The river has carved a grandiose cut though the Sierra Del Carmen Mountains to create Boquillas Canyon.   The ribbon of water disappears around a bend as the canyon walls become so steep that even the most brazen trailblazer is forced to give up.  A couple donkeys graze on the Mexican river flat.  Rock wrens busily probe crevices and eroded holes for tasty tidbits.  A Mexican national serenades each newcomer in hopes of a bit of coin into his collection jar on the US side.  There are more trinkets for sale here too.