SANITARY SURVEY OF DRAINAGE BASIN OF POTOMAC RIVER. 233 
at Little Falls. Depending on their height, they cause slack water, 
which extends from 4 to 11 miles above each. At Great Falls is a 
dam above which, on the Maryland side, is the intake of the Wash- 
ington aqueduct. 
The velocity of flow is extremely variable. It has been esti- 
mated ® that it takes from four to seven days for the water to travel 
from Cumberland to Great Falls, a_ distance of 176 miles. 
The United States Geological Surve}^ has recorded a variation in 
the volume of discharge of the river at Point of Kocks, Md., ranging 
from 218,700 to 900 cubic feet a second. 
In Table No. 1 is given the monthlj^ run-off for 1906 to September 
1, contrasted with the normal, based on data fora period of eleven 
years. 
Table No. 1. — SJwuing monthly run-off at Point of Rtchs, Aid. 
[Drainage area, 9,654 square miles.] 
DEPTH IN INCHES. 
Year. 
Jan. 
Feb. 
Mar. 
Apr. 
May. 
June. 
July. 
Aug. 
Sept. 
Oct. 
Nov. 
Dec. 
Normal a 
Mean, 1906 b 
1.298 
1.790 
1.848 
.552 
2.421 
1.900 
2.008 
2.600 
1.350 
.662 
0.946 
.810 
0.741 
.527 
0.651 
1.820 
0.324 
0.514 
0.419 
1.079 
CUBIC FEET PER SECOND. 
15,200 I ' I 
I I ! _ 
a Calculated from data for eleven years, 1895 to 1905, in Bulletin No. 3, Geological Series, Virginia 
Department of Agriculture and Immigration. 
b Obtained from U. S; Geological Survey. 
In Table No. 2 is given the mean monthly precipitation, and in 
Table No. 3 the mean monthly temperature for the Potomac basin 
from January to September, 1906. 
In order to bring out such relation as may exist between the pre- 
cipitation, run-off, temperature of the Potomac basin, and the preva- 
lence of typhoid in Washington, their curves and that for the typhoid 
mortality of Washington have been plotted in figures 1, 2, 3, and 4. 
Table No. 2. — Precipitation. 
[Depth in inches.] 
Year. 
Jan. 
Feb. 
Mar. 
Apr. 
May. 
June. 
July. 
Aug. 
Sept. 
Oct. 
Nov. 
Dec. 
Normal 
1906 
2.54 
2.66 
2.99 
1.07 
3.35 
4.49 
3.00 
2.42 
4.00 
2.28 
4.00 
6.26 
3.96 
3.88 
3.58 
8.41 
2.78 
2.70 
2.45 
2.84 
a Captain Gaillard, Annual Report of the Chief of Engineers, U. S. A., 1894, cited 
in manuscript of the U. S. Geological Survey on the hydrography of the Potomac. 
Mean, 1906 ,14,990 ^ 5,116 15,900 22,440 . 5,538 ' 7,007 4,406 
