_How a valley gets a stream._--Valleys may become somewhat deep and long and wide without possessing permanent streams, though from their inception they have _temporary_ streams, the water for which is furnished by showers or melting snow. Yet sooner or later, valleys come to have permanent streams. How are they acquired? Does the valley find the stream or the stream the valley? For the answer to these questions, a brief digression will be helpful. [Illustration: Fig. 12.--A typical river system of the Coastal plain type.] In cultivated regions, wells are of frequent occurrence. In a flat region of uniform structure, the depth at which well water may be obtained is essentially constant at all points. If holes (wells 1 and 2, Fig. 13) be excavated below this level, water seeps into them, and in a series of wells the water stands at a nearly common level. This means that the sub-structure is full of water up to that level. These relations are illustrated by Fig. 13. The diagram represents a vertical section through a flat region from the surface (s s) down below the bottom of wells. The water stands at the same level in the two cells (1 and 2), and the plane through them, at the surface of the water, is the _ground water level_. If in such a surface a valley were to be cut until its bottom was below the ground water level, the water would seep into it, as it does into the wells; and if the amount were sufficient, a permanent stream would be established. This is illustrated in Fig. 13. The line A A represents the ground water level, and the level at which the water stands in the wells, under ordinary circumstances. The bottom of the valley is below the level of the ground water, and the water seeps into it from either side. Its tendency is to fill the valley to the level A A. But instead of accumulating in the open valley as it does in the enclosed wells, it flows away, and the ground water level on either hand is drawn down. [Illustration: Fig. 13.--Diagram illustrating the relations of ground water to streams.] The level of the ground water fluctuates. It is depressed when the season is dry (A' A'), and raised when precipitation is abundant (A'' A''). When it is raised, the water in the wells rises, and the stream in the valley is swollen. When it falls, the ground water surface is depressed, and the water in the wells becomes lower. If the water surface sinks below the bottom of the wells, the wells "go dry;" if below the bottom of the valley, the valley becomes for the time being, a "dry run." When a well is below the lowest ground-water level its supply of water never fails, and when the valley is sufficiently below the same level, its stream does not cease to flow, even in periods of drought. On account of the free evaporation in the open valley, the valley depression must be somewhat below the level necessary for a well, in order that the flow may be constant.