70 BULLETIN 1017, tr. S. DEPARTMENT OF AGRICULTURE. 
point by the greater width and excess of depth available as noted 
above. The continuous spoil banks would necessitate the use of 
flood gates for the small drains entering this section of the main 
channel, and the construction of spoil-bank levees along the lower 
parts of the larger ditches that enter the outlet channel. Such an 
arrangement would make possible the drainage of the low, wet areas 
along the Bois de Sioux, and only under the most extreme conditions 
of flow, which seldom occur, would there be any danger of flooding 
the lands adjacent to the outlet channel. 
THE SECONDARY DITCH SYSTEM. 
The secondary ditch system is designed pirmarily to provide outlet 
for the local drainage which will be necessary in the development t 
-of the wet lands along the Bois de Sioux River. The ditches are e 
placed also with the view of relieving the upper part of the main i 
channel from the run-off from the adjacent land, leaving its entire el 
capacity free for the removal of the discharge from the reservoir, j 
The proposed locations of the seven laterals are shown on figure 3, j 
sheets 2, 3, and 4. All would have final outlet into the main channel, J 
with the exception of No. 7, which would discharge the water from J 
Big Slough into the reservoir above the dam, thus permitting it to ) 
be controlled within the reservoir. These lateral drains are designed I 
with capacities great enough to carry any probable run-off from their 
drainage areas and, because of construction limitations, are in most ; 
cases oversized. 
BRIDGES. 
On account of the enlarged channel along the Bois de Sioux, it 
will be necessary to replace most of the present bridges with larger 
ones. Several of the steel spans can be used at their present loca- 
tions by readjusting their elevations and constructing approaches, 
and some of the present pile trestles can be utilized as approaches 
for new steel spans at the same locations. As the design of highway 
bridges is in most States controlled by the State highway depart- 
ment, no consideration has been given to the design of the bridges 
necessary, but it is assumed that steel bridges with concrete abut- 
ments will be used. It is estimated that six spans 60 feet to 80 feet 
in length, and 14 spans 20 feet to 28 feet long, will be necessary. 
LOCAL DRAINAGE. 
To gain full benefit from the proposed improvements, it will be 
necessary to supplement them by local drains in all parts of the af- 
fected areas. The secondary ditch system as planned herein will, 
to a certain extent, drain the land in the immediate vicinity of the 
ditches, but the main object of these ditches is to provide outlet for 
