Lake Como: why the lake is a Y, and why it is so deep

Lake Como is around 410 metres deep. Its surface sits just under 200 metres above sea level, which puts the bottom of the lake more than 200 metres below the level of the sea - in the middle of Lombardy, a long way from any coast.

The lake is long and narrow: 46 kilometres from Gera Lario in the north to Como in the south, and in places only 650 metres across. What makes it recognisable on any map, though, is the shape. It is an upside-down Y - one arm in the north that splits into two towards the south, with a promontory between them and Bellagio at its tip.

The depth and the shape have the same explanation. Ice.

Three branches and a promontory

The lake has three named parts. The northern one is the Colico branch, or simply the upper lake, and it is the most open of the three. The south-western arm is the Como branch and the south-eastern arm is the Lecco branch. The authority that manages the lake, the Autorità di Bacino del Lario e dei Laghi Minori, describes it in exactly those terms: an inverted Y, with Bellagio on the point of land that divides the three branches.

That is why Bellagio is the best-known place on the lake. Stand there and you are at the junction, looking at water in three directions.

Ice that dug below sea level

During the ice ages a glacier flowed south out of the Valtellina and the Valchiavenna, the valleys where the rivers Adda and Mera enter the north end of the lake today. Where the ice met the mountain in the middle of the valley it split and carried on in two troughs. Those troughs are now the Como and Lecco branches, and the mountain between them is the promontory with Bellagio on it.

A glacier erodes differently from a river. A river cuts down towards the level it drains into. A glacier is pushed forward by the weight behind it and can grind a valley out far below that level. When the ice melted, the valley was left deeper than its surroundings and deeper even than the sea, and water filled it.

A lake whose floor lies below sea level is called a cryptodepression. According to the basin authority, Lake Como at 410 metres is the fifth deepest lake in Europe. Other sources give a few metres more, but everyone agrees it is the deepest lake in Italy. The deepest water lies in the Como branch, between the villages of Argegno and Nesso.

An arm that goes nowhere

There is one detail that only makes sense once you see the shape from above. Water comes in from the north, but it leaves the lake in one place only: through the Adda at Lecco, at the end of the south-eastern arm. The Adda is the lake's only outflow.

So the Como branch - the arm the lake is named after - is a dead end. From the promenade in Como it looks exactly like the other arm. From above the difference is obvious: two troughs that look alike, but only one of them leads on to the plain.

The Como branch is also where you find the lake's only island, Isola Comacina, a fortified site in the Middle Ages.

Mountains straight into the water

The same ice that dug the depth also polished the sides. That is why the mountains around the lake so often rise straight out of the water with no broad shore in between, and why the villages sit wherever the slope allows. The shoreline runs to 170 kilometres, and the width varies from 650 metres to just over four kilometres.

The narrow, deep valley steers the wind as well. By day the breva blows up the lake from the south. At night the tivano comes down from the north. Both follow the same valley.

Why the lake reads so clearly in relief

On our Lake Como 3D topographic map the water is cut out of the relief. The window is centred on the point where the arms meet, with Bellagio, Menaggio and Varenna inside the frame and the mountains rising out of the lake all around. The terrain is 25 x 25 cm in a frame that measures 28 x 28 cm on the outside, in oak or black, with or without a name plate.

It is a good example of what makes a place readable in three dimensions. It is rarely height on its own. It is contrast: dark water against steep sides, and an outline that belongs to no other place. If you would rather have your own lake or valley, you choose the window yourself in FRMD by You.