Per reader Allen R., from Spell of Catastrophe, page 55:
A river's tributaries feed into the river not out of the river. By definition they are always upstream. According to Wikipedia http://en.wikipedia.org/wiki/Tributary the term you need is 'distributary'.
The passage in question:
"The river takes a long curve out to the east in this area and then loops back to the west; the city's right at the eastern-most edge of this bulge. About fifteen miles downstream to the south the Oolvaan splits into its three primary tributaries: the Greater, Lesser, and Equivocal Oolvaans."
... and here's the updated text:
About fifteen miles downstream to the south the Oolvaan splits into its three primary distributaries: the Greater, Lesser, and Equivocal Oolvaans."
“Wait – what are you talking about, distributaries? You mean tributaries, right?”
“No, Max,” Roni said patiently, if with an air of some superiority. “The tributaries feed into the river, not out of the river. Distributaries flow away from the main stream.”
"Well, you learn something every day, sometimes,” Max grumbled. “Who makes these things up, the Imperial Archivist? But the Equivocal Oolvaan’s the one that floods, right?"