River weed refers to cannabis which has been inadvertently brought into a river, lake or any large body of water, making it incredibly soggy. Even after drying the weed out, it still tastes like shit.
The phrase "dirty old river weed" may be a vague throwback at the phrase "dirty old foot hash."
This dirty old river weed tastes like a wet dog.
Yeah, let's just roll up a river blunt!