IRRIGATION" IN NORTHERN COLORADO. 71 
The earth embankments are of various dimensions. Crests are 
from 8 to 16 feet wide and usually carry a roadway. Outer slopes 
range between 2 to 1 and 4 to 1. Inner slopes are steep or flat, de- 
pending on whether they are well protected against wave action. 
The slopes paved with concrete are usually 1 to 1 or 1-| to 1, while 
slopes with no protection are often as flat as 4 or 5 to 1. The free- 
board maintained on the dams ranges from 1 to 15 feet and is 
usually a compromise between water requirements and safety. When 
possible it is the custom to fill the reservoirs only partly full in the 
early spring to permit them to pass through the period of high 
winds with a safe freeboard. After the danger from high winds is 
past they are topped out and the water is raised to a point on the dam 
which would be decidedly unsafe under a continued high wind. 
The erosive action of waves on earth embankments, illustrated in 
Plate XVI, figure 2, is so destructive that some sort of protection is 
always provided if possible. A few of the smaller dams are protected 
by brush laid on the slope and held by stakes and wire. Many are 
protected by a loose rock riprap laid on the upper part of the slope, 
where wave action is most destructive. On the whole, this protection 
seems to give as much satisfaction as any. Its chief fault is that the 
rock is continually settling and slipping down the slope, making it 
necessary to add more rock until a condition of stability has been 
reached. In the case of the Cache la Poudre Reservoir more or less 
rock has been dumped on the slope every year for 20 or 25 years and 
a condition of strict stability has not yet been attained. A few slopes 
are protected by a rock riprap hand laid on a cushion of gravel. This 
type is also subject to dislodging and settling, and requires con- 
siderable repair work to keep it in good shape. A number of dams 
are protected by concrete pavements about 6 inches in thickness and 
reinforced with wire mesh or iron rods. Some of these are laid on in 
sheets without joints, but the majority are in strips running from 
toe to crest. A few are supported by ribs running up and down the 
slope at intervals. Owing perhaps to the short slopes covered, there 
have been no total failures of this type so far, but local failures are 
common. These failures start with cracks opened by expansion and 
contraction through which the water is able to dig a cavity in the 
slope under the pavement. After these cavities are formed it is 
supposed that the force of the waves either smashes in the unsup- 
ported pavement over the cavity or compresses the air in the cavity 
sufficiently to produce an outward bulge of the pavement and con- 
sequent failure. Plate XVII, figure 1, shows a break in the pavement 
of Terry Lake resulting from a crack. 
As the great majority of the reservoirs in the valley are supplied 
through canals and with few exceptions the drainage area immedi- 
ately above develops only a small amount of water, wasteways are 
