Bulletin 1.pdf - California Department of Water Resources - State of ...

WATER RESOURCES OF CALIFORNIA 623
records assembled are recent data collected from well drillers by the
Geological Survey and cooperating agencies, but some material was
obtained from publications of the Geological Survey, the Oalifornia
Division of Water Resources, and the Oalifornia Division of Mines. Of
the 6,000 well logs assembled by the Geological Survey, about 3,600 were
collected by the survey, 2,000 by the Bureau of Reclamation, and 400
by the Oalifornia Division of Water Resources. About 4,800 of these logs
were located in the field: 3,200 by the Geological Survey, 900 by the
Bureau of Reclamation, and 700 by the Oalifornia Division of Water
Resources in connection with their investigation in the Sutter-Yuba area.
In the drillers' logs, gravel, sand, clay, and volcanic rocks are usually
distinguished, and the more complete logs mention the color, coarseness,
hardness, degree of cementation, and other easily identifiable characteristics
of the sediments. In interpreting these logs, it must be remembered
that different drillers attach different meanings to the various terms used
for identifying sediments. •
Peg Model
A peg model of the Sacramento Valley, based on drillers' logs, was
constructed to help in recognition of hydrologic units and geologic features.
Each well log was represented by a wooden peg a quarter of an
inch in diameter, mounted on a base map of the valley. A vertical scale
of 50 feet to the inch and a horizontal scale of 4,000 feet to the inch were
used on the model. Eight major lithologic types were recognized, and
each was distinguished py a different color on the pegs. Three datum
planes, the land-surface altitude as interpolated from topographic sheets,
sea level, and 1,000 feet below sea level, were marked on each peg. Each
peg in its respective location was set in a hole bored in a wooden table so
that the top of the table represented 1,000 feet below sea level.
At the scale used for the base map (4,000 feet to the inch) it was
neither practical nor efficient to attempt to utilize all the well logs. Therefore,
in places where wells were closely spaced, it was decided to select
the deeper logs and not to make pegs for logs of wells closer to each other
than about 1,500 feet. With this selection, about 3,000 well logs were
utilized in the model. These same logs were used for the estimates of
specific yield and storage capacity described beyond.
To estimate the storage capacity of the valley, it was necessary to
subdivide the near-surface sediments into hydrologic units. The peg model
has been of great value in making this subdivision. In addition, much
was learned from the peg model about such geologic information as continuity
of sand and gravel strata, depth to bedrock, and the extent and
position of volcanic flows and of buried gravels of ancient stream courses.
Selection of Depth Zones
At the request of the California Division of Water Resources, the
storage capacity of the water-bearing deposits of the Sacramento Valley
has been estimated for three depth zones: 20 to 50 feet, 50 to 100 feet, and
100 to 200 feet below the land surface. The only exception to this threezone
treatment was for the area south of Marysville between the floodplain
deposits of the Sacramento River on the west and the channel of
the 'Feather River on the east. The Oalifornia Division of Water Reilources
reports that water of poor quality exists there at relatively shallow depths
and that it is not generally practicable to draw down the water level to