Dodyg wants to represent rivers in OPML. This is good. We already do this in the worldoutline software.
Here's an example of the OPML. And here's the worldoutline rendering.
You can see there are seven attributes on the river node, however only two pertain to it's "riverness" -- type and url. The others are used by the content management system and the worldoutline software.
So for a river node, the type must be river and there must be a url attribute that points to the JSONP for the river.