The north San Francisco Bay area valleys contain a mixture of urban and agricultural lands. Population growth since the 1970's has been rapid and urban areas have replaced much of what was formerly agricultural land. Nonetheless, agriculture remains important to the local economy. Orchard crops are a significant part of the total agricultural output but are being replaced in many areas by vineyards. Many formerly unplanted hillsides now grow wine grapes.
Ground-water supplies in the area are limited by local availability and, to a degree, by the quality of the water. Most of the water used in the area is surface water, much of which is either derived from the Russian River or imported from the Central Valley. Ground water, however, remains the primary source of supply for agriculture, stock watering, and domestic uses and is an important source for municipal supply.
The five drainage basins of the north San Francisco Bay area valleys are structural troughs filled to great depths with marine and continental sediments and volcanic deposits. The basins each have a flat to gently rolling valley floor formed primarily on alluvial fan deposits. The slope of the fan steepens near the foothills at the base of the surrounding mountains. Some streams that drain mountain valleys are perennial only in their upper reaches because the water table falls below the level of the streambed during the dry season. The lower stream reaches are seasonally intermittent.
The North San Francisco Bay area has a Mediterranean-type climate, characterized by moderate temperatures and markedly seasonal precipitation that falls primarily during late autumn to early spring. Precipitation amounts are dependent on altitude, with average annual amounts that range from less than 20 inches in some valley locations to more than 60 inches in the higher elevations of the Coast Ranges.