▁subject ▁to ▁season al ▁variations ▁in ▁flow . ▁ ▁The ▁Tag us ▁is ▁the ▁longest ▁river ▁on ▁the ▁pen ins ula ▁and , ▁like ▁the ▁Dou ro , ▁flows ▁west wards ▁with ▁its ▁lower ▁course ▁in ▁Portugal . ▁The ▁Gu ad iana ▁river ▁b ends ▁south wards ▁and ▁forms ▁the ▁border ▁between ▁Spain ▁and ▁Portugal ▁in ▁the ▁last ▁stretch ▁of ▁its ▁course . ▁ ▁Mountains ▁The ▁terrain ▁of ▁the ▁I ber ian ▁Pen ins ula ▁is ▁largely ▁mountain ous . ▁The ▁major ▁mountain ▁systems ▁are : ▁ ▁The ▁Py ren ees ▁and ▁their ▁fo oth ills , ▁the ▁Pre - Py ren ees , ▁crossing ▁the ▁is th mus ▁of ▁the ▁pen ins ula ▁so ▁completely ▁as ▁to ▁allow ▁no ▁passage ▁except ▁by ▁mountain ▁road , ▁trail , ▁coast al ▁road ▁or ▁tunnel . ▁An eto ▁in ▁the ▁Mal ad eta ▁mass if , ▁at ▁ 3 , 4 0 4 ▁m , ▁is ▁the ▁highest ▁point ▁▁ ▁The ▁Cant ab rian ▁Mountains ▁along ▁the ▁northern ▁coast ▁with ▁the ▁massive ▁P icos ▁de ▁Europa . ▁Torre ▁de ▁Cer red o , ▁at ▁ 2 , 6 4 8 ▁m , ▁is ▁the ▁highest ▁point ▁ ▁The ▁Gal icia / Tr ás - os - Mont es ▁Mass if ▁in ▁the ▁North west ▁is ▁made ▁up ▁of ▁very ▁old ▁heavily ▁er oded ▁rocks . ▁P ena ▁Tre vin ca , ▁at ▁ 2 , 1 2 7 ▁m , ▁is ▁the ▁highest ▁point ▁ ▁The ▁S istema ▁Ib ér ico , ▁a ▁complex ▁system