Elevations in feet.

shows the capacity curve of the reservoir, by means of which the amount of water stored can be ascertained for any depth. The drainage area tributary to this reservoir is 161 square miles. The table on page 54 shows the estimated monthly discharge of Arroyo Seco at the Pettitt ranch, 4 miles below. The discharge here will be practically the same as at the gaging station.

FIG. 14. Capacity curve of Foster reservoir site, showing capacity, in thousands of acre-feet, with the water surface at various levels.

The reservoir site is situated in the terraced valley of Arroyo Seco, and is a long, narrow, steep-sided stream channel, the bottom of which is covered with washes of granitic bowlders and sand. Pl. VI, A, shows the general character of this channel looking upstream from below the dam site.

At the dam site the canyon has a width of 250 feet at the level of the stream and about 900 feet at the 615-foot contour, the top of the

dam. The rock at the dam site is the Monterey shale. The specific gravity of an ordinary specimen of the shale is 2 and the weight per cubic foot is about 125 pounds. These shales are hard and close textured where not weathered. In weathering they do not crumble into dust or sand as do ordinary rocks, but simply break into smaller fragments. At this point the shales are evenly stratified in thin beds which strike N. 75° W. and dip 40° SW. Rock is exposed in the bed of the stream at several places at the dam site, and it is probable that it is but a few feet to bed rock at any point. No borings have been made to bed rock, however. Pl. VIII, A, is a view of the north side of the canyon at the dam site, and Pl. VIII, B, is a view of the south side.

PROPOSED FOSTER DAM.

The dam at this point, as designed, is to be of the rock-fill type. Three different classes of material are available for the construction

of a dam here-(1) the Monterey shale on the south side of canyon; (2) the granitic bowlders and sand in the bed of the stream both above and below the dam site; (3) gravel, sand, and soil on the north side of the canyon. It was not considered safe to build an earthen or sand and gravel dam in the main channel of a torrential stream of this character, so the loose rock-fill type was chosen. It will be necessary to excavate a spillway in any case, and as located the excavation for the spillway will about make the fill for the dam. The relative position of the dam and spillway is shown in fig. 15,

The dam will have a maximum height of 125 feet above the stream at the lower toe of the dam, as shown in fig. 16. The upstream slope is to be 13 horizontal to 1 vertical and the downstream slope 1 horizontal to 1 vertical. The top width is to be 20 feet and the extreme length about 900 feet. The crest of the dam is to be 15 feet above the level of the spillway. This may seem excessive, but so little is known of the floods on this stream that it was not considered wise to make it less. In the future, if observations show that it is advisable, the level of the spillway can be easily raised.

The dam is to be made water-tight by a 13-foot layer of asphaltic concrete laid on the upstream slope of the dam. This concrete is to be covered with 34 feet of hand-laid rubble to protect it from the heat of the sun and weathering. The asphaltic concrete is to be made of sharp, angular broken rock and sharp sand, cemented with refined asphalt, properly tempered. The amount of asphalt should be reduced to the smallest amount that is required to secure a water-tight layer.

FIG. 16.-Maximum cross section of proposed Foster dam.

Concrete of this character, if laid on a 13 horizontal to 1 vertical slope and protected, will certainly be permanent.

A cut-off wall of concrete is to be carried down to bed rock at the upstream toe of the dam. The asphaltic concrete is to join this wall, making the entire upstream slope of the dam water-tight.

The spillway channel is to be excavated in the Monterey shale at the south end of the dam. It is to be 200 feet wide on the bottom and will discharge approximately 29,000 second-feet with the water 12 feet above the level of the crest. It is believed that this spillway will be permanent without a protective pavement. It should be deepened somewhat on the convex side to throw the current away from the south end of the dam.

The outlet from the reservoir is to be through a tower and tunnel near the south end of the dam, as shown in fig. 17. The tower is to be of concrete, is circular in plan, and will contain the valves for controlling the flow of the water from the reservoir. Four intake pipes 3 feet in diameter will be placed in this tower. The valves will be