The ground-water surface is the name given the upper surface of ground water, the level below which all rocks are saturated.  In dry seasons the ground-water surface sinks.  For ground water is constantly seeping downward under gravity, it is evaporated in the waste and its moisture is carried upward by capillarity and the roots of plants to the surface to be evaporated in the air.  In wet seasons these constant losses are more than made good by fresh supplies from that part of the rainfall which soaks into the ground, and the ground-water surface rises.

In moist climates the ground-water surface (Fig. 24) lies, as a rule, within a few feet of the land surface and conforms to it in a general way, although with slopes of less inclination than those of the hills and valleys.  In dry climates permanent ground water may be found only at depths of hundreds of feet.  Ground water is held at its height by the fact that its circulation is constantly impeded by capillarity and friction.  If it were as free to drain away as are surface streams, it would sink soon after a rain to the level of the deepest valleys of the region.

Wells and springs.  Excavations made in permeable rocks below the ground-water surface fill to its level and are known as wells.  Where valleys cut this surface permanent streams are formed, the water either oozing forth along ill-defined areas or issuing at definite points called springs, where it is concentrated by the structure of the rocks.  A level tract where the ground-water surface coincides with the surface of the ground is a swamp or marsh.

By studying a spring one may learn much of the ways and work of ground water.  Spring water differs from that of the stream into which it flows in several respects.  If we test the spring with a thermometer during successive months, we shall find that its temperature remains much the same the year round.  In summer it is markedly cooler than the stream; in winter it is warmer and remains unfrozen while the latter perhaps is locked in ice.  This means that its underground path must lie at such a distance from the surface that it is little affected by summer’s heat and winter’s cold.

While the stream is often turbid with surface waste washed into it by rains, the spring remains clear; its water has been filtered during its slow movement through many small underground passages and the pores of rocks.  Commonly the spring differs from the stream in that it carries a far larger load of dissolved rock.  Chemical analysis proves that streams contain various minerals in solution, but these are usually in quantities so small that they are not perceptible to the taste or feel.  But the water of springs is often well charged with soluble minerals; in its slow, long journey underground it has searched out the soluble parts of the rocks through which it seeps and has dissolved as much of them as it could.  When spring water is boiled away, the invisible load which it has carried is left behind, and in composition is found to be practically identical with that of the soluble ingredients of the country rock.  Although to some extent the soluble waste of rocks is washed down surface slopes by the rain, by far the larger part is carried downward by ground water and is delivered to streams by springs.