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.
