The extent of the ecoregion is defined largely by the watershed of the Río Papaloapan and its tributaries that drain into the Gulf of Mexico. Major tributaries include Río Santo Domingo, Río San Juan, and Río Trinidad. There are also tributaries of the Laguna de Alvarado, including Río Blanco, Río Otopa, and some basins like Laguna Mandinga, Laguna de Catemaco, and Presa Miguel Alemán.
The Stikine River originates in the Cassiar and Stikine Mountains of northwestern British Columbia and drains a 51,000-square-kilometre basin before crossing the border with the United States on the Alaska Panhandle and discharging through several channels into the Pacific. The upper portion of the river basin is a semi-arid plateau of up to 1,900 metres in elevation,