Why Do Beavers Build Dams?

Beavers are master engineers in the animal world. They are able to shape the environment around them to suit their needs. You’ll already know that beavers are most famous for building dams. But why do beavers build dams?

Beavers build dams to slow the water flow and create a pond in shallow water such as a stream, river, or creek. Stopping the water helps the beaver create a moat-like structure around their home to keep them safe. The dam makes the water around the beaver lodge deep enough to suit their semi-aquatic lifestyle.

That’s beaver dams in a nutshell. But if that’s left you with a few more questions then don’t worry. This guide will take you through everything you want to know about beaver dams.

Let’s get started.

What is a beaver dam?

Before we look into why beavers build dams, it’s best to start with what a dam is.

A beaver dam is a structure they create from wood, branches, twigs, rocks, grass, vegetation, and mud. The beavers will carry this material to the narrowest part of the waterway and start to build.

Piling these building materials on top of one another creates a wall in the water.

First, they start at the edge, where the currents are weak. They usually start with narrow branches such as willow. They pack these down with rock and mud. As they bring more and more branches along the beaver will weave them together for a strong structure.

Beavers will construct the dam to accommodate the current of the water. A slow-flowing waterway usually is sealed with mud so no water gets through the dam at all.

Yet, a dam built on a fast-flowing waterway will usually allow a small trickle of water through the structure. This is to prevent the water pressure from eroding the dam over time.

Why Do Beavers Build Dams?

We’ve already discussed that beavers build dams to create their own pond ecosystem. Yet the main purpose of a beaver dam is to control the water.

The two main outcomes the beavers want from a dam is to:

  • Make the water deep
  • Slow the flow of water

Making the water deep allows the beavers to create a barrier to surround their home.

Slowing the flow of the water also helps to prevent damage to the beaver lodge and create a still water environment around their home.

These aims will help you to understand why beavers need to build a dam. Let’s look at why controlling the water is important to the beaver lifestyle.

1. Predator protection

The main reason that beavers need a deep water pool is to create a protected waterway around their home that keeps predators away.

A beaver lodge is situated in the middle of the pool of water created by the dam. Beavers dive under the water to reach the entrance of their lodge.

This means that if an animal wanted to attack the beaver lodge it would need to be able to swim a good distance and dive deep underwater to get inside.

Most beaver predators are not great swimmers. It would also waste too much energy and body heat for them to attempt to attack a beaver lodge.

A deep pool of water also allows beavers to move in and out of their home completely undetected above the surface. This gives them further protection from hungry predators near the waterside.

This habitat design is so successful that, unlike most wild animals, predator attack is low down on the list of threats to a beaver’s lifespan.

2. Food Supply

The pool created by the dam allows the beavers to store a cache of food close to their homes. The beavers will pile up twigs and branches as a food cache outside the entrance of their lodge.

This cache allows them to access food in times of danger or when resources are low. Keeping a stash of food underwater means that beavers can collect a supply in summer, when food is plentiful, and access it in winter when there is less food around.

Storing food underwater is much easier for the beavers as they are faster in water than they are on the land. That means a food supply in water is much safer for them than continuously collecting it from the land.

A beaver diet is also made up of aquatic plants. The pools create help a variety of underwater plants to grow and supply the beaver with more food.

3. Maintain teeth

Creating dams is a lot of hard work for beavers who have to continually cut down trees and branches to form the structure. Once the dams are built the beavers need to maintain them to ensure it’s watertight and not breaking down.

Cutting down trees to keep up with the building and maintenance of dams is good for the beavers’ dental health. Beavers need to keep their teeth well maintained by cutting wood. Otherwise, their teeth continue to grow and cause physical deformities that are bad for their health.

4. Survive winter

Deep pools of water are less likely to freeze over in the wintertime. That means that beavers can remain active in the water during winter, even under thick layers of ice.

The still water helps to maintain the temperatures within the beaver lodge. This keeps them nice and cozy and protected from the harsh winter weather.

Do beavers live in a dam?

Beavers do not live in a dam. The dam structure is more like a wall that helps to create a moat that surrounds the beavers’ actual home.

It’s a common misconception that beavers live in their dam. That’s usually because beavers can come and go easily from their lodge without being seen. Yet you can sometimes see beavers building and maintaining their dams, so it’s often assumed that’s where they live.

A beaver lives in a home known as a lodge. There are two types of the lodge: a conical lodge and a bank lodge. The conical lodge is the type that requires a dam.

Conical lodges need a small island area which the beavers live on. They will then create a lodge structure over the island to keep out predators and keep in the heat.

You can check out more about beaver lodge in my guide to where beavers live.

Do beavers always build dams?

Beavers don’t always build a dam around their lodge. That’s because sometimes the water which they live in has to be suitable to build a dam.

The first problem might be that the water is too deep. That means it would be too difficult for them to build an island for their lodge.

Another reason is the water may be too fast flowing. That means that the dam will quickly become damaged by the constant water movement.

In these situations, the beavers can either find a more suitable waterway to build their dams and lodges. Or they can build a bank lodge at the side of the river.

How do beaver dams affect the environment?

Beavers are a keystone species which means their presence in an area has a massive effect on the local ecosystem. The reason that beavers have such an impact is because of the way they engineer an environment to suit their needs.

For some species, the modifications that a beaver makes have a really positive effect. For others, it has a negative effect.

Let’s look at just how beaver dams impact their environment.

Why are beaver dams good for the environment?

Create biodiversity

The wetland areas of ponds and marshes that emerge from beaver dams create a lot of habitat for a wider range of animals to live.

For example, a fast-flowing river would not have frogs living in it. Yet a beaver dam creates a habitat suitable for the frog to live in. In turn, this creates a food source for other animals such as raccoons.

It should be noted that there are some animals who would have previously thrived in the river environment who may be displaced by the beaver dam. Yet the diversity the dam creates appears to be larger than the impact of the animals who have lost habitats.

Beaver dams also create plant diversity around the edges of the water and also as aquatic plants in the water. This increase in vegetation provides an excellent food source for animals such as moose and bison.

Reduce flooding

Just like man-made dams, a beaver dam works well at holding back excess water during heavy rainfall.

Flood prevention from dams usually works better when there are a series of beaver dams together to slow the flow.

A longstanding, strong beaver dam is much more likely to hold back a flood than a newly constructed dam with a lot of weak areas that can easily burst open.

Improve water quality

Beaver dams can improve the local water quality of faster-flowing rivers and streams.

The faster the water flows the more likely it is to pick up sediment from nearby fields and roads.

Once the water reaches the beaver dam the flow of the sediment is slowed down. Beaver pools can filter out the sediment and allow the freshwater to flow through the dam.

This gives a much better water quality downstream past the beaver dam.

Some sediment such as fertilizers may be beneficial to the beaver pool by encouraging the growth of aquatic plants.

Reducing drought

Climate change is having a massive impact on the environment around us. One of those which impact us most is drought.

Drought means that humans and animals have less water to drink, keep clean, grow food and stay cool during hot weather.

Beaver dams can reduce the effects of drought by pooling water that takes longer to dry out. This water can slowly leak into rivers and streams or the ground which would otherwise be affected by the drought.

This stops animals from losing their habitats, keeps crops growing, and creates a water supply for humans.

This means beaver dams can reduce the impact that the drought has on a specific area.

Why are Beaver Dams Bad for the Environment?

Timber damage

Beavers need to cut down a lot of trees to eat, build and maintain the health of their teeth.

Cutting down trees can be problematic for the environment as it can cause a loss of habitat for some animals.

Half felled trees can also be a safety concern if they fall without warning.

Beavers may also impact the industry of an area if they are cutting down trees that would usually be sold by a business to the construction industry.

Landowners may also be left with the cost of paying to have beaver felled trees uprooted from their land.

Crop damage

For the beavers, slowing the flow is the point of the dam. Yet for areas downstream that rely on the flow of water that could be a problem.

Hoarding the water further upstream means less water for crops and plants that rely on it downstream.

Yet beavers can also cause temporary flooding to areas. Usually, this is due to moving around their construction materials, damaging or blocking pipes, or the dam causing an overflow.

For farmers that live near a fresh water source, this will be a huge problem for crops. Large fields that normally rely on the water source will produce poor crop quality and yield if they are dry or flooded. In turn, this impacts the farmers’ livelihood.

How big are beaver dams?

Beaver dams aren’t a standard size and will be constructed to meet the specific needs of each waterway. We’ve already discussed how beavers usually try to make the dam in the narrowest part of the water. This helps make it much easier for the beaver to build and maintain the dam.

The average dam is between 1-3 meters (3-9ft) tall, but it is not unusual for them to be as high as 5m (16ft). Dam length can range from a few meters across right up to hundreds of meters.

The largest beaver dam in the world is 350m (2790 feet) long in Canada’s Wood Buffalo National Park. It’s so big that it can be seen from space.

The average beaver dam will last for 2-3 years but if properly constructed and well maintained can last up to 30 years.

Final thoughts

Now you know that beavers build dams to control the water around them for a better home environment. Beavers don’t wait to find the perfect place to set up home, they go and create it for themselves.

The pools created from a beaver dam are the perfect environment to survive. Including protection from predators, places to store food, and winter protection.

Beaver dams are amazing feats of work that provide a lot of benefits to other animals and the surrounding environment.

Yet beaver dams can also cause problems for landowners and farmers, which can result in them being considered pests.

Regardless of whether beavers are a friend or foe, it’s an amazing achievement for an animal of its size.

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