Over the week-end I mentioned the www.enaturalist.org website that I found that is really great! Here's the question and answer I asked at the site. I had my answer in less than a day from posting the question! Very nice! I was pretty sure we had seen a beaver so it was nice getting that confirmation!
Q: We have a river that goes through our property here and I was walking down by the river and there's a place where there's a "waterfall/dam". The river is probably 10-15 feet wide but fairly shallow at that spot and there are lots of sticks etc. placed to make a dam that looks like about a 3" waterfall. By the river there are some kind of shrub (that I'm not sure what they are - they grow all over the place up here like a weed and are about 10-15' high and thick and take over), and there were lots of them that had been chewed off to make the dam. Some of them looked like they had been chewed off as high as 3-4' off the ground. One day we were there when I was walking away and was quite a distance away I looked back to see an animal swimming down the river. My son was closer to the river and saw it but not really clearly. Then I heard the slap - sounded like a large log being thrown into the water and the animal disappeared. Would this be a beaver or a muskrat?
A: What a great description of what you experienced and the photos are helpful too! Really makes my ability to help with identification, so much easier.
No question about it. You saw and heard a Beaver. I've often thought that tail slap sounds like someone dropping a boulder off of a bridge - quite a sound. Beavers use the tail slap to announce danger and it is quite effective. Your Beaver probably was spooked by your son. Muskrats don't make dams and their tails are more rat-like and can't do the water slap.
As to the shrubs along your shore, from what I can tell from the photos, these look like one of the alders, probably Speckled Alder (Alnus rugosa). Alders are shrubby, grow in very wet locations and help stabilize banks and also are able to fix nitrogen from the air. So although these plants may seem a bit of a nuisance, they really are valuable parts of the environment.
I'm guessing that if your Beavers stay around, you're going to have more chewing around your river and that the dam will increase in size. The pools that Beavers create are especially rich with animal and plant life and research has shown that streams below these ponds are much more fertile and therefore more wildlife-friendly as a result - the fishing improves too.
Thanks for writing and I hope this solves your "mystery".
Our Journey Into The Country - A Faith Walk