weather than in dry. So soon as a valley gets a permanent stream, the process of enlargement goes on without the interruption to which it was subject when the supply of water was intermittent.
