I'd never heard that before. The creek would provide an escape route in either direction, down to the river or upstream to semi-open country outside the beltway. I didn't know there was a local population in Rock Creek Park itself.
With the Potomac, Rock Creek, the Anacostia, and the Anacostia tributaries, DC has some pretty good wildlife corridors. We do have some interesting critters wandering around from time to time.