us, the surface flow will receive and transmit the
water into the mass only through the cracks and fissures in
the rock. Pervious materials, such as sandstones, sands,
gravels, and cracked or fissured rocks, induce seepage, retard
runoff, and, if such deposits are underlaid with an impervious
bed, provide underground storage which impounds water away
from the conditions which permit evaporation, and hence tends
to increase runoff and equalize flow. On the other hand, if
such pervious deposits possess other outlets outside of the
stream channel and drainage area, they may result in the
withdrawal of more or less of the seepage waters entirely from
the ultimate flow of the stream. Coarse sands and gravels
will rapidly imbibe the rainfall into their structure. Fine
and loose beds of sand also rapidly receive and transmit the
rainfall unless the precipitation is exceedingly heavy under
which conditions some of it may flow away on the surface.

Many of the highly pervious indurated formations receive water
slowly and require a considerable time of contact in order to
receive and remove the maximum amount.

In flat, pervious areas, rainfalls of a certain intensity are
frequently essential to the production of any resulting stream
flow. In a certain Colorado drainage area, the drainage
channel is normally dry except after a rainfall of one-half
inch or more. A less rainfall, except under the condition of a
previously saturated area, evaporates and sinks through the
soil and into the deep lying pervious sand rock under the
surface which transmits it beyond the drainage area. Such
results are frequently greatly obscured by the interference of
other factors, such as temperature, vegetation, etc.

* * * * *

The natural storage of any drainage area and the possibilities
of artificial storage depend principally upon its topography
and geology. Storage equalizes flow, although the withdrawal
of precipitation by snow or ice storage in northern areas
often reduces winter flow to the minimum for the year. Both
surface and sub-surface storage sometimes hold the water from
the streams at times when it might be advantageously used.
Storage, while essential to regulation, is not always an
advantage to immediate flow condit