Some Notes on River Placement

https://io9.gizmodo.com/a-map-of-all-the-rivers-in-the-united-states-and-nothi-513353739

Follow the link.  Rivers go literally everywhere except the very deepest, driest deserts.

As a side note, most of the waterways shown on this map would not generally show up in a campaign map.  It's important to keep in mind, though, as you drill down into smaller area maps, that there should be more rivers, streams, etc added to feed into the larger ones already on the map.