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How deep is Loch Lomond?

How Deep is Loch Lomond?

Lots of visitors want to know how deep is Loch Lomond. It’s just one of these Scottish questions. It’s the third-deepest loch in Scotland, after Loch Morar and Loch Ness.

The depth of Loch Lomond was a questionfirst addressed by the Hydrographic Department of the Admiralty in Britain, who did the first surveys in the 1860s. The splendidly named Captain H.C. Otter used a lead-weighted line and a rowing boat for his soundings.

Loch Lomond panorama from Duncryne Hill, looking north

Loch Lomond panorama from Duncryne Hill, looking north.

This Loch Lomond panorama is a September pic., as you can see by the autumnal rowan berries. The Highland Boundary Fault runs through the islands in the middle distance. The loch is confined to a deep, narrow trench in the far distance. Closer at hand, you can see how it spreads out in the gentler terrain of the Lowland edge.



The Royal Society of Edinburgh was an enthusiastic supporter of efforts to find out more about the depth contours of many of Scotland’s lochs. However, as the 19th century went on, issues arose as to who would fund such a project.

In fact, you get the impression that somebody in the Treasury soon pointed out to the Admiralty that Loch Lomond wasn’t actually the sea and, as such, wasn’t going to have much in the way of battleships floating about in it, so could they please restrict themselves to salt-water surveys on the grounds of cost…

Anyway, the experienced oceanographer Sir John Murray took up the cause and went on to create the ground-breaking Bathymetrical Survey of the Fresh-Water Lochs of Scotland, 1897-1909 – a work that still has value today.

In it, we learn that 623ft / 190m is answer to the question about how deep is Loch Lomond. This survey is also, presumably, the original source of the oft-quoted other facts about Scottish lochs: such as the water volume of Loch Lomond is 92,805 million cubic ft (second largest in Scotland after Loch Ness); the surface area of Loch Lomond is 27.45 sq miles (largest in Scotland with Loch Ness second) and so on. (There are some more loch facts in the fact box, in the right column.)

However, there has been an update on our knowledge of the depth of Loch Lomond. Survey work by the British Geological Society in early 2009 found that the north of the loch is a deep trench with very steep sides, the result of glaciation.

How deep is Loch Lomond? Well, this is the shallow southern end.

Monster speculation at Balloch, south end of Loch Lomond.

Authentic local talk bubbles from Loch Lomond.
Read on below to find out why they’re looking for a monster


As well as answering the query how deep is Loch Lomond, the original Bathymetrical Survey also looked into another phenomenon seen in Loch Lomond as well as other lochs (lakes) right across the globe. There is an ancient saying about Loch Lomond as having ‘a wave without a wind, a fish without a fin, and a floating island’. The first part of the old saying may refer to a ‘seiche’ – a scientific term from a Swiss French dialect word. It now means a standing wave in an enclosed body of water. (Think of it as a ‘slosh’ – that’s the word that’s used for this phenomenon on the North American Great Lakes.) The key here is that the water body should be enclosed and that the wave is observed irrespective of the weather conditions.

As for the other two parts of the rhyme, personally, I like to think that the fish without fin is the Loch Lomond monster – that’s the one that never quite caught on in the way that our chum up on Loch Ness did. Many years ago, at the peak time for the old monster mania that did so much for tourism in the Scottish Highlands, there was even a report of a monster on Loch Lomond being seen from the footplate by a train crew. (This was in the days of steam and I expect it was only a seiche.)

Finally, as for the floating island, if it isn’t a monster hump (and, no, I’m not being serious at all here), then it might be the mats of vegetation that can break off the banks after waves or storms.

Signposts by Loch Lomond, at Tarbet

Signposts by Loch Lomond, at Tarbet.

Loch Lomond has been loved to bits, not just because of that well-known song ‘The Banks of Loch Lomond’ (words on that link) but because of the fact it lies within easy reach of the Clydeside conurbation, the geographer’s phrase for Glasgow and round about. The area is within the Loch Lomond and Trossachs National Park.

On Loch Lomond 'Cruise Loch Lomond' is just one of a number of operators. Top of Ben Lomond, centre

On Loch Lomond ‘Cruise Loch Lomond’ is just one of a number of operators. Top of Ben Lomond, centre

Naturally, in an area so beloved of tourists, the ‘infrastructure’ is well developed. There is a good choice of Loch Lomond cruises.

The depth of Loch Lomond is just one of a ton of Scottish facts on this site. Follow the link for more. Or for far more than you need to know about Scotland’s largest loch see our facts about Loch Lomond page. Or go back to the Scotlandinaweek home page.

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