I can only guess that it was a mink. It popped out near the edge of the river and ran along the bank and into some rocks. It had a gait like no animal I have ever seen in the area. Another person who saw it said it looked like a weasel but it was almost entirely dark brown, black even, and the descriptions I have found pint to mink. Any thoughts? Mink in the Bronx? The habitat in that area seems appropriate.