The valleys are located mostly in the Cascade Mountains, the northern Sierra Nevada, and the Modoc Plateau. All the valleys are drained by tributaries or the main stems of the Klamath and the Sacramento Rivers. The primary land uses in these sparsely populated valleys are agriculture and grazing; some land is devoted to timber.
Climate among the valleys varies with altitude and distance from the Pacific Ocean. Generally, the valleys nearest the ocean and at lower altitudes have a moderate climate. Precipitation amounts vary with altitude and local physiography. The average annual precipitation ranges from less than 20 inches in Butte Valley near the Oregon border to as much as 80 inches in some valleys of the Sierra Nevada.
The most common use of water withdrawn from the northern California alluvial-valley aquifers is irrigated agriculture. Surface water provides the largest source of supply, but ground water is a significant percentage of the total water withdrawn, especially in dry years. Municipal and industrial supplies in most of the valleys depend primarily on ground water. Because ground-water withdrawals currently (1995) do not exceed natural recharge in these valleys, additional development is possible.
The complex geologic history of northern California is characterized by extreme tectonic and volcanic activity. The oldest exposed rocks date from the middle part of the Paleozoic Era and are in the Salmon and the Cascade Mountains and the Sierra Nevada. Mesozoic marine rocks also are in these mountains; for the most part, these rocks are metamorphosed sedimentary and volcanic rocks. The lithology of the rocks in these areas provides evidence of an ocean that once extended much farther inland than does the modern Pacific Ocean and a subduction zone that was located as far east as the Sierra Nevada, which predominately consists of granitic intrusive rocks. Mesozoic intrusive rocks also make up part of the Sierra Nevada.
The valleys in the interior of northern California are in structural troughs or depressions that resulted from the folding and faulting of crystalline rock. The troughs are partly filled with permeable sediments, principally of Pleistocene and Holocene age, that were eroded from the mountains and deposited as alluvial fans or lake deposits, or both. In some of the basins in the Cascade Mountains, volcanism produced basalt, pyroclastic flows, and tuffs that were deposited along with the basin-fill sediments. These volcanic and pyroclastic rocks were subsequently covered with alluvial material. The maximum thickness of unconsolidated permeable deposits in the valleys ranges from approximately 300 to 1,700 feet.
Ground water in the valleys is contained mostly in the alluvial-fan and lake deposits that fill the basins. Small to large amounts of ground water are stored in fractures and joints of volcanic rocks. In some basins, however, volcanic rocks store, transmit, and yield large amounts of water. Whether the ground water is under unconfined to confined conditions depends upon depth and the amount of fine-grained materials.
The aquifers are recharged by runoff from the surrounding mountains, seepage from streams, precipitation on the valley floor, irrigation return, or subsurface flow through fractured crystalline rocks. Ground water leaves the valleys by evapotranspiration, as stream discharge in the valleys that are drained by rivers, and, in some cases, by subsurface flow through permeable bedrock.
Well depths vary among the valleys, but most wells are from 50 to 500 feet deep; those completed in volcanic rock, however, can exceed 1,300 feet in depth. Well yields vary widely, and depend on the permeablilty of the water-yielding material in which the well is completed. Yields range from less than 100 gallons per minute in alluvial-fan and lake deposits to as much as 5,000 gallons per minute in intensely fractured volcanic rocks.
Although isolated areas have highly mineralized water, ground water suitable for most uses is widely available in these valleys. Locally, some of the water contains objectionable quantities of sodium or sulfate.