Oh, and the river is made of oil.
"Titan is the only other world we know of that has stable liquid on its surface," Jia-Rui Cook of NASA's Jet Propulsion Lab writes. "While Earth's hydrologic cycle relies on water, Titan's equivalent cycle involves hydrocarbons such as ethane and methane."
The river stretches more than 200 miles from its "headwaters" to a large sea, which is also likely made of raw hydrocarbons.
Cassini's visual and infrared mapping spectrometer confirmed liquid ethane at a lake in Titan's southern hemisphere known as Ontario Lacus in 2008, Cook says.