that was taked last year 4/7/10 at extreme low tide, probably way up river--Roland Park? I'm trying to picture places where the street is between the houses and the water (Mayflower, Cambridge) but this isn't it.
Right now I'm seeing dark patches of an algae similar to slur on the bottom at my house near Hampton Blvd Bridge.
there are also storm drains going into the river that would cause an anomoly on the bottom like that. Dredging, too, but this one isn't dredging I don't think
The pic is from the mouth of a creek in the western branch of the Elizabeth River. Yeah I think it must be some sort of algae or debris, because I just found another pic of that spot taken at a different time and it wasn't there anymore.