When you swim from one state to another state; e.g., from Maryland to Virginia and back again, that is interstate swimming.
I am pleased to report that current conditions for interstate swimming in the Washington, DC area are excellent. The temperature of the Potomac River is in the mid 70's, the river is low, and the water is relatively clear.
Yesterday I swam solo from Maryland to Virginia and back again. This is not as crazy as it sounds, because you can see the bottom much of the way across, and you can touch the bottom at frequent points in the crossing.
FYI, the health of the Potomac River now is better than it has been in years past. Plant and fish life are in greater evidence than they have been in years past. Another member of the Interstate Swim Team has reported a rash, which may or may not be related to interstate swimming. I have not had any problems.