Why are beavers obsessed with dams? - Glynnis Hood

Nestled in the remote forests of northern Canada sits the world’s longest beaver dam. Curving around the water like a castle wall, this 850-meter-long structure is large enough to be seen in satellite imagery. The dam and the generations of North American beavers that maintain it have dramatically transformed the region, creating a pond containing roughly 70,000,000 liters of water. This is a suitably sizable home for the creators of this woodland kingdom.

But even dams 1/100th the size of this one can have huge impacts on their environment. So how exactly do beavers redesign the forest, and how do they build these impressive structures in the first place? Consider this beaver in the northwestern US. Standing just under 2 feet tall, he’s a proud representative of the world’s second largest rodent species. While he’s at risk to predators on land, once he’s built a lodge, he’ll have a massive moat to keep him safe.

But he can’t just build a dam anywhere. Following the sounds of running water, our beaver searches heavily wooded areas to find a medium-sized stream that’s not too steep or too deep. After abandoning one construction site due to its rocky floor, he finds a stream with a soft, muddy bottom. Combining vegetation, mud, and sticks, he creates a small bank along the stream’s edge.

Then, using a bite almost 3 times stronger than any other mammal of this size, our beaver chews nearby logs into sturdy sticks. He then rolls them into the water and spikes them down into the soft streambed. Beaver dams come in several shapes, but our beaver opts for a concave dam to dissipate the forceful flowing water, and layers in large rocks to reinforce areas where water flows the strongest.

Depending on a dam’s length, the stream’s speed, and the number of beavers on the job, these devoted architects can build shockingly fast. In some cases where humans tried to remove dams, beavers have rebuilt them overnight—sometimes rebuilding larger than they were before. Like most dams, our beaver’s project is just a couple meters long. And working alone, this dam could take several days to complete.

But once the structure spans the channel, his watery home begins to fill up. As the pond grows, he’ll extend the dam to block water flowing around the sides. However, some is permitted to leak downstream, releasing pressure on the dam and regulating the pond’s water levels. The larger the pond, the larger the beaver’s territory.

And since they can hold their breath for up to 15 minutes, beavers can easily access food along the shorelines. Throughout the fall, our beaver builds up an impressive supply for winter—while also looking for someone to share it with. Beavers are fiercely territorial, but they also bond for life. When the pond freezes over, our new beaver couple splits their time between making trips to the food cache with their lodge’s private entrance and starting a family.

Come summertime, the juveniles will help reinforce and expand their dam, gather food, and watch their younger siblings. After 2 to 3 years, these young beavers will disperse to find territory and mates of their own. But their ancestral dam can last for decades. Maintenance work is continued by descendants of the original colony, or new beavers that move in when the reigning family leaves.

There’s certainly no shortage of neighbors—some regions have as many as 40 beaver dams per kilometer of stream. This is great news for surrounding wildlife that rely heavily on these semi-aquatic engineers. Lodges can serve as nesting sites and refuges for various species of waterfowl. Beaver channels also connect bodies of water, increasing the biodiverse areas between water and land.

Humans benefit from beaver construction projects too. Their ponds help replenish groundwater stores, in part by creating large expanses of surface water. And just like their manmade counterparts, beaver dams slow floodwaters. So just by following their natural instincts, these ecosystem engineers create huge impacts downstream.