Some rivers are small enough that they can be added with just a line. But how are wider rivers added? They are polygons that can stretch hundreds of miles. How do you stop and start editing or does the entire river somehow have to be added at once?

The answer is to use the tag waterway:riverbank and build the river up from polygons that are next to each other. This diagram shows:

Ways in a river

Seems simple enough. But what about bridges. The bridge:yes page explained. Just cut up the road and mark the section on the bridge with bridge:yes. The layer option was a bit ambiguous so to be sure I added layer:1 for the bridge.

So I tried added a section of the Pantano Wash in Tucson. After waiting 15 minutes or so the result was added to the map:

A Wash in OSM

Looking good. Now to add some more.