All the basins are drained by streams that are perennial only in their upper reaches. The lower reaches become dry in summer because of infiltration where they are underlain by permeable deposits. The ground-water flow system in most basins is essentially self-contained, and interbasin transfer of water is minor.
Ground-water movement generally followed surface-water drainage under natural, or predevelopment, conditions (fig. 109). The discontinuous nature of deep water-yielding materials makes it impossible to construct accurate basinwide water-level maps. However, the ground-water flow pattern in deep aquifers is likely to be similar to that of shallow aquifers. Withdrawals alter the direction of ground-water movement locally and can affect significant changes in regional flow patterns if withdrawal rates are relatively large. Present-day (1995) flow patterns, in general, do not differ significantly from those of predevelopment conditions except locally near withdrawal centers. Withdrawal in the past, however, has reversed the freshwater gradient and induced the intrusion of saltwater in the lower parts of the Napa, the Sonoma, and the Petaluma Valleys.