Does the Potomac river above Dahlgren get brackish do to the tide?

Yes. Although there are some LMB in the Dahlgren area above the 301 bridge it is really more known for its striper fishing. There is good LMB fishing in the Port Tobacco/Nanjemoy creek areas though. Skeet Reese one the BASS tournament fishing in Nanjemoy. The area from Aquia down to the 301 is really a transition zone where you get progressively more brackish water. Below the 301 you need a saltwater license.