ridge has an altitude ranging from 3,300 to 3,800 feet, and runs in a general northwest-southeast direction for many miles, being one of the main ridges of the Mount Diablo Range. These elevated ridges of this portion of the Mount Diablo Range are generally without forest covering, and often without even brush, bearing only scattered pine and oak trees. (See Pl. X, B.) Many large areas are covered only with a growth of grass or wild oats. In some of the valleys, as Priest Valley, there are large scattered oak trees. These ridges and elevated valleys, which embrace an area of 150 square miles, are in the region of greatest precipitation in the Mount Diablo Range, and the greater part of the run-off from San Lorenzo Creek in ordinary years is from this portion of the drainage basin, the mesa to the west yielding little or no run-off.

All of the streams that rise along the southwest slope of the Mount Diablo Range in Monterey County have eroded deep channels into the soft, sedimentary rocks of the mesa, the erosion being favored by the folded and broken condition of the strata in a narrow zone along the southwest slope of the Mount Diablo Range. The tendency of these streams-Peachtree, Gaviota, and Chalone creeks-to follow this folded zone is apparent on inspection of the drainage lines shown on Pl. I. The contour of the country is such that these streams receive but little drainage from the mesa which slopes southwestward to Salinas River. Peachtree Creek has recently been robbed of a portion of its drainage basin by Gaviota Creek, formerly one of the small mesa streams. This change of drainage was accomplished by the progressive erosion upstream of the canyon of Gaviota Creek until it finally cut through the dividing ridge and diverted the water from 40 square miles of the upper drainage area of Peachtree Creek. The comparative recency of this change is shown by the fact that Gaviota Creek is now rapidly cutting a deep gorge through the upper part of its course in the mesa, having already sunk its bed several hundred feet below the former bed of Peachtree Creek, and by the additional fact that the valley of Peachtree Creek below the diversion is fast filling with débris brought down by small lateral streams.

Below Lonoak San Lorenzo Creek has eroded a valley 500 to 800 feet deep, and has reached granite bed rock below the soft sedimentary rocks of the mesa (see Pl. XI, A).

From the scant information at hand it seems probable that the upper portion of this drainage area in the Mount Diablo Range receives an average annual precipitation of at least 20 inches, with a probable maximum for wet years of 40 inches, and a minimum of 10 inches for very dry years (see table on p. 43). During December, 1900, and from January 1 to April 30, 1901, and during 1902, a record was kept of the discharge of this stream at the Mathews ranch. From these observations the table below of discharge of San Lorenzo Creek has been prepared. The diagram shown in fig. 23 is a graphic illus tration of the discharge of this stream during the season of 1901–2.

FIG. 23-Discharge of San Lorenzo Creek at Mathews ranch from September 1, 1901, to August

30, 1902.

Estimated monthly discharge of San Lorenzo Creek at Mathews dam site, 5 miles east of Kings City.

NOTE.-Gage heights and discharge measurement for 1901 are given in WaterSupply Paper No. 66, page 155; rating table on page 178, same paper.

During the summer of 1900 the lower part of this drainage basin was explored by Prof. Charles D. Marx. The Mathews site was discovered and a reconnaissance survey was made. During the summer of 1901 Mr. II. E. Green resurveyed the reservoir and dam site, designed the dam, and made estimates of the cost.

MATHEWS RESERVOIR.

The Mathews reservoir site is in the canyon of San Lorenzo Creek, about 6 miles northeast of Kings City. The dam site is in the NW. sec. 24, T. 19 S., R. 8 E., M. D. M., and the reservoir site extends eastward about 2 miles. The topographic map of the reservoir is shown in fig. 24. The altitude of the stream at the dam site is 440 feet. The survey was made for a dam that would raise the water 100 feet, to the 540-foot contour. Such a dam will impound 16,131 acrefeet of water.

Area and capacity of proposed Mathews reservoir on San Lorenzo Creek.

The accompanying table shows the area and capacity of this reservoir at the various contours. Fig. 25 shows the capacity curve of the