western ▁boundary ▁with ▁T oc ant ins ▁state . ▁The ▁Par na í ba ▁River ▁forms ▁the ▁eastern ▁boundary ▁of ▁Mar anh ão , ▁but ▁it ▁has ▁one ▁large ▁trib ut ary , ▁the ▁B als as , ▁entirely ▁within ▁the ▁state . ▁Other ▁rivers ▁in ▁the ▁state ▁include ▁the ▁T uri ass u ▁( or ▁Tur ia çu ) ▁which ▁runs ▁just ▁east ▁of ▁the ▁G ur up i , ▁empty ing ▁into ▁the ▁Ba ía ▁de ▁T uri ass u ; ▁the ▁M ear im , ▁P ind ar é , ▁and ▁Gra ja ú , ▁which ▁empty ▁into ▁the ▁Ba ía ▁de ▁São ▁Mar cos ; ▁and ▁the ▁It ape cur u ▁and ▁Mun im ▁which ▁dis charge ▁into ▁the ▁Ba ía ▁de ▁São ▁José . ▁Like ▁the ▁Amazon , ▁the ▁M ear im ▁has ▁a ▁por or oca ▁or ▁tid al ▁bore ▁in ▁its ▁lower ▁channel , ▁which ▁greatly ▁inter fer es ▁with ▁navigation . ▁ ▁The ▁western ▁coast line ▁has ▁many ▁small ▁indent ations , ▁which ▁are ▁usually ▁mask ed ▁by ▁islands ▁or ▁sho als . ▁The ▁largest ▁of ▁these ▁are ▁the ▁Ba ía ▁de ▁T uri ass u , ▁facing ▁which ▁is ▁São ▁João ▁Island , ▁and ▁the ▁cont igu ous ▁b ays ▁of ▁São ▁Mar cos ▁and ▁São ▁José , ▁between ▁which ▁is ▁the ▁large ▁island ▁of ▁São ▁Lu ís . ▁This ▁ind ented ▁sh or eline ▁is ▁home ▁to ▁the ▁Mar anh ão ▁mang ro ves , ▁the ▁tall est ▁mang ro ve ▁for ests