Screaming or howling in Del Norte County, California

It was the last weekend of September during the fall salmon run of fishing season on the lower Klamath River 2001. It was the end of summer, the river was extremely low and I'd just finished cleaning my salmon. It was about 10 oclock at night and I drove up the river to Blake's river bar. This is where the road ends. We were sitting in my truck with the windows down when we heard a sound that I'd never heard on the river or anywhere for that matter. My attention was immediately taken as we could hear an extremely deep howling sound that sounded something like a whoooooaaahhhh. I was a little freaked out because I have never heard anything like this. I remember trying to put a body with the sound and not coming up with anything. My friend and I looked at eachother and said "what the hell is that?" The hair was raised on the back of my neck and I felt very fearful, even though I knew the thing was across the river. We waited and didn't say anything and within about 3 seconds we heard the same sound again. This time it went whooooooahhhhh, whoooooahhhhh, whoooooahhhh, three times. It was a very deep almost like a moaning sound. Immediately after the sound stopped I heard the same noise, except this one sounded like it was coming from up around the corner, about a half mile, and on the other side of what is called Turup flat. This one sounded more faint because is was coming from on the other side of this densely wooded area where the river bends. It seemed clear to us that they were calling back and forth to one another. The one on the top of the river called back only twice and the one across the river sounded off about 4 times. This entire thing only took about 20 seconds and then it was done. I couldn't really explain what I was thinking or feeling after they stopped but I was very shaken and just sat there for about thirty seconds waiting to see if there was going to be any more sounds. I turned my lights on after I thought that it was over and couldn't see anything because the fog was to dense. We were both sure that we had never heard an animal like this before. Between the two of us we have about 60 years of night fishing and hunting in this area and this was a first. We both talked about it and felt that whatever it was that they could have come in from either side of the river because the river was so low that it would have been easy to wade across and there wasn't littel traffic in the area. The river was also very warm and it was also full of 30-50 pound salmon that were heading up the river. It was a great time to fish if that's what they doing.