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The highest contour of the reservoir is at an altitude of 975 feet, as determined by aneroid observations. The drainage area tributary to the reservoir is 22 square miles of steep mountain slopes covered with brush. The altitude of the area ranges from 900 feet at the dam site to almost 6,000 feet on the slopes of Santa Lucia Peak. .
The dam as designed was to be of sandstone rubble, with a maximum height of 175 feet and a length of 400 feet on top. The bed rock is hard sandstone, and the estimated depth below the bed of the stream to solid rock is 15 feet. Sandstone suitable for the construc
tion of the dam is available in the cliffs above, at both ends of the dam. The character of the canyon at the dam site is shown in Pl. VII, B. The irrigable lands are in the Salinas Valley, 10 miles below. The following table shows the estimated storage capacity and cost of storage at this location.
Estimated storage capacity and cost of storage at Leigh reservoir.a
No measurements of the run-off from this drainage basin have been made. An examination of the discharge of Arroyo Seco, of which this stream is a tributary, shows that in 1901–2, a season of about average rainfall, the run-off was 0.78 second-foot per square mile (see table on p. 54). When this factor is used for computing the run-off of Los Vaqueros Creek a total run-off of approximately 10,000 acrefeet is given. As the rainfall at Los Vaqueros Valley is somewhat below the supposed average for this altitude, it is assumed that the run-off will be but 75 per cent of the amount computed as above, or approximately 7,500 acre-feet.
This will fill the reservoir to about the 925-foot contour. It seems probable that the drainage basin will supply this amount in a season of average rainfall, much more in a wet season, and but little in a dry season. It is believed that this will eventually be a valuable adjunct to the irrigation systems taking water from Arroyo Seco.
The dam should be built high enough to impound all the flood waters, the excess over 7,500 acre-feet being held over, if desirable, for the next year. The impounded water can be dropped into the channel of the creek and picked up again in Arroyo Seco at the head works of the canals without serious loss of water, so the only expense will be that of storage.
A cheaper dam, of the rock-fill type, could be constructed here, which would reduce the cost to probably one-third of the above estimate.
The proposed Foster reservoir is on Arroyo Seco, 12 miles southwest of Soledad in an air line and about 17 miles by the wagon road (Pl. II). The dam site is in the NW. sec. 23, T. 19 S., R. 5 E., M. D. M., and the reservoir site extends southwestward about 2 miles. A survey of this location was made by the writer in the summer of 1902. A topographic map of the reservoir is given as fig. 13. The altitude of the stream at the dam site is 490 feet by aneroid observations.
a MS. report of Prof. Charles D. Marx.
The survey was made for a dam which would raise the water 110 feet above the bed of the stream, or to the 600-foot contour. Such a dam will impound 16,710 acre-feet of water. The table below shows the area and capacity of this reservoir at the various contours. Fig. 14
Area and capacity of proposed Foster reservoir, Arroyo Seco.