[45] The Silt Problem and Storage Reservoirs. 13 
the water is drawn off very rapidly, so that the current does the work- 
This cleaning is done abont once every four years, and costs only about 
$50.00. The deposit is frequently thirty to fifty feet deep at the upper 
end of the reservoir. 
The Villar reservoir impounds a portion of the flow in the Lozoya 
river and furnishes additional supply to the Isabella IT Canal, from 
which it is drawn for irrigation, and for domestic use in the city of 
Madrid. The length of this reservoir is about the same as the Alicante, 
but the gorge above the dam is much wider, the capacity of the reser- 
voir being about 14,000 acre feet. It is not subject to the same rate of 
silting up as is the Alicante reservoir, however. The dam has two sluic- 
ing galleries, with inlets of twenty square feet each, and are set one hun- 
dred and sixty- two feet below the overflow line. The sluicing outlets 
are opened and closed by gates operated by hydraulic power, but the 
opening is small in comparison with the cross section of the lake above 
and the side slopes of the latter are flatter than the Alicante reservoir, 
so that opening the gates only removes the silt from the natural chan- 
nel of the old stream and high banks are left on each side. T>v means 
of ditches built on each side, and a little above the flow line of the res- 
ervoir, water is conducted from a point higher up the stream ; hv cut- 
ting the hanks of this ditch and trailing the water across the banks of 
silt they are able to wash it down into the old channel, and then out. 
This moves the silt quite rapidly, hut requires a large amount of water, 
which, on account of the smallness of the feeding stream, cannot always 
be well spared. 
At the Lozoya reservoir a large flat boat is used to which is attached 
a large disc wheel, which revolves horizontally, and which can he raised 
or lowered. To the face of this wheel narrow wings or teeth are 
attached and these heat the sediment into fine mud that can he moved 
with a small current. The steam engine that propels the boat also 
drives this wheel. The apparatus does good work whenever the amount 
of water flowing through the reservoir is considerable; it wastes no 
water, and can he used whenever it is thought desirable to do so. 
Col. Nettleton states that he has seen attempts in Old Mexico to 
sluice out reservoirs that had been three hundred years filling with silt, 
the dams being cut from top to bottom. Notwithstanding wide open- 
ings were made the results accomplished were insignificant, only a. sin- 
uous channel being cut through the sediment as wide as they could 
afford to carry the material to the stream that was winding through 
thirty or forty feet of hard packed silt. Perhaps not more than 5 per 
cent, of the material was removed in this way. 
Col. Nettleton adds that “the Spanish people have learned to clean 
their reservoirs often, rather than let them go as long as these people 
in Mexico did.” 
