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.

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).

A city in a region of copious rains, built on the narrow flood
plain of a river, overlooked by hills, depends for its water
supply on driven wells, within the city limits, sunk in the sand a
few yards from the edge of the stream. Are these wells fed by