Are there any waterfalls you can swim in Northern Virginia?

I know this may sound like a pretty funny question, but I've always wanted to swim in a waterfall. I haven't been able to find out whether this opportunity exists in my area, but it'd be great if one of you could help me out. Does anyone know of any 'swimmable' waterfalls in or near the Northern Virginia area? Near Fairfax or Washington D.C.?