Lake, Mere, Tarn and Water – The Difference

A question I am often asked in the hiking world especially when it comes to the Lake District. What is the difference between a lake, a mere and a tarn? You can go further and see that some lakes are called water, not lake. And when does a pond become a lake?

The famous trick question. How many lakes are there in the English Lake District? The answer (for pedants) is one. Well, one that is ‘called’ a lake, Bassenthwaite Lake. The rest are called mere and water etc like Windermere.

I thought it would be good to explain here a lot of the differences, whilst trying to not get too technical. There is a clue to a lot of it in the above riddle and answer, etymology. Our language and the naming of things has derived down the ages from so many influences from Vikings to Germanic for example. Let us take a look.


A definition, a lake is a static or slow moving basin of water surrounded by land apart from the river or rivers that flow in and out of it. That’s it really. You could call nearly all of them all lakes technically.

The first question should be, what is the difference between a pond and a lake, when does it become a lake? There is no rule that defines it ‘properly’. The nearest scientific answer you get is a pond is standing water, and small enough to allow photosynthesis to rooted plants at the bottom. What we know as lakes, are too deep, too cold and the sun can’t get all the way down, so it is a lake not a pond.

So, the largest lake in England is? Windermere, here we go.


Windermere, Grasmere, Buttermere, Martin Mere to name just a few examples. Technically a mere is a lake that is really shallow in relation to its size (breadth). Take Martin Mere for example, if you have visited it you see a large body of water, yet its depth only ranges from 100 to 150 mm (4 – 6 inches.

Science will tell you also a mere is a lake without a Thermocline. Without trying to get too technical, a simple way to explain that is, think of a layer of water that separates the rough cold water at the bottom and the calm warm water at the top. It cannot occur until certain depths are in place and the temperature is right in the atmosphere.

Windermere is a complicated one because it is not as shallow as many meres and in ‘some’ warmer parts of the year it has a thermocline, but not always.

Etymology has played a part here too. The word mere comes from Old English ‘mere’ which meant lake or ‘sea’ in Old Saxon, a broad term for a body of water. Time and many many generations and language differences can make it all more confusing.

Take the Mediterranean Sea for example:

The Dutch say – Middellandse Zee

However the Germans say – Mittelmeer

Plus the Germans call a lake a sea, eg Bodensee or Zell am See in Austria

In language, mere and sea have diversified in time and geography, we just used it a certain way in general. A shallow but large lake.

Lake, Mere, Tarn and Water - The Difference


Hikers and walkers in Britain will know a tarn, high on the mountains and hills, a body of water that has been left over from the ice age. It is a lake, just a glacial lake. The word comes from Old Norse, Tjörn which meant ‘pool’.

Lake, Mere, Tarn and Water - The Difference

There are examples outside the Lake District for example Malham Tarn. Although I love that tarn, I will dispute the often used claim that it is the highest lake in Britain at 377 m. I would easily go for Red Tarn on Helvellyn at 718 m for starters.

For the pedants, weren’t Windermere, Derwentwater etc all glacial lake left over from the ice age? Are they tarns therefore? This is where etymology and geography and altitude have all played a part in the definition and confusion.


Derwentwater, Rydal Water etc. What about these beautiful and famous bodies of water. Lakes not called lakes but water. No science behind it, it all comes down to whoever named them way back in time most likely. Named as simple as possible.

Lake, Mere, Tarn and Water - The Difference

Bassenthwaite Lake

Let us go back one moment to Bassenthwaite Lake, the only ‘lake’ in the lake district and only because of how it was named. It was only called Lake since Victorian times!

Through history it was recorded as variations on Bastunthwaite Water and later Broadwater before only within the last couple of centuries changing to Lake on somebody’s whim. Perhaps a local land owner.

What About Lochs, Loughs and Llyns Etc?

Irish, Scottish, Gaelic and Welsh have all contributed to the many variations in names of our bodies of water in the UK.

Often, lakes in Wales are translated to Llyn (eg Llyn Tegid)

Ireland, Lough (eg Lough Neagh)

and in Scotland of course, Loch (eg Loch Lomond)

That is way too simple a definition of course. For instance in Scotland a loch is not just a lake but also a long sea inlet, that many would define as a fjord. A story for another day 🙂

Lake, Mere, Tarn and Water - The Difference


For me, all the lakes are lakes and all have differing names thanks to our diverse history and cultures arriving over time. Forget what the name is on the map and go out there and enjoy the beauty of them all. A name cannot take that beauty and memories created, away from a place.

Written by Paul Steele

Paul is the founder and Editor of the site. An avid hiker and trekker. Travel, adventure and photography are passions that he combines to make his articles here. Likes to see the positive in everything.


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  1. I hadn’t heard of mere either. But, I am across the Atlantic “pond” lol and in the center of that. There is a park in Nashville, Tennessee, named “Grassmere.” It’s not that big, but is a small zoo. Mom and I discovered a love for otters there. I don’t recall any body of water, but then, that was about 30 years ago that we went. This article is VERY helpful!

    • thanks for the kind words. Yes interesting the use has spread around the world in places 🙂

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