atra ▁province , ▁it ▁r ises ▁in ▁Bar isan ▁Mountains ▁( B uk it ▁Bar isan ) ▁and ▁me and ers ▁slowly ▁east wards ▁to ▁join ▁Mus i ▁River ▁at ▁K ert ap ati , ▁Pale mb ang . ▁The ▁O gan ▁ranks ▁as ▁the ▁third ▁longest ▁river ▁in ▁South ▁Sum atra ▁( be hind ▁Mus i ▁and ▁Kom ering ). ▁The ▁river ▁either ▁borders ▁or ▁cut s ▁through ▁the ▁reg ency ▁of ▁South ▁OK U , ▁OK U , ▁OK I , ▁and ▁O gan ▁Il ir . ▁ ▁O gan ▁and ▁Peg agan ▁tribe ▁lived ▁along ▁the ▁O gan ▁and ▁its ▁trib ut aries . ▁Most ▁were ▁pe asant ▁or ▁merchant . ▁The ▁river ▁served ▁first ▁as ▁vital ▁transport ation ▁ar tery ▁and ▁communic ations ▁link . ▁Form ed ▁from ▁thick ▁layers ▁of ▁this ▁river ' s ▁s ilt ▁depos its , ▁the ▁O gan ▁River ▁Valley ▁is ▁one ▁of ▁the ▁most ▁fert ile ▁agricult ural ▁regions ▁of ▁the ▁province . ▁ ▁Geography ▁The ▁river ▁flows ▁in ▁the ▁southern ▁area ▁of ▁Sum atra ▁with ▁pre domin antly ▁tropical ▁rain for est ▁climate ▁( design ated ▁as ▁Af ▁in ▁the ▁Kö ppen - Ge iger ▁climate ▁classification ). ▁The ▁annual ▁average ▁temperature ▁in ▁the ▁area ▁is ▁ 2 4   ° C . ▁The ▁warm est ▁month ▁is ▁October , ▁when ▁the ▁average ▁temperature ▁is ▁around ▁ 2 6   ° C , ▁and ▁the ▁col dest ▁is ▁January , ▁at ▁ 2 2   ° C . ▁The ▁average ▁annual ▁ra