Since the Potomac is patrolled by a multitude of lower flying helicopters (including coast guard and park police) and is also route for DCA, drone flying is restricted. It is a Fairfax county park, which has allowed drone flights at other parks. But since rules regarding drones is always changing, i recommend double checking with FAA and fairfax park authority.
Not sure but not allowed because i never seen drone there
Not sure, please check with their office
To many trees, but no where does it say you can't
Unlikely, research the allowable fly radius around Dulles airport
Doubtful
I suspect not, but I can't say definitely.
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