Usumacinta (ōˌsōmäsēnˈtä) [key], river, c.600 mi (970 km) long, formed at the Guatemala-Mexico border by the Chixoy and Pasión rivers and flowing NE through Tabasco state, Mexico, to the Bay of Campeche. It is navigable for c.300 mi (480 km) upstream by small boats and is used to move logs and chicle downstream. Near its mouth some of the channels of the Usumacinta merge with the Grijalva River.
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