I'm guessing someone didn't want the body found. I have heard stories of dogs that can scent the area where someone's body lies under fairly deep water. It's very possible (IMO) that someone (whoever *that* may be... ahem...) moved the body around until the pressure was off and then dumped her in Rock Creek Park after the searching in that area was over.
The body could have been laying in someones basement, sealed in plastic and doused with lime.
In a year, the body would thouroughly decompose and be fairly easy to move to the park.
The DC police are stupid but the park was the center of their investigation.
I don't believe she was killed there.