In the south central part of Texas is an upland covering an area
of 14,000 square miles, and known as the Edwards Plateau. At the
southeastern foot of this elevated tract there is no end of gushing
springs, which form the headwaters of the San Antonio and San Marcos
rivers. In a big State like Texas, the rainfall of one locality often
varies a good deal from that of adjacent regions. But, according to
a bulletin of the United States Geological Survey, the fluctuations
of discharge of the streams just mentioned correspond closely to
the rainfall up on the plateau, from which it is inferred that some
invisible connection exists between the springs and the upland. The
bulletin declares that this similarity has been found to hold true for
dry and wet years alike. The Edwards Plateau is a flat, grass covered
upland. The rain which falls upon it does not flow off in surface
streams, but sinks into the porous soil, and eventually finds its way
underground to the bold scarp line of the region, where it bursts out
in abundant springs.
The San Antonio River has its source in one of these artesian springs,
and between it and the wells driven to supply water to the city of
San Antonio there seems to be close connection, shown in their mutual
changes, which indicates that their waters have a common source. It
was recently noticed that when the wells were steadily drawn upon for
twenty-four hours the water level of the head lake of the river fell
several inches, but that on shutting off the wells the lake regained
its level in about one day. So intimate is the relation between the
flow of the wells and that of the river that it is always possible
to tell how high the water will rise in the former by observing the
river’s height on a gauge rod placed upon its bank.