dam ▁curr ents ▁on ▁the ▁Columbia ▁efficiently ▁carried ▁f ry ▁to ▁the ▁ocean , ▁but ▁the ▁introduction ▁of ▁d ams ▁and ▁res er voir s ▁changed ▁the ▁flow ▁of ▁the ▁river , ▁forcing ▁the ▁young ▁fish ▁to ▁ex ert ▁much ▁more ▁energy ▁to ▁sw im ▁through ▁s lack ▁waters . ▁In ▁addition , ▁many ▁fish ▁are ▁killed ▁by ▁the ▁dam ▁tur b ines ▁as ▁they ▁try ▁to ▁sw im ▁further ▁down stream . ▁It ▁is ▁unclear ▁exactly ▁how ▁many ▁fish ▁are ▁killed ▁in ▁the ▁tur b ines , ▁but ▁old ▁estimates ▁range ▁between ▁ 8 - 1 2 % ▁per ▁dam . ▁If ▁a ▁fish ▁h atch es ▁high ▁up stream ▁they ▁will ▁have ▁to ▁sw im ▁through ▁multiple ▁d ams , ▁leading ▁to ▁possible ▁cum ulative ▁losses ▁of ▁over ▁ 5 0 - 8 0 % ▁of ▁the ▁migr ating ▁f ry . ▁E ff ort s ▁to ▁make ▁tur b ines ▁sa fer ▁for ▁fish ▁to ▁pass ▁through ▁have ▁significantly ▁reduced ▁fish ▁los es ▁to ▁near ▁pre - dam ▁levels . ▁While ▁h atch eries ▁appear ▁to ▁be ▁quite ▁successful ▁for ▁some ▁species ▁of ▁fish , ▁their ▁efforts ▁to ▁increase ▁fish ▁populations ▁will ▁not ▁be ▁effective ▁until ▁up ▁and ▁down stream ▁migration ▁is ▁improved . ▁There ▁is ▁no ▁one ▁solution ▁to ▁impro ving ▁the ▁sal mon ▁and ▁tr out ▁populations ▁on ▁the ▁Columbia ▁as ▁it ▁is ▁the ▁cum ulative ▁effects ▁of ▁the ▁d ams , ▁s lack - water ▁res er voir s , ▁loss ▁of ▁habitat , ▁poll ution ,