REPORT OF INVESTIGATIONS NO. 48


 0 5 10 15 20 25 30 35 40 45 50 55 60
 CUMULATIVE RAINFALL, FEET
 Figure 11.-Double mass relation between estimated rainfall at Lake
 Jackson and the resulting rise of the lake level.

runoff, and by a tendency for the relation line, as shown in figure
11. to drift persistently in one direction. In figure 11, however,
the relation line shows no such tendency, indicating that to date
the total runoff from the lake basin has not been altered appreci-
ably by urbanization.

 LONG-TERM FLUCTUATIONS OF LAKE LEVEL

 The hydrologic significance of the recent water-level fluctua-
tions of Lake Jackson may be appraised by considering past
fluctuations. Unfortunately, systematic observations of the water
level of Lake Jackson, or of any comparable lake, were -not made
prior to 1950. Other hydrologic parameters, such as rainfall and
ground-water levels, have been observed for longer periods of