66 
BULLETIX 1017, U. S. DEPARTMENT OE AGRICULTURE. 
Table 15. — Storage capacity for the proposed Lake Traverse detention reservoir 
south of White Rock, S. Dak. 
Contours. 
within 
desig- 
nated 
contours. 
972 
973 
974 
975 
976 
977 
978 
97^ 
Billions 
of sq.ft. 
0.610 
.680 
.749 
.811 
.873 
.915 
.956 
.976 
Mean 
areas = 
volumes 
when 
lfoot 
deep. 
Total 
storage 
above 
elevation 
972. 
BiUions 
Billions 
ofcu.ft. 
ofcu.ft. 
0.000 
0.645 
.645 | 
.715 
1.360 
.780 
2.140 
.842 
2.982 
.894 
3.876 
.936 
4.812 
.966 
5.778 j 
i Areas 
within 
Contours. i desig- 
I nated 
contours. 
BWions 
j of sq.ft. 
980 ! .996 
981 ' 1.017 
982 1.037 
9813 : 1.065 
984 1.092 
985 1.121 
986 1.149 
990 1.243 
Mean 
areas= 
volumes 
when 
lfoot 
deep. 
Billions 
ofcu.ft. 
Total 
storage 
capacity 
above 
elevation 
972. 
Billions 
ofcu ft. 
6.764 
7.771 
8.798 
9.849 
10.928 
12.035 
13. 170 
The relations of inflow, storage, and outflow are shown in the mass 
diagrams of figure 20. It will be noted that, in these diagrams, the 
15-day periods of no outflow have been taken in each case at a 'time 
when their effect in producing high reservoir stages would be 
greatest. 
In diagram E of figure 20, the estimated mean annual run-off, 
" Inflow," was obtained by multiplying the mean monthly, 1909-1917, 
run-off of the Minnesota Wild Rice by 1.6, the ratio of the watershed 
areas, and deducting the loss by evaporation. The curve for the run- 
off of 1916, diagram A, figure 20, the year of maximum precipitation 
over the Lake Traverse watershed, was obtained by plotting as run- 
off for each month, 20 per cent of the precipitation over the watershed 
for the preceding month with the exception that 20 per cent of the 
total precipitation for December, 1915, and January and February, 
1916, was plotted as the March, 1916, run-off, the January and Febru- 
ary run-off being merely nominal. This ratio (20 per cent) of run- 
off to precipitation is the mean of those that prevailed immediately 
after the great storms on the Bed River watershed as indicated by 
figure 19, and is considerably higher than is indicated for normal con- 
ditions by the rather meagre data available. Diagram E shows that 
with an outflow of 1,000 second- feet, starting May 1. the reservoir in 
a mean year would be brought to low stage by about the middle of 
June and that from that time on the outflow would at all times be 
less than 1.000 second- feet. The maximum storage required would 
be less than 2 billion cubic feet. In the case of run-off such as is 
estimated for 1916 (diagram A) all run-off prior to May 1 could be 
stored and the reservoir closed for 15 days during the summer, and 
yet the reservoir could be emptied by the middle of December. In 
that case there would be a maximum storage of 4.3 billion cubic 
feet, which would not raise the water above elevation 978. 
In diagram B of figure 20. maximum spring run-off (January- 
June) was plotted in combination with the summer and fall run-off 
