The Humboldt River is a body of water in Liberty City State which separates Algonquin island from Broker, Dukes and Bohan with Charge Island and Colony Island located on the river. It is named after the explorer Horatio Humboldt.
The river is implied by the documentary "A History of Liberty City" to be heavily polluted before even the establishment of the city itself, due to the natives using it effectively as a sewer.
It's real life counterpart is the East river.
In Grand Theft Auto: Chinatown Wars, the protagonist, Huang Lee, is left for dead at the bottom of the river.