GROUND WATER IN NORTH-CENTRAL TENNESSEE

SPBINGS 93
roof above its channel is breached by collapse or by solution. (See
pi. 5, B.} Tubular springs representative of each of these classes are
very numerous in most parts of the region, and many typical examples
are described in the tabulated data and the county reports
(pp. 124-233).
The discharge of tubular springs in north-central Tennessee ranges
from one gallon or less to many thousand gallons a minute. No springs
of the first magnitude discharging 100 cubic feet a second (45,000
gallons a minute) are known to exist within the region, but on Sep­
tember 10, 1927, Hurricane Rock Spring, in Humphreys County
(No. 181, pp. 161-162), discharged at the approximate rate of 60 cubic
feet a second (27,000 gallons a minute). This spring may attain first
magnitude during periods of maximum discharge. Of the springs
that were visited by the writer during July, August, and September,
.1927, 57 discharged more than 100 gallons a minute each, and 12 dis­
charged more than 1,000 gallons a minute. However, most tubular
springs fluctuate greatly in yield, the maximum observed discharge
of one of these springs during the four months of the field investiga­
tion being about twenty times the minimum observed discharge.
The discharge of certain springs presumably those that are fed by
solution channels of steep gradient or small storage capacity and those
whose source is largely in local intermittent run-off increases to
several times the normal flow in the course of a few hours after a
heavy rain and may decline with almost equal rapidity. The dis­
charge of other springs presumably those that issue from solution
channels of flat gradient and large storage capacity as well as those
whose source is not primarily from local run-off is relatively uni­
form from day to day, although it may vary considerably from season
to season. All the springs of an area may not attain their maximum
rate of discharge at the same time, so that the relative magnitude of
two or more springs can not be determined accurately if the discharge
of each is measured only once, even if the measurements are taken at
approximately the same time. The true average discharge of the
variable springs can be determined only from a continuous and accu­
rate record of discharge extending over a period of several years.
At many localities tubular springs constitute the only source of large
water supplies for municipal or industrial purposes. In view of their
variability, however, these springs should not be developed as a
source of supply without trustworthy information as to the quantity
of water that may be available from them in dry seasons.
The tubular springs in north-central Tennessee differ noticeably in
the temperature of their water. In springs whose discharge is mod­
erate and not highly variable the temperature is relatively constant
and is approximately equal to the mean annual temperature of the
region, 58° to 60° F. Springs derived from bodies of surface water