Backcountry Science: Why Don’t Streams Freeze in Winter?

Today’s post is the first in a recurring series of pieces on the science behind common natural phenomena experienced in the Appalachian backcountry. Each post will take a seasonal theme, with this post the first in a series of pieces on the science behind winter in the southern and central Appalachians. Other posts in the series can be found as they are added by browsing the “backcountry science” category tagged to this post.

It’s something almost every hiker has seen when they’re out in the woods during an Appalachian winter: you’re trudging along through several inches of snow and come across a stream crossing. Instead of being frozen over, though, the stream is flowing…forcing you to make a feet-wet ford with temperatures sometimes well below the freezing mark. What’s going on here? Why do so many Appalachian streams never seem to freeze?

In reality, this is a bit of a trick question – technically, any stream can freeze. Even an icicle, after all, is simply a very slow-moving, minute stream of water that has frozen solid. The real question one has to think about when pondering why some Appalachian streams stay liquid while others turn to ice is instead under what conditions any stream will actually freeze over.

Understanding how to address this question requires a little bit of basic physics. First off, it requires a relatively high amount of energy to cause a change in water temperature, thanks to a property known as specific heat. In terms of freezing, this means that the temperature of a body of water will generally remain a bit warmer for a bit longer than will the air or land temperature around that body of water. Think about it: you’ve probably already experienced this same phenomenon in the reverse direction. What happens if you jump into a pond or river just a week or so into the changeover to warm weather in the spring? Even if the air temperature has warmed considerably, it takes a bit more time – and more sustained warming – for the water temperature to catch up. This means that, generally speaking, a body of water will not freeze completely over until temperatures have been well below freezing for a sustained period of time.

But wait – this still doesn’t make sense. Even if it takes a bit longer for water temperatures to drop to the point to allow a stream to freeze, shouldn’t that mean that all streams should freeze over at roughly the same time? Why would a large mountain river stay flowing while its smaller tributaries freeze solid? The answer again lies in physics. For any stream of moving water to freeze, it takes more than a simple drop in temperature. Heat must be lost at a rate that exceeds the rate of replacement: the rate at which flowing water is replaced by water of potentially higher temperatures upstream.

Because it takes such a relatively high amount of energy to cause a change in water temperature, fast-moving water (such as that in a waterfall) often never sees heat loss exceed flow rate to a point where the stream will freeze over completely. This requirement explains why still ponds and sluggish streams tend to freeze quickly, while nearby mountain cascades do not. In reality, freezing becomes a close dance between air temperatures and streamflow. The cooler the temperatures and the slower the flow, the more quickly a stream will freeze. Add depth into that mix – the deeper the water, the more buffered against freezing the bottom of the  water body becomes – and you can begin to see that stream dynamics become a bit more complex than whether or not the temperature is below 32 degrees outside.

The aforementioned characteristics, among many others, also make water uniquely suited to harboring life. Because it takes a relatively high amount of energy to heat water, aquatic habitats are a remarkably stable place (chemically speaking) for organisms to thrive. In fact, the same characteristics mentioned above also allow many aquatic organisms to survive harsh winters by spending the colder months at the bottom of streams and ponds, buried in mud and buffered against the much harsher temperatures near the surface. Moisture is also one of the major climatological features driving our region’s incredibly high salamander diversity, and some parts of southern Appalachia harbor more aquatic organisms than any other river systems of comparable size in North America. In this sense, water forms one of the region’s most important natural features – frozen or not.

Get out and experience the science mentioned in his post yourself with hikes in and along central Appalachian streams:


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