Scientists deduce that the river, which is in Titan's north polar region, is filled with liquid hydrocarbons because it appears dark along its entire length in the high-resolution radar image, indicating a smooth surface.
"Though there are some short, local meanders, the relative straightness of the river valley suggests it follows the trace of at least one fault, similar to other large rivers running into the southern margin of this same Titan sea," said Jani Radebaugh .... http://www.jpl.nasa.gov
