In my experience, Riverway has only the one client, NRG /Reliant. The agency is also biased towards Hispanics. On my assignment, I found Hispanics were paid $1 more per hour than I was. I also found that several of my Hispanic coworkers had FAILED their pre-employment testing but were given chances to take the tests until they passed. On the other hand, if I'd have failed (English speaking and non-Hispanic) I probably wouldn't have gotten past the first interview.
My assignment was abruptly ended -- and no, I'm not bitter. It was more than a year ago and I've moved on. My review is unbiased.
Like all agencies, you would get one assignment with them. After that, it's as if you do not exist.