Depth and width of the rivers in the Greenbelt are covered on page 55 of the first book in the AP. I think that's the only place in the AP where river measurements are explicitly stated, but maybe you can use that as a guide for the rivers in the River Kingdoms, especially if any of the Greenbelt rivers also run through the River Kingdoms. I know that there are more details given for the River Kingdoms in one of the appendices in one of the volumes of this AP. Maybe you can scour those to see if there's any measurements given that you can use.
