BUTTE BASIN RECHARGE, NORTHERN SACRAMENTO VALLEY, CA

With a surface area of about 2600 square kilometers, the Butte Basin is one of the largest aquifers in the Northern Sacramento Valley. Not surprisingly, recharge to the Butte Basin is poorly understood. Conventional wisdom assumes that recharge occurs in the foothills, where the main water-bearing unit, the Tuscan Formation, crops out. However, observations of foothill hydrology made during the winter and early spring indicate that incoming precipitation infiltrates the thin soil cover and then runs off as shallow subsurface flow and into adjoining creeks.

Based on these observations, we hypothesize that recharge to the Butte Basin occurs primarily where perennial creeks draining the foothills cross more permeable strata within the Tuscan Formation. Permeable strata include Tuscan B unit interbeds and the Chico Monocline, a series of en echelon faults and fractures that offset the foothills from the valley proper. According to our conceptual model, during periods of high flow water moves from the creeks into the groundwater, thus recharging the aquifer, across these permeable strata. Conversely, during low flow periods creeks gain water from groundwater in these locations.

We tested our model on Deer Creek, a small tributary to the Sacramento River that crosses both the Tuscan B unit and the Chico Monocline in a single location. Based on the results of a single low-water gauging day, our hypothesis seems to be holding; discharge at the gauging location downstream of more permeable strata is 8.8 % greater than the upstream discharge. Depending on the difference in water-surface elevation between the creek and the adjacent water table, we estimate that volumes on the order of 2.5 million cubic meters may recharge the Butte Basin from Deer Creek annually.