20 
BULLETIl:^ 181^ U. S. DEPARTMENT OF AGEICULTUEE. 
Time of rising and falling of streams under minimum storage conditions as indicated by 
hydrographs resulting from practically continuous rains before but unaffected by rainfall 
after maximum stage was reached. 
Name of river. 
Observation station. 
Date of maxi- 
mum stage. 
Time 
rising. 
Time 
falliQg. 
Maximum 
stage. 
Flood 
stage. 
Tombigee 
Columbus, Miss 
do 
do 
do 
Apr. 20,1893 
Feb. 29,1908 
Mar. 28,1908 
July 13,1910 
Apr. 24,1911 
Dec. 9,1912 
Apr. 11,1911 
Apr. 27,1911 
Apr. 19,1910 
Nov. 30, 1899 
Feb. 9, 1907 
Dec. 27,1908 
June 27,1909 
July 22,1912 
May 28,1910 
July 15,1905 
Feb. 15,1905 
Feb. 16,1905 
Days. 
6 
f 
5 
? 
4i 
3 
7 
8 
f 
3 
6 
5 
Days. 
6 
4 
5^ 
f 
5 
3 
7 
10 
4 
? 
6 
5 
f 
Feet. 
17.5 
10 
21 
19 
26 
25.5 
32 
31.5 
10 
16.5 
45 
25 
42 
17.5 
12.8 
20.7 
20.7 
29.4 
Feet. 
33 
33 
33 
33 
33 
33 
33 
41 
Do 
Do 
Do 
Do 
do 
Do .. - 
Aberdeen, Miss 
.... do 
Do 
Do 
Cochrane, Ala . . . 
Do 
Fulton, Miss 
17 
Do 
Demopolis, Ala 
do 
do 
do 
35 
35 
35 
35 
35 
Do 
Do 
Do 
Do 
.. . do 
West Pearl 
Pearl River, La 
12 
32 
Flint 
Montezuma, Ga 
Pera, Ala 
20 
Pea 
Total 
96 
lOli 
The above table indicates that the time of falling is slightly greater 
than that consumed in rising, as may be seen by comparing the 
total number of days in each case. However, in the method under 
discussion, if the time of falling be taken equal to that of rising the 
result will tend toward a run-off rate that is too large rather than 
too small, and there will thus be introduced a factor of safety which 
is especially desirable in planning levee systems. Therefore, in the 
following computations the assumption is made that the time of 
falling after the maximum rate of run-off is reached will be the 
same as the time of rising, and that a uniform rate of rising and 
faUing is maintained, the latter assumption also being on the side 
of safety. 
If a hydrograph of the Pearl River for the storm period of 1902 
be constructed, showing the river to perform in accordance with 
the foregoing conditions, it would consist of a triangle whose base 
represents 10 days, as shown by the line a u in figure 4, B. The 
lower portion of the hydrograph would probably conform quite 
closely to the curve u n whose rate of falling is slightly greater than 
that indicated by curves representing this stage of subsidence during 
other periods. Thus the hydrograph afunr would represent the 
performance of the stream from March 26 to May 12 under improved 
drainage conditions, and assumhig that no rainfall occurred which 
affected its decline. 
The total run-off for the storm period of 1902 is represented by 
the area maoqs as shown on the actual hydrograph of the Pearl 
River. An inspection of this hydrograph shows the decline of the 
river to have been materially affected by rainfall after April 1, which 
