Interior Region in USA Question?

I need a major body of water for the Interior Region and I already have the Colorado River and the Great Salt Lake. All I need is one more. Also what states are in the Interior Region? Is it AR, CO,ID, MT, NV, NM, UT< and WY? Because my friend did a research and found CA, OK, and other states but those are in the West Region. Please tell me what is correct.