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Leave it to Beaver – How Beavers Change the BWCA

When I study nature, I love to learn about the superlatives. I appreciate how the tallest mountains, the biggest trees, or the oldest animals have a way to serve as ambassadors, helping people to appreciate the natural world even if they don’t understand the “nitty-gritty” science of everything. It's with this mindset that I set off onto an interesting study of the huge role beavers play in the BWCA. One of my favorite places in the whole wide world is the BWCA. In my time guiding and taking personal trips, I am consistently looking for new lenses through which to appreciate it and, in turn, help others to love it more too. And beavers are a huge part of the BWCA being the way it is today.

In my study, I primarily set out with two goals:
“What does the impact of beavers look like on a landscape level?"
and “What are beavers capable of building? How big can they build?”

To answer the first question, I needed to choose a study area. I wasn’t going to map EVERY beaver dam in the BWCA, but I needed to map a big enough area to lend perspective. Since this was about the superlative, I also wanted to choose an area with a large beaver population which has been sustained over time. The best I could think of was the very western end. With an intricate network of rivers, streams, and marshes, it’s sort of the perfect beaver habitat. I ended up essentially mapping every beaver dam from Meander Creek to Crane Lake inside of the BWCA border (and including waters which flow into the BW.) I was only mapping dams which were easily discernible on the air photos (about 30 ft long and longer) and this yielded nearly 800 beaver dams in my 42,000 acre study area! Check out the map here.

Beyond this, I also wanted to study how large of dams beavers could build and how long those dams last. Incredible articles like this one have described how beaver dams can lest centuries, even a millennia in one case. And this article connected to Wood Buffalo National Park in Canada claims the world’s longest beaver dam at 2542.65 ft long. With those stats in mind, I kept track of every beaver dam in three “large” sizes within my study area and then screened the rest of the BW for other superlatives (though my screening was far from in depth.) The three sizes noted in my data are 4-500 ft, 500-1000 ft, and long than 1000 ft in length. For any dams longer than 500 ft, I delved into a more detailed history including original construction date (if discernible), any notable additions, years of damage (or failure), years of replacement (if applicable), watershed, and an identification number to keep track of each individually. I used multiple historical air photo databases for discovering this data, but my air photos only go back to 1948 for this region. Also, as a bi-product of this study, I learned a lot about different shapes of beaver dams as well as individualism in styles and different beaver families' preferences for redundancy (building lots of dams to catch every drop of water.) I also learned how to spot beaver dams based on characteristic shapes even more than a century after they were built.

Here are some highlights from the data:

I found evidence of approximately 15 different 1000+ ft dams in the study. Of these, two are “questionable” in terms of “is this whole thing a dam or is it multiple”? Once beaver dams reach the century+ mark, things become increasingly unclear as they essentially become land masses unto themselves with deep soil and trees. Of the 15 suspected 1000 fters, 8 were built before 1948 (and, in some cases, well before.) But some have been built this century already which blew my mind. How can beavers build something that is 1/5 of a mile long in 20 years? Of the less than 1000 fters, there were about 90 between 500-1000 ft and 60 between 400-500 ft. And the watersheds with the most beaver dams over 500 ft? Meander Creek, Nugget Creek, and Gaunt Creek. Probably not surprisingly, Nugget and Gaunt combine to become Beaver Stream which flows into the Loon River. I was blown away again and again by the beavers. Sometimes they built crazy interwoven dam structures which defy my understanding. Sometimes beavers returned 70 or 80 years later and either rebuilt an ancient dam or made a giant connector between old structures. It also seemed like certain beavers were masters at their craft while others were poor at best (and their creations lasted for a comparatively short time.) All in all, it’s incredible the level of impact the Beavers have had considering that, during the fur trade, this population would have been under a tremendous amount of pressure. They obviously have rebounded and what we see today is a very healthy beaver population in the BWCA.

Check out some examples and highlights below from the study.

Thanks for reading.

For those who didn't click open the map, here is a screen shot of the study area with dams highlighted in yellow

Beavers do an incredible job of using their landscape. This beaver held back an entire lake by building an 80 ft dam at the narrowest point. I also observed beavers using islands in a similar way. Why build a 1000 ft dam when an 80 can do the job?

This interwoven beaver “jungle gym” is one I still can’t explain. This entire structure, 1700 ft of dam in total, was built within 20 years from the 90s into the 2000s. I still don’t fully understand what purpose all the weaving accomplishes.

In this photo, we see three large beaver dams. The two purple ones were built before 1948 and both measure over 400 ft long. In the 90s, another beaver family came along and added the 559 ft gold dam onto the west side, essentially making a 1000 ft structure out of the original.

This is a fascinating example of what I meant when I said “redundancy” The beavers built an incredible network of dams in very close proximity to each other within a short time frame. The reasons for this behavior probably vary, but it’s still fascinating all the same.

The largest dam in my data that’s still very clear. This nearly 1200 ft dam was started in the 1970s with major additions in the 2000s.

In this photo, the beavers commandeered an old road grade, building their dam on top of the grade.

This photo shows roughly how beavers work their way up watersheds. As young beavers set out to find their own space, they follow the creeks and rivers out from their family groups. What's amazing to me is the spots that beavers seemingly had to walk for miles overland to reach.

This photo is a prime example of how beaver dams weather over time. There are seven different dams visible in it all of different ages and conditions. The amount of water being held is part of evidence of how "healthy" a structure is. The structure itself can persist for a long, long time after it stops holding water which makes marking them on a map challenging and fascinating.

This photo demonstrates how new dams built downstream can essentially make old dams obsolete by flooding them out of their roles. Even still, the old structures, if well built, persist. In some occasions, I was able to observe through the air photos how old dams can become viable again if the new dam failed and ceased to flood out its predecessor.

This is one of the most fascinating examples of beavers adding to older structures. The three golden dams are all over 600 ft long and were built in the 50s and 60s. Then in the 70s and 80s the beavers built an 850 ft long connector between the two (orange) followed by another 400 fter in the early 2000s (purple.)

This is another example of beavers adding onto an older structure. The peach colored line is the original pre-1948 dam which was crossed in an “x” shape by a new 800 ft dam built in the 1970s.

And, as for me, this is the most beautiful dam in the whole study. Did this beaver go to engineering school? Where most of the other dams seem to serpentine and go every which way, this one makes an almost perfect curve. And the quality of building shows. These two photos were taken nearly 80 years apart from each other and the dam has hardly changed or lost a drop of water.

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