Something walking upright in Cowlitz County, Washington

This is a story that I've never put down on paper, although I've told many people about it over the years. I was young a the time, but I am sure of what I saw, so I suppose it's time to report it. You'll forgive me if the details are sketchy, it's been about 16 years (I'm 28 now). I was steelhead fishing with my father on the Kalama river in southwest Washington state. My dad left me at a bend in the river (unfortunately, I was 12, so I don't know exactly where) and went down to fish out of sight from me. I drift-fished for a while, with no luck. Then I heard an odd "whack-whack" sound, and looked upriver. Across the river, about a 50-70 yards away, I saw a large, black, bipedal figure with a fish (maybe a steelhead?) in its hands. I say hands, not claws, because it repeatedly raised the fish overhead and brought it down on a rock, apparently to kill it. It moved its arms like a brachiating ape, overhead and down in a pretty large half-circle. What struck me most, other than the odd way of killing a fish, was how upright and thin the figure appeared—I'm no naturalist, but I'd seen Grizzly Adams enough (and spent enough time hunting with my dad) to know what a bear looked like, and this was not a bear. I suppose it could have been a swarthy human dressed all in black whacking a fish against a rock, but why? Being 12, I pretty much froze and watched. I remember thinking I wasn't going to catch any fish if I didn't cast the line again, but I didn't want to attract the attention of whatever it was across the river. It's difficult to reckon time in this instance, but I would guess it was only a couple of minutes before the creature took the fish and walked back up toward the woods.I didn't actually see it go into the woods, because I reeled my line in as soon as it started to move away and was intent on finding my dad. As soon as I got my line in, I ran down around the bend and told my dad what I'd seen. He told me it was probably just another fisherman and to go back and get my fishing pole. I realize that a 16 year-old report may seem dubious, but I know what I saw wasn't any kind of animal I'd seen before. I was unable to get across the river (my dad couldn't see the logic in it) to look for tracks, to my eternal regret.