▁in ▁river ▁flows ▁led ▁to ▁the ▁run ▁dro pping ▁to ▁about ▁ 3 , 0 0 0 - 7 , 0 0 0 . ▁Some ▁sources ▁put ▁the ▁historic ▁populations ▁as ▁high ▁as ▁three ▁hundred ▁thousand , ▁but ▁this ▁is ▁highly ▁unlikely ▁because ▁of ▁the ▁limited ▁habitat ▁available ▁in ▁the ▁waters hed . ▁ ▁Const ruction ▁of ▁d ams ▁in ▁the ▁San ▁Jo aqu in ▁waters hed ▁has ▁led ▁to ▁nearly ▁all ▁of ▁the ▁good ▁sp aw ning ▁streams , ▁located ▁higher ▁in ▁the ▁mountains ▁near ▁the ▁head w aters ▁of ▁the ▁San ▁Jo aqu in ▁and ▁its ▁trib ut aries , ▁being ▁cut ▁off ▁to ▁the ▁sal mon . ▁The ▁remaining ▁areas ▁left ▁for ▁the ▁sal mon ▁to ▁spawn ▁in ▁are ▁und es irable . ▁Prior ▁to ▁the ▁building ▁of ▁the ▁Fri ant ▁Dam ▁in ▁ 1 9 4 4 , ▁the ▁San ▁Jo aqu in ▁River ▁was ▁believed ▁to ▁have ▁over ▁ 6 , 0 0 0 ▁miles ▁of ▁stre amb ed ▁suitable ▁and ▁available ▁to ▁sp aw ning ▁sal mon . ▁However , ▁in ▁ 1 9 9 3 ▁it ▁was ▁estimated ▁that ▁there ▁was ▁just ▁ 3 0 0 ▁miles . ▁ ▁This ▁significant ▁loss ▁of ▁suitable ▁sp aw ning ▁habitat ▁is ▁due ▁to ▁lack ▁of ▁sp aw ning - s ized ▁gra vel ▁rec ruit ment ▁from ▁later al ▁and ▁up stream ▁sources ▁due ▁to ▁the ▁installation ▁of ▁man made ▁bar riers ▁that ▁block ▁this ▁gra vel ▁from ▁passing ▁naturally ▁down stream . ▁Without ▁rec ruit ment ,