iments ▁onto ▁the ▁marine ▁depos its . ▁Com pression al ▁forces ▁along ▁the ▁boundary ▁of ▁the ▁North ▁American ▁and ▁Pacific ▁Pl ates ▁between ▁ 2 - 4 ▁MY A ▁resulted ▁in ▁the ▁u pl ift ▁of ▁the ▁Coast ▁R anges , ▁creating ▁an ▁en closed ▁bas in ▁today ▁known ▁as ▁the ▁Central ▁Valley ▁and ▁resulted ▁in ▁the ▁San ▁Jo aqu in ' s ▁present ▁path ▁to ▁the ▁sea . ▁ ▁Because ▁of ▁its ▁highly ▁per me able ▁nature , ▁the ▁San ▁Jo aqu in ▁River ' s ▁valley ▁is ▁under lain ▁by ▁one ▁of ▁the ▁largest ▁aqu if ers ▁in ▁the ▁Western ▁United ▁States . ▁The ▁aqu ifer ▁underlying ▁the ▁San ▁Jo aqu in ▁River ▁and ▁Tul are ▁Bas in ▁is ▁estimated ▁to ▁hold ▁nearly ▁ ▁of ▁water , ▁of ▁which ▁about ▁half ▁can ▁be ▁p ump ed ▁econom ically ▁or ▁is ▁clean ▁enough ▁for ▁human ▁use . ▁The ▁aqu ifer ▁receives ▁in ▁excess ▁of ▁ ▁of ▁in flow ▁per ▁year , ▁mostly ▁from ▁precip itation ▁and ▁ir rig ation ▁water ▁se ep age . ▁Con cent r ation ▁of ▁ch lor ide ▁and ▁other ▁min er als ▁generally ▁increases ▁from ▁east ▁to ▁west ▁across ▁the ▁bas in . ▁ ▁History ▁ ▁Ind igen ous ▁people ▁Arch ae ological ▁finds ▁near ▁the ▁southern ▁end ▁of ▁the ▁San ▁Jo aqu in ▁Valley ▁suggested ▁that ▁humans ▁first ▁arrived ▁in ▁the ▁region ▁as ▁early ▁as ▁ 1 2 , 0 0 0 ▁but ▁no ▁later ▁than ▁ 5 , 0 0 0 ▁years ▁ago . ▁The