i have been really happy with gaia gps
there's both a free and paid version. i use it (paid version) year-round on an old iphone 5s dedicated for navigation and photos.
you'll need to download maps of the area you'll be navigating in advance, but after that's done you can switch to airplane mode and use the app.
you can import GPX files. rivermaps provides them (free) for some of the major western rivers here
. they're a nice addition to a hard-copy rivermap.
overall i'd recommend it, although i found there was a bit of a learning curve.
good luck with your search!