Due to the final results of DWR’s groundwater basin boundary modifications in 2019, the Santa Maria River Valley – Arroyo Grande (No. 3-12.02) Subbasin (AG Subbasin) and Santa Maria River Valley – Santa Maria (No. 3-12.01) Subbasin (Santa Maria Subbasin) were established as separate subbasins within the previous Santa Maria River Valley Basin (No. 3-12). The Arroyo Grande Subbasin is approximately seven miles long, oriented in a northeast-southwest direction, extending from Lopez Dam to the adjudicated area boundary (approximately coincident with the Wilmar Avenue Fault and Highway 101). The tributary valley of Tar Springs Creek is about three miles long, oriented east-west, and joins Arroyo Grande Creek about three miles upstream of Highway 101. Land surface of Arroyo Grande Creek valley extends from an altitude of about 380 feet Above Mean Sea Level (AMSL) at the base of Lopez Dam to about 100 ft AMSL at the bottom of the valley. Tar Springs Creek Valley extends from an altitude of about 360 ft AMSL to 160 ft AMSL at the confluence with Arroyo Grande Creek. Mountain ridges on the north side of the valley rise steeply to elevations of over 1500 feet AMSL near Lopez Dam.