As much as I like the idea of it being something cool like a meteorite or dino poop, I suspect @Horn is closer to the real truth. That close the river bank I suspect it is some really old (possibly 100+ or so years) industrial slag from something. From the picture it looks a lot to me like it was poured there.
Way to deflate a guys dream. Guess I better not quit my job just yet to sell sasquatch hunts yet. Thanks @Horn