GROUND WATER IN NORTH-CENTRAL TENNESSEE

OCCURRENCE OF GROUND WATER IN LIMESTONE 77
are compared with surface streams that are in the corresponding stage
of development (pp. 78-82).
The major ground-water conduits are not all single channels pass­
ing directly from point of intake to point of discharge, for some large
springs that issue from them are relatively invariable in yield and
discharge clear water even during floods. Hurricane Rock Spring (No.
181, pp. 161-162), in Humphreys County, is of this sort. Where such
conditions exist the system of channels must store temporarily enough
water to clarify the turbid inflow, even though the gradient directly
from area of inflow to point of discharge may be relatively steep.
Obviously, this storage capacity can not exist unless the channels
ramify so widely that they are very flat or unless they comprise many
voluminous pools of nearly static water.
The volume of water thus stored temporarily in cavernous rocks
may be truly enormous, as is shown by the behavior of the Major
Johnson group of springs on the Carlsbad irrigation project, New
Mexico. These springs are about 3% miles southwest of the McMil-
lan Dam, 60 feet below the crest of its spillway, and 40 feet below the
lowest point of the floor of the reservoir. The discharge of these
springs was 272 second-feet in 1921, when the reservoir was full, and
it has been shown that under those conditions the springs were supplied
chiefly by water that leaked from the reservoir into cavernous rocks. 61
As soon as the reservoir becomes empty, however, the discharge of
the spring group begins to decrease, and in the course of about a
month it declines to a minimum of approximately 40 second-feet,
this minimum discharge being the normal ground-water discharge of
the spring basin as long as the reservoir is empty. The decline is
due to cessation of leakage from the reservoir and to gradual drain­
ing of the water stored temporarily above the normal water table.
On the assumption that the rate of decline is uniform throughout the
month, about 300 million cubic feet (2,250 million gallons) of water
must be stored temporarily in the cavernous rock that drains into
this spring group. This volume of water would fill a reservoir 1 mile
square to a depth of nearly 11 feet.
On the other hand, some trunk channels must pass directly from
intake to outlet, for the discharge of many tubular springs increases
considerably and becomes extremely turbid for a brief period after
a heavy rain. Such springs are common in north-central Tennessee,
and a typical example is the municipal spring at Murfreesboro,
Rutherford County (No. 439, pp. 187-188).
" Meinzer, O. E., Benick, B. O., and Bryan, Kirk, Geology of No. 3 reservoir site of the Carlsbad irri­
gation project, New Mexico, with respect to water-tightness: U. S. Qeol. Survey Water-Supply Paper
580, pp. 23-24, 1927.