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34 BULLETIN 1358, U. S. DEPARTMENT OF AGRICULTURE 
basin is limited to that carried by the amount of water required to — 
fill it, whereas with a spillway at the lower end a part of all the silt | 
in the overflow water also is deposited. 
SETTLING BASINS AND CHECK DAMS 
Small dams located above the main reservoir and forming settling 
basins stop many little freshets and receive their silt, and even in 
high waters fulfill the same mission in lesser degree. Mud in small 
settling basins dries more quickly than in the main basin and there- 
fore can be removed more readily. Fencing of the settling basin not 
only prevents trampling and thus facilitates cleaning but alco fosters 
a rank growth of vegetation which makes it a better silt trap. One or — 
more settling basins 1 may be placed above a reservoir, depending upon — 
the need and cost. They may vary from very small structures to 
reservoirs of considerable size, which can hold water for several 
months. (Pl. XVII.) The larger dams should follow the lines of | 
construction given for main dams, including spillways, since they — 
must withstand the first shock of all freshets, and if washed out the © 
accumulated material would enter the main reservoir. . 
Inexpensive check dams, 2 or 3 feet high, too smal! and often toe 
porous to form real settling basins, have been used as silt-contro] — 
aids, but not.as generally as their effectiveness merits. Such check | 
dams stop considerable silt and are also effective in preventing the © 
channel from being cut into deep gullies (79). They may often be | 
placed to advantage in series in the main channel above reservoirs, 
but are of especial value in short side draws where slopes are steep 
and erosion apt to be increased by stock trails. Brush piled in side 
draws and held down by rocks or anchored by wire or stakes grad- 
ually becomes solidly embedded and stops much silt. Unanchored 
brush is likely to wash out. 
Loose rock walls often do service for years in small drainages, but 
check dams located in main watercourses must be built more str ongly. 
A few logs laid on the upstream side of conveniently spaced trees, 
or otherwise anchored, are very effective checks. In one instance a 
9-foot woven-wire fence reinforced by a loose rock wall resisted 
floods for years, thus preventing the development of a gully and 
excluding from the reservoir many tons of sand and silt. Heavy 
woven-wire fencing (18 or 20 inch) alone will prove effective if sup- 
ported by plenty of deep-set posts on the downstream side. Leaves, 
grass, twigs, and débris gradually catch in the meshes and a dam is 
soon formed. Above a very old earthen reservoir in Arizona, built 
in the early eighties, a picket fence of juniper posts set close together 
forms an effective silt check. Silt washed down the short drainage 
has banked up against the fence to a maximum depth of 3 feet. 
The watershed in this case is ver y small and consequently the fence 
has never been subjected to excessive water pressure. 
Although the construction of such small dams to check channel 
cutting and retard siting has cost limitations, several can be con- 
structed for the cost of maintaining for one day up-to-date * tank 
crews ” like those shown in Plate xe Figure 2, and Plate XVIII, 
Figure 1. 
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