Tuesday, January 21, 2014

Don't Feel Like a Human, Think Like a Beaver

I was taken aback by how low the ice was behind the East Trail Pond dam on December 3:

Here’s how the water settled behind the dam on November 6:

We had a cold November with ice thickening more than usual and the beavers were cutting trees below both ends of the dam. Although I couldn't see them on the 3rd, I assumed the beavers made some holes in the dam to weaken the ice so they could break through it easily and get to those trees. 

On the 9th I saw that they had dug a hole through the far south end of the dam to get to some trees they cut just below the dam.

Most of the pond had an imposing sheet of ice that I probably could have walked on but I didn’t try.

It’s easy to see that breaking the ice would not be easy for the beavers, though I've seen evidence that beavers can gnaw escape holes in ice 6 inches thick.

There was a more discreet hole in the north end of the dam which appeared to be letting just enough water leak through to make it easy for the beavers to keep a hole open in the ice just behind the dam.

The south end of the pond is much shallower that the north so I thought there was no danger of more water draining out there and that the leak at the north end of the dam could easily be patched. Usually we have enough warm days in December so the character of the ice can change before the usual January deep freeze.

Between December 9 and Christmas we had several nights and even a few days of below 0F temperatures. Plus we had an ice storm that kept me from hiking in the woods. When I saw the dam again on December 29, the ice had completely collapsed behind the dam:

That allowed the beavers to keep a hole open so they could cut down trees and collect branches on a slope northeast of the pond.

Then in the next two weeks we had more punishing cold and I didn’t get out to the pond again until January 15 after a January thaw that didn’t melt all the snow and ice but melted enough to fill the creeks with rushing water. The beavers' reaction to the thaw was to widen the hole in their dam.

In other years they put a hole through the middle of the dam so that they could get to cattails and bushes below the dam. Ice often covered the hole on the dam side and I usually didn’t see it in all its glory until late January or February.

Like most humans I find still water behind a dam innately pleasing. Some biologists have suggested that beavers innately try to stop flowing water with dams and that the same drive forces them to patch their dams whenever there is a leak, a compunction that human beaver trappers use to their advantage. So my initial reaction to a hole in a dam and water flowing through it is unease. Not only are my own instincts offended but I fear that the beavers are being bested by the elements, or trappers, or otters or muskrats. The last two have their own reasons for putting holes in dams.

However, when I watched the beavers in the East Trail Pond, I’ve learned that I am watching a rather resourceful family. So when I think I am observing something awry, I step back, try to think like a beaver and not feel like a human, and soon I see that they are in complete control.

The East Trail Pond sits in the middle of a small watershed about two miles with a drop in elevation of about 100 feet. However, unlike most beaver ponds the creek that runs through the valley does not enter the beaver pond towards its rear. The creek enters right behind the north end of the dam, as the photo below with the footbridge over the creek at the far left shows.

When I first began hiking in the valley in the late 1970s beavers built a dam in a narrow part of the valley. The remnants of the dam are still there backing up a small pool of water. Here’s what that dam looked like on March 24, 2003.

Otters had put a hole in the dam in late January and by March the hole through it was rather big but the beavers soon patched the hole and returned the pond to its usual magnificence. Here is how the water brimmed the dam on May 8:

Here is a photo from May 27 taken from beside the dam looking back up the valley.

The ridge where I often sit and watch the beavers today is in the hazy background of the photo. This fall, 2013, after the leaves were down, I took a photo from that ridge looking back to where the old dam had been just to left as you look at the two pines almost in the middle of the photo.

Beavers left the pond in 2004, and soon I’ll write a post on that. I didn’t see a beaver nor signs of one in the valley until in 2008 one beaver built a lodge on top of the old dam but only stayed a few months and did not make any repairs on the dam evidently content with a small deep pool of water behind the dam (half the size of a basketball court.)

The wide mid-section of the old pond turned into meadow save for two small creeks snaking through it. The upper section of the old pond is naturally low ground and there were pools of water supporting a lush crop of ferns and winterberry shrubs.

I was not surprised when I heard a beaver gnawing in that thick vegetation on June 10, 2010. Back when beavers flourished in the grand old pond, one of their lodge was tucked back in that lush zone. Here is a photo lifted from a video that I took in September 2002.

When beavers moved back in the pond in 2010 they began making small dams just to the south of the where the creek crossed under the foot bridge.

Then the beavers began making a dam below their pool.

I expected the beavers to manage this the way I’ve seen done in other valleys. They would build a dam just below their new dam and when water backed up into a new pool, they would breach the dam behind it. In this way they more or less keep the creek centered through a radiating series of progressively longer dams. I knew the creek flowed through a series of depressions. I used to see otters fish in them when the large pond still existed. And it looked like the beavers were going to do that, methodically reclaiming those pools much to my delight and the otters‘

In the photo above you can tell where the damp soil is by the line green cattails going to the left. The cattails in the center and right of the background are drying out.

To my surprise the beavers continued their dam into the dry area of the valley.  Below are two photos I took in mid-October showing their progress. First a view from the footbridge: 

And then from out just below the pool of water looking back toward the footbridge:

I suppose that one could make the case that beaver dam-making shows instinct adjusting to geography. The nature of the terrain dictates the direction of the dam. As I saw the direction the dam took, I tried to recall my many winter hikes tracking otters through the valley. I was well acquainted with that pool where I first heard the beavers in 2010 and where other beavers had a lodge 10 years before. I also knew of an old bank lodge along the nearby south shore of the pond and there was another pool there. And when those two pools filled up as water coming down the ridges collected there, water found its way to the main creek by two meandering channels, one below the ridge to the north and a smaller one a bit off from the south ridge.

Back in May 2003 I got a photo showing the how the channel on the north side of the upper end of the pond connected with the main pond formed behind the large dam.

So I could say that the beavers brilliantly engineered a dam to put those two pools into play. But by and large most of the area behind the dam was higher than that below the dam. There was a gentle slope down to the east and north. A photo I took on December 19 as I sat on the ridge south of the new pond gives pretty good sense of the lay of the land before the beavers built their dam and the relative heights of the near, maybe one foot, and far ends of the dam, a little over two feet.

The beavers who first developed this valley in the late 1970’s made the brilliantly engineered dam. I think the design of this dam was informed less by instinct and geography, and more by memory, which made it brilliant in another way.

These beavers were no stranger to the creek they dammed.

The East Trail Pond is at the conjunction of four large valleys. The largest valley extending about a half mile to the north supports the main creek and drains to the south. Another valley from the east supports a smaller creek. The valley to the west drains to the west. The beavers now in the pond spent years living in that valley to the west in small ponds they made habitable by dredging. They made dams but more to cozy the water up, than to back up any streams.

In 2007 they moved into a pond on the other side of a short but dramatic granite ridge that divided the valley to the west of the creek. Beavers had abandoned that valley in 2001 after park officials put perforated pvc pipe through their dam.

In two years the new beavers engineered a magnificent pond. They used the dam the other beavers had abandoned using their dredging skills to stuff the conduit pipe with mud. A sheet of water almost the size of a narrow football field stretched to the west.

Where the creek flowed in from the north water also backed up but not so grandly.

The beavers built a dam where the creek entered the growing pond affording them  a  smaller pond convenient to the narrow valley filled with small trees that the creek ran down.

So the creek ran through what became the pond to the north and then turned to the southeast and had a short straight run to the dam which the beavers built higher and higher until it was about five feet high.

Then the dam failed twice in the spring of 2009, a few weeks after the photo above was taken. After the first failure in April the pond gave the appearance of complete devastation.

But the beavers were still there.

They repaired the dam.

And it failed again worse than before.

The footbridge over the creek just north of the pond they would develop in 2010 appears in the background of the photo above.

I blame three factors for the failures of their dam in 2009: rains from a series of thunder storms that brought water down the creek so that the water level of the pond brimmed the top of the dam; two years of dredging mud behind the dam to patch the perforated pvc pipe which weakened the base of the dam; and strong gusts of wind coming down the valley to the west which pushed the sheet of water over the dam.

So in 2010 the beavers decided not so much to dam as to tame the creek that had twice destroyed their dam. They extended the creek to the south with their dam and forced it to flow a few feet up hill. The dammed water would extend placidly to the west but only two thirds as far as the western reach behind the dam that failed. 

Also the dam would completely be of their own making primarily with grass and mud, limited in height to about three feet and not anchored on any trees or rocks, no immovable or impenetrable object. At their convenience to relieve pressure on the dam or get access through it when the pond was ice covered, they could dig a hole through it. Then they could patch it as easily as they had made it.

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