The Triadelphia Reservoir is located in Laurel, MD and just outside of the 495 beltway. There are several put-in spots with easy car access and boat ramps. The reservoir is split in two parts by the Brighton Dam, where you can purchase your $3 day permit as early as 7 am in the morning, weekends included.
There are already a couple of reports, so I wont repeat. The important thing is, that the reservoir is again open for paddling and, besides kayaks, sail boats and the like, only boats with electric motors are permitted so it is quiet.
Going north-west from Brighton Dam (the put-in is a bit off the dam in this direction) you have about 4-5 mile one way till the end of the reservoir. There are many shallow areas along both shores where you can practice wet exits, rolling etc. in shallow water with ample space where the bottom is flat. The water seems clean (should be it is a source for drinking), so try to keep it this way. You can also paddle the area below the dam (I have not yet done this).
A good place to go flat-water paddling when the Potomac is flooded!
Get all the info you need at the WSSC web site: http://www.wsscwater.com/info/wrup.cfm
Boat ramps, parking, recreational areas with picnic tables, playgrounds.
$3 day, $30 year. PFD required on the craft, not necessarily to be worn (check regulations for updates)
Dam is located at 2 Brighton Dam Road, Brookeville, Maryland 20833; and T. Howard Duckett Dam is located at 7400 Brooklyn Bridge Road, Laurel, Maryland 20707. This is an easy (on the weekends!) about 15-20 miles drive from Washington DC, depending on where you are located.
Maps available for free at the Brighton Dam office, where you buy the permits as well.