Before development began, the aquifer system was under steady-state conditions in which natural recharge balanced natural discharge. Ground water in the shallow part of the aquifer system flowed from areas of high altitude at the valley margins, where most of the recharge took place, downgradient to discharge into rivers and marshes near the valley axis (fig. 79). The aquifer system was recharged primarily by streams emanating from the Coast and Cascade Ranges and the Sierra Nevada. Most of the recharge was in the northern and eastern parts of the valley. Precipitation falling on the valley floor during the rainy season provided only a small part of the total recharge. Ground water that was not evaporated or transpired by plants discharged either into the Sacramento and the San Joaquin Rivers that drained to San Francisco Bay or into the Tulare Basin from which it was eventually removed by evaporation or transpiration. The areas of recharge and discharge in the Central Valley before development are shown in figure 80.
Under predevelopment conditions, the hydraulic head in the shallow water-table aquifer where water entered the aquifer system at the valley margins was greater than the head in the deeper confined aquifer; thus, ground water moved downward (fig. 81). Conversely, the head gradient was reversed where water left the aquifer, typically by discharge to surface-water bodies, and the hydraulic head in the water-table aquifer was less than that in the confined aquifer. The difference in hydraulic head created upward movement of the ground water toward rivers and marshes (fig. 81). Precipitation that fell on the valley floor and was not lost to evapotranspiration recharged the water-table aquifer and moved down the head gradient toward the rivers and surrounding marshes.
Upward vertical flow to discharge areas from the deep confined aquifer was impeded by confining clay beds, which caused a pressure head in the deep parts of the aquifer system. Because of the pressure head, wells that penetrated the deep aquifer in low-lying areas near the rivers and marshes flowed during the early years of ground-water development in the valley.
