They are different critters. Hellbenders, as far as I know, are only found in streams.
Also similar to hellbenders are mudpuppies (Necturus spp.); mudpuppies, however, have external gills, which are red and plumelike, behind the head on both sides. They have 4 toes on each foot. As adults, mudpuppies reach only 8–13 inches in length. Hellbenders lose their external gills once they reach 4 or 5 inches long, they have 4 toes on the forelimbs and 5 toes on the hind limbs, and their total length may reach 24 inches.