62 BULLETIN 1017, U. S. DEPARTMENT OF AGRICULTURE. 



The information collected shows very clearly that the highest 

 floods are those caused by melting snow, combined in some instances 

 with rains at a time of the year when the ground is more or less im- 

 pervious to water. However, the floods that have caused the greatest 

 loss to crops have been those caused by heavy rains over the watershed 

 during the period from May to August. The natural drainage is not 

 capable of removing the run-off from heavy storms, such as occurred 

 in 1916, in time to prevent flooding and serious damage to crops. In 

 many instances where an attempt has been made to relieve this condi- 

 tion by constructing ditches, it has not been possible to secure a satis- 

 factory outlet or the drains have been dug too shallow and too small. 



^HE PROBLEM. 



In order to relieve the conditions just described, it is necessary 

 either to provide channels of sufficient capacity to remove the run-off 

 from storms and spring thaAvs without flooding or so to control this 

 run-off that the drainage channels shall not be overtaxed. A combi- 

 nation of the two methods is recommended as most advantageous. A 

 number of different plans for accomplishing these results were given 

 careful study. No discussion will be given of the several plans con- 

 sidered but not adopted. 



In reaching conclusions as to the conditions which must be met 

 in planning the improvements extensive and detailed studies were 

 made of all available precipitation and run-off records. These 

 studies and the methods used in estimating requirements have been 

 discussed, and only those phases that apply to this project in par- 

 ticular will be taken up here. 



In planning for the drainage of this watershed provision has been 

 made only for outlet drainage. Only those works have been in- 

 cluded that will require joint action of the three States interested. 

 Some suggestions are made as to lateral drainage, but it is believed 

 that these local drains can be constructed most economically under 

 the drainage laws of the State in which the drains are located. 



THE DETENTION RESERVOIR. 



The reservoir, by means of which it is proposed to control the run- 

 off from the Lake Traverse watershed, would include that part of 

 the present bed of the lake between Browns Valley, Minn., and the 

 north line of sections 24 and 33, T. 128 N. Along this line, which 

 is about 4 miles south of White Bock, S. Dak. (see PL V, fig. 

 2), it is planned to build the earth dam extending to high ground 

 on both sides. The proposed sluiceway for controlling the outflow 

 is located at the intersection of the dam with the Boise de Sioux chan- 

 nel, and the spillway for protecting the dam from damage by over- 



