Hello Bryan,
I was reading the other day and it seems that Autodesk has developed a new extension for AutoCAD Civil 3D, called “River & Flood Analysis”, and it promises a lot of new capabilities to help the use of Civil 3D alongside with HEC-RAS. I never tested it before and my advice would be the same as Chris’s. Either way, this River analysis extension is only included in a specific autodesk package for the Civil 3D, the “Infrastructure Design Suite 2015”. I hope that helped!
Cheers,
Mateus