At the stop sign turn right onto Route 642. Go 1.3 miles and bear right onto Furnace Road. Follow that 0.8 miles to the stop sign and turn right onto Ox Road. If you are carrying rowers in your car you should be able to drive at least part of the way to the boathouse area to drop them off. Then you will be directed to available parking lots.
Parking lots fill quickly and a satellite lot is set up almost a mile back up the road.
In that case shuttle bus service is provided for a nominal fee to Sandy Run. Always plan to arrive well ahead of the races you want to watch, allowing plenty of time to get from the lots to the park and grandstand area. If you know Georgetown and Washington's West End, then you know how to get to this venue. The race starts north of the Key Bridge and ends at Thompson's Boathouse south of the bridge. In which case park around the Watergate and walk to the river.