The flow of springs varies much less during the different seasons of the year than does that of surface streams. So slow is the movement of ground water through the rocks that even during long droughts large amounts remain stored above the levels of surface drainage.
Movements of ground water. Ground water is in constant movement toward its outlets. Its rate varies according to many conditions, but always is extremely slow. Even through loose sands beneath the beds of rivers it sometimes does not exceed a fifth of a mile a year.
Fig. 26. Geological Conditions favorable to Strong Springs
a, limestone; b, shale; c, coarse sandstone; d, limestone; e, sandstone; f, fissure. The strata dip toward the South, S. Redraw the diagram, marking the points at which strong springs (ss) may be expected.
In any region two zones of flow may be distinguished. The upper zone of flow extends from the ground-water surface downward through the waste mantle and any permeable rocks on which the mantle rests, as far as the first impermeable layer, where the descending movement of the water is stopped. The deep zones of flow occupy any pervious rocks which may be found below the impervious layer which lies nearest to the surface. The upper zone is a vast sheet of water saturating the soil and rocks and slowly seeping downward through their pores and interstices along the slopes to the valleys, where in part it discharges in springs and often unites also in a wide underflowing stream which supports and feeds the river ([Fig. 24]).
Fig. 27. Fig. 28.