low . ▁ ▁Human ▁activities ▁have ▁replaced ▁or ▁alter ed ▁over ▁ 9 5   percent ▁of ▁the ▁historic ▁wet lands ▁as ▁well ▁as ▁the ▁California ▁o ak ▁wood land ▁habitat , ▁which ▁originally ▁occurred ▁along ▁stream ▁and ▁river ▁cor rid ors ▁in ▁the ▁fo oth ills , ▁and ▁the ▁t ule ▁grass ▁that ▁once ▁th riv ed ▁in ▁huge ▁stands ▁on ▁the ▁edges ▁of ▁mar sh es ▁and ▁la kes . ▁Some ▁of ▁the ▁rich est ▁remaining ▁mar sh ▁habit ats ▁are ▁in ▁the ▁Sac r amento - San ▁Jo aqu in ▁D elta , ▁which ▁despite ▁significant ▁agricult ural ▁and ▁infrastr ucture ▁development , ▁has ▁retained ▁many ▁of ▁its ▁original ▁sw amps ▁and ▁back w aters . ▁Before ▁the ▁ 1 9 th ▁century , ▁the ▁delta ▁was ▁a ▁region ▁of ▁numerous ▁islands ▁of ▁nut ri ent - rich ▁pe at , ▁all uv ial ▁depos its , ▁w inding ▁channels ▁and ▁water ways . ▁Since ▁then , ▁most ▁of ▁the ▁D elta ▁islands ▁have ▁been ▁cultiv ated , ▁and ▁consistent ▁use ▁of ▁the ▁land ▁has ▁resulted ▁in ▁subs idence , ▁in ▁some ▁cases ▁up ▁to ▁. ▁Water ▁divers ions ▁from ▁the ▁rivers ▁feed ing ▁it ▁have ▁increased ▁sal inity , ▁which ▁has ▁in ▁turn ▁caused ▁decl ines ▁in ▁fish ▁populations ▁that ▁once ▁th riv ed ▁in ▁the ▁region . ▁ ▁As ▁defined ▁by ▁the ▁World ▁Wild life ▁Fund , ▁the ▁San ▁Jo aqu in ▁River ▁waters hed ▁is ▁part ▁of ▁the ▁Sac r amento - San ▁Jo aqu in ▁fresh