Saturday, February 11, 2012

Why Beavers Breach Their Own Dam

As I followed a fresh otter trail to the new East Trail Pond dam,


I feared I would see a hole in the dam made by the otter to drain water out of the pond leaving pools of water under the ice where it could easily catch fish and pollywogs. I could see the stain of water on the ice below the dam, a good sign of a leak. I have seen some huge otter holes in dams. This is how a hole the otters started in late December 2009 at the nearby Lost Swamp Pond looked in mid-February.

But the usual scenario didn't seem to be playing out in this pond this winter. The otter went up and over the dam, not through a hole in the dam. It went over the pond ice to the beaver lodge where I lost track of it. There was a small hole in the ice next to the lodge. Maybe it dug there to get down to where the fish are.

Otter holes usually start small and relatively high along the dam just under the water line and then, as the winter wears on, the otters, and rushing water, widens a trench. The hole in this dam this year was big from the start. Since all the snow has not melted yet, I don't have a definitive portrait of the hole in the dam. But lying on the dam and sticking my head, and camera, as close to the hole as I can get, it looked to be two feet wide and almost as high.

Over the years, I've seen evidence that beavers put holes in their own dams, which is to say, a dam had a hole it, no otters had been around, and the hole was much bigger than what a muskrat would make.

The clincher is seeing evidence that a beavers uses the hole. I found a hole in the dam of Meander Pond in January 2010. Here is a photo of a branch a beaver cut and dragged to the hole.

That doesn't prove that a beaver came in and out of the hole, but a few days later I came up to the dam while a beaver was below it. Here's a short video of a beaver running up rushing water and going into the hole

The same beaver family that made the hole in Meander Pond dam is now living in the East Trail Pond, where I just discovered that hole in the dam.

Soon after the hole was made, the East Trail Pond probably lost 75% of its water, as did Meander Pond back in 2010. The beavers used the hole in the Meander Pond dam to get to a small grove of alders and some ash trees below the dam. So, for the past few weeks, every time I checked the East Trail Pond I checked for evidence of beavers coming out of the hole and foraging below the dam, which they occasionally did last winter and earlier this winter before the pond iced over.

Last winter the water slowly drained out of the pond. That was a snowy winter and the ice on the pond never got that thick. It was easy to make holes in the ice and they made one next to the north ridge

and two more between that hole and the lodge about 30 meters away.

This is the warmest winter we've had in years and as of the first week in February we've had only a few inches of snow. However, we did have a few nights when the temperature got below 0 F, below minus 20 C. The pond soon had 6 inches of thick ice. The beavers had to gnaw through ice to make a hole so they could forage on the wooded slope north of the pond.

As far as I could tell, the beavers gnawed the ice from under, after swimming 30 meters or so under water. During the first three weeks of January this was the only hole the beavers used to get out and forage. Yes, they had a cache of branches and logs near their lodge in the middle of the pond, but over the 10 years I've watched this family they always foraged outside of the pond throughout the winter.

Once the water drained out of the pond through the hole in the dam, I was able to get some great photos of what the pond looks like under the ice.

While a beaver could easily climb out of the hole and eat the woody shrubs on the pond, they also cut some small segments of shrub under the ice.

Soon enough I saw a beaver use the hole. On February 7, one carried a small log it cut off a red oak trunk, that the beavers cut up on the ridge, down to the hole in the ice.

After the beaver climbed down the hole, I went quietly on the ice over to the slope and as I expected heard the beaver gnawing on the log for the next half hour. I could see the water under the hole moving as it gnawed. And before a mink ran along the ice up to the hole, I could hear the beaver's heavy breathing -- something beavers do when they are not pleased. You might not be able to hear the breathing in the video.

I'll go back in a few days and see what happened to the log. I've noticed that there is a growing pile of leftovers under the ice next to the hole.

We human's are a bit naive in our thinking about beavers. Perhaps because we calculate so much benefit from them for ourselves, we forget that for beavers a dam and the pond behind it are the means to an end. Especially in the winter when the ice on a pond is thick, the beavers' life is made easier by compromising the dam and draining the pond.

I took a photo of the hole that shows how secure a beaver must feel under it, and how insulated it must feel from the harsh winter above.

Meanwhile as winter wears on, the ice is collapsing all around the pond and some nice holes in the ice have opened up behind the dam.

But I've seen no evidence that beavers have used those holes. The hole on the north shore of the pond is too convenient. They don't need another outlet.

After the Spring thaw, the beavers patched the hole in Meander Pond dam back in 2010. Here's the hole on March 2.

Here's the repair job on March 19.

If the beavers choose to stay in the East Trail Pond, the dam there will be good again by April.

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