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In the southeast corner of Monterey County, at Parkfield, the rainfall is 11.29 inches. Over the Carriso Plain, probably the most arid region in the Coast Range, the rainfall is light, being but 8.01 inches at Simmler and about 5 inches at Goodwin.
The average monthly precipitation at six stations in the Salinas Valley is shown in fig. 3. The height of each of the small black columns represents the average amount of rain for the corresponding month. This diagram shows graphically the contrast between the distribution of the rainfall in summer and winter.
To summarize, it may be said that the distribution of rainfall in Monterey County depends upon the topography. The region may be divided into three topographic zones or belts in which rainfall conditions are similar. The most western belt is the Santa Lucia Range, the central is the Salinas Valley, with the mesa lands on the northeast side, and the eastern is the Mount Diablo Range.
In the Santa Lucia Range the rainfall varies from about 18 inches at sea level to 50 inches along the summit of the range, decreasing on the northeastern slope to about 18 inches at an altitude of 1,000 feet. In the upper Salinas Valley the rainfall is about 10 inches, decreasing gradually toward the southeast. From the incomplete records in the Mount Diablo Range it seems probable that the average rainfall is from 20 to 25 inches in Monterey County.
The main trunk stream of the Salinas Valley is Salinas River, which, with its tributaries, drains an area of 4,780 square miles. (See Pl. I.) The drainage basin is 150 miles long, extending from southern San Luis Obispo County to the Bay of Monterey, in Monterey County. The western tributaries of the Salinas have their sources in the elevated, well-watered Santa Lucia Range, while its eastern tributaries drain the low mesa lands and the western slopes of the Mount Diablo Range. The river drains all the area lying between the Santa Lucia Range on the southwest and the Gabilan and Mount Diablo ranges on the northeast except the inclosed basin east of San Juan Creek known as the Carriso Plain.
The following table shows the areas of the drainage basins of the river and its tributaries:
Areas of drainage basins of Salinas River and its tributaries.
Sq. miles. Total drainage of the Salinas River....
4,780 Drainage of Salinas River above gaging station near Salinas ----
4,084 Drainage of Salinas River above Estrella Creek Total drainage of Nacimiento River ---
380 Drainage of Nacimiento River above gaging station near Bryson
171 Total drainage of San Antonio River
342 Drainage of San Antonio River above Pinkerton dam site ........
322 Drainage of San Antonio River above gaging station near Jolon.. Total drainage area of Arroyo Seco above Arroyo Seco canal No. 3.
218 Drainage area of Arroyo Seco above gaging station at Pettitt ranch
215 Drainage area of Arroyo Seco above Currier dam site
184 Drainage area of Arroyo Seco above Foster dam site .. Total drainage area of Los Vaqueros Creek above Pettitt dam site.. Drainage area of Los Vaqueros Creek above Leigh dam site.
22 Drainage area of Estrella Creek ----Drainage area of Carriso Plain ....
170 Total drainage area of San Lorenzo Creek above Mathews dam site.. 235 Drainage area of Chalone Creek ...
128 These areas were determined by planimeter measurements from the Monterey and San Luis Obispo atlas sheets.
From San Miguel, in San Luis Obispo County, northwest to Wunpost, in Monterey County, the Salinas flows in a deep, wide canyon, which it has eroded several hundred feet below the level of the surrounding mesas. At Wunpost the mesa recedes on the northeast side and the river valley widens out somewhat into a long, narrow, terraced alluvial plain, with a width ranging from 1 mile at the upper end to 4 or 5 miles at Soledad. From Soledad to the Bay of Monterey the alluvial plain and bordering detritus slopes are from 6 to 30 miles wide, with terraces far less pronounced than those in the upper portions of the valley. Each of the terraces is an ancient flood plain of the stream. For this distance the stream flows in a broad, sandy channel from 500 to 1,500 feet wide, into which the waters sink in summer to reappear again at some point below. From Soledad to the ocean the stream is perennial, being fed by these underground waters. When wet the fine sand in the river bed becomes quicksand, making the stream dangerous to ford, especially in winter. The photographic views reproduced in Pl. III, and Pl. IV, A, show the general character of the stream during the dry season.
The large volume of water discharged by this stream during the rainy season has enabled it to trench its channel to a depth considerably below the alluvial plains on both sides of the river. For the same reason the grade of the stream is very slight, being but 1.33 feet per mile from the ocean to the gaging station at the Monterey road bridge; 5.5 feet per mile from the Monterey road bridge to Soledad and 6.5 feet per mile from Soledad to Bradley, where the bed of the stream is at an altitude of about 500 feet. These grades are approxi
mate and are given merely to show the low gradient of this, the main drainage line of the region.
This stream is torrential in character, having very large flood discharge during the rainy season and being practically dry during the summer except in its lower portion. The winter flow is probably at all times sufficient for the needs of winter and spring irrigation.
That portion of the drainage basin of the Salinas that lies south of San Miguel, comprising 591 square miles, is on the northeast slope of the Santa Lucia Range, in the region of heaviest rainfall, and the run-off here is undoubtedly large.
A record of the discharge of the main stream was kept at the Monterey road bridge from January 8, 1900, to July 31, 1901. From these observations the following table has been prepared.
Estimated monthly discharge of Salinas River 4 miles south of Salinas.
NOTE.-Gage heights and discharge measurements for 1901 are given in WaterSupply Paper No. 66, page 154.