The aquifer is recharged primarily by runoff from the hills that surround the stream valleys and by seepage from the upper reaches of streams. Minor recharge is by lateral movement of water from adjacent rocks and by direct precipitation. Deeply-buried sediments are recharged by precipitation where they crop out and by leakage from shallower water-yielding beds to which they are hydraulically connected, especially where withdrawals from the deep sediments are sufficient to cause a downward hydraulic gradient. Ground-water movement in the surficial deposits is generally toward the coast (fig. 106), where the water mostly discharges into estuarine reaches of the rivers; some water discharges directly into Humboldt Bay or the Pacific Ocean, or is withdrawn by wells. Water in the deeper sediments is discharged by vertical flow to shallower deposits where the hydraulic gradient is upward, or is withdrawn by deep wells.