system
Given the following post, respond as if you're commenting on Reddit:
human
Why do beavers build dams?

First: I'm unaware of an "ask an animal expert" subreddit, so I hope this still fits here. Second: The most common answer as discovered by google searching suggests that beavers are seeking calmer waters to build their home. This doesn't sound like the end of the story to me. Surely if a beaver simply wants  calmer water, they'd seek calmer water. I feel like there has to be more to the behavior, so I'm curious.
gpt
When asking why animals do things we can divide the question into proximate and ultimate causes.  For example, the proximate cause of you eating is that you are hungry.  The ultimate cause of you eating is that you need energy and nutrition to live and grow.

The ultimate cause of beavers building dams is to provide a suitable safe habitat for them to live.  Beavers live in lodges, and prefer to have the entrance under several feet of water to prevent predatory entry and allow the beaver to exit under the ice in winter.  Beavers are also faster swimmers than walkers, and are safer in the water.  

Where a suitable body of water and embankment exists, beavers often _won't_ build dams.  Instead they will just burrow into the bank and use the pond to access nearby trees.  But in a narrow, fast flowing stream or a location without an embankment, they will construct a dam to raise the water level several feet.  Then they can construct a lodge in the middle of the water with a suitable entrance.

So that's the ultimate reason; beavers need a safe place to live

But there's also a proximate reason.  Beavers are probably about as smart as your average mammal, which means they can't work through the rather abstract chain of logic listed above, any more than your dog knows it needs to eat because of energy and nutrients.  Beavers need a direct reason to build dams, a reason that will help them build dams in the right place and time and keep them repaired.

So what's the proximate reason?  Beavers have a strong instinct to stifle the sound of running water.  In a famous experiment a researcher recorded the sound of running water and played it out in a field near a beaver dam.  He came back a day later and found the speaker buried under a pile of mud and logs and debris until the sound couldn't be heard.  That's the basis of dam building.  If there's a shallow area with water running over it, they'll start dumping logs and twigs and mud on it.  If there's a gap in the dam with water splashing out, they'll fill it up.  All this  dumping debris to silence the sound of running water means that eventually the flow is stopped and a nice, calm pond remains.