Mayor Adrian Fenty at the 2010 Nation’s Triathlon. Photo by BWKP

This Sunday is the annual Nation’s Triathlon, which means road closures around the Kennedy Center, Tidal Basin, Clara Barton Parkway, and Hains Point. (The full list is here.)

And short of a massive rainstorm that overwhelms the city’s sewers and floods the Potomac River with human waste (hey, it happens often enough), participants in the triathlon will again be swimming in the Potomac River.

Wait, can they do that? Isn’t it illegal? Well, yes, it is. But they’ve gotten permission to do so over the last few years, and recently D.C. moved to codify an annual exemption from the swimming ban for events like the Nation’s Triathlon.

Organizers just have to hope that there isn’t a big storm—enough rain could flood sewage into the river, forcing city officials to cancel the swimming portion of the race. (It happened a few years back, and the triathlon became a duathlon.)