Most of the Grass Valley Creek drainage basin is located within the Shasta Balley Batholith. This Jurassic batholith ranges in composition from a quartz diorite to granodiorite and is deeply weathered. Cenozoic alluvial deposits accumulated over much of the area. Remnants of terrace deposits are scattered on hillsides above the present-day creekbed. Quaternary alluvial deposits are deposited within the tributary drainages and creekbed; a variable depths of slopewash cover most of the slopes.
At the damsite, Grass Valley Creek meanders across on alluvium-filled flood plain which is mapped as Qal and varies from 240 to 460 feet wide and consists mostly of loose to firm silty sand and poorly graded sand with silt. On the hillsides, quartz diorite, mapped as Jqd, is usually covered by slopewash, mapped as Qsw. The slopewash consists mostly silty sand. Remnants of terrace deposits, mapped as QTt, are scattered on hillsides above the present-day creek bed and consist of mostly silty sand, poorly graded to silty sand and sandy lean clay. outcrops of quartz diorite and core is differentiate on the basis of weathering which ranges from decomposed to slightly weathered.
The water table slopes toward Grass Valley Creek from the adjacent quartz diorite ridge. Active springs, seeps and a swamp area are confined to the channel area. Water level readings in the abutments indicates water levels rises within the abutments to elevation 2770.0 feet in left abutment drill hole LA-1 and to elevation 2780 in auger hole AP-411 in the right abutment. Water levels in the channel are generally controlled by the water level of Grass Valley Creek.
Permeabilities calculated from drill and auger holes water tests indicate that the alluvium (Qal) ranged from 2152 to 204,688 feet per year, the terrace deposits ranged from 0 to 2796.9 feet per and the quartz diorite ranges except in drill hole DH-114, from 0 to 302 feet per year. In drill hole DH-114, permeabilities up to 1,165.4 were reported in the quartz diorite.