The Patuxent is actually to the west of Elkridge MD. It is the Patapsco that flows through Elkridge. There are some small falls that you could possibly navigate is a plastic kayak but I wouldn't want to try taking a fiberglass or composite boat through there on the Potapsco. Bedsides tne Patapsco is kind of the alimentary canal of west Baltimore, flowing by the old sanitary landfill and the 695 Baltimore harbor tunnel thruway. Very much less than scenic.
As for the Patuxent, the upper reaches have a lot of sand banks and blow downs as possible obstructions. I have not paddled there.
From the Patuxent State Park on down to Solomons Island, which is a least two thirds of the river, it should be clear.