The RÃÂo San Pedro is a river of Guatemala and Mexico. Its sources are located in the Guatemalan department of El Petén at . The length of the river in Guatemala is 186 km (116 mi). The river flows westwards until it reaches the Mexican border and crosses into the state of Tabasco at . From there it continues northwest and joins the Usumacinta River. The Guatemalan part of the San Pedro river basin covers an area of . The river has red mangroves.