From Exalted - Unofficial Wiki
The widest river in the Scavenger Lands, the Yanaze is formed by the confluence of the Yellow River and Gray River at Nexus. It flows westward, emptying into the Inland Sea at Lookshy. The River of Tears joins it from the north some 180 miles east of the sea.
Based on the map of Creation found in the main rulebook, the Yanaze is between 40 and 60 miles in width for most of its length. This is very wide for a river, but on the other hand, this is Exalted! It's forty miles wide at the mouth, 130 miles wide at Nexus, and some 628 miles long (give or take a few miles).