Not sure where in NW Iowa you're talking, but here are a couple of the most reliable I'd recommend...
1) Storm Lake. A bit hit or miss, but some dandy cats in the lake.
2) Little Sioux River. Be a bit of a problem right now due to very high water levels, but it has a bunch of nice catfish in it...as well as walleyes and other species.
Good luck :way:
NWBuck