Mexico's largest lake is four and a half metres deep

A lake of eleven hundred square kilometres sounds like a deep body of water, and the image the mind forms is of an alpine lake: walls dropping sheer and a dark sheet whose bottom you cannot see. Chapala is nothing like that.

Here the figure that rules is not the surface but the bottom, and it explains the whole lake: its behaviour, its vulnerability and even the shape of the region around it.

01 A sheet, not a reservoir

Chapala measures some 1,112 square kilometres as a natural reservoir, with a total surface of around 114,659 hectares, 86 per cent of it in Jalisco and 14 per cent in Michoacán. It is by far the largest freshwater body in the country.

And its mean depth is 4.5 metres, with a maximum of 7. That is: less depth than a two-storey building is tall, spread over an area larger than many whole provinces.

The ratio between those two figures is what defines the lake. A sheet like that behaves differently: it warms and cools fast, it is stirred from top to bottom by the wind, it evaporates a very high fraction of what it holds and it changes size visibly from one year to the next.

02 A widening of the country's longest river

The explanation for that shape lies in how it fills. Chapala is neither a glacial lake nor a flooded crater: it is a shallow tectonic basin occupying the floor of a depression, and a river runs through it.

That river is the Lerma, one of the longest in Mexico, which collects water from much of the central plateau and reaches the lake's eastern end. Chapala works, in practice, as a widening of it.

That is why the lake's level depends less on the rain falling on it than on what reaches it by river, and that makes it sensitive to everything happening hundreds of kilometres upstream.

03 Where the water goes

The water leaves at the opposite end, in the lake's northwest, and in doing so changes name: from there on it is the Santiago river. The Lerma-Santiago pair forms one of the country's longest river systems, and Chapala is the hinge in the middle.

What is interesting is what the Santiago does next. The plateau where the lake sits ends at an edge, and the river takes advantage of it: it incises and excavates. The result is a canyon that in some stretches reaches 600 metres deep.

That canyon is not in the middle of nowhere. It runs along the northern edge of Guadalajara, and from several neighbourhoods you can walk to a viewpoint and see the river five hundred metres below.

04 And the step where the city is

With those two elements, the lake and the canyon, you understand where Guadalajara is and why. The city occupies a plateau at 1,566 metres, with the lake some fifty kilometres to the south and the river's cut to the north.

The relationship between the three is direct and current: Chapala supplies around 60 per cent of the drinking water the metropolitan area consumes. That is, the city drinks from the lake, and the lake depends on a river arriving from the other side of the plateau.

And the city's northern edge is, literally, the edge of the plateau. Guadalajara does not end in wasteland but in a precipice, which surprises anyone arriving expecting a plain.

05 What a flat lake does not forgive

The lake's geometry has a consequence visible on the ground: a very wide, very shallow body of water loses an enormous proportion of what it holds to evaporation, above all in a region where in April relative humidity drops to 46 per cent.

And the second consequence is geometric. In a flat basin, a metre of level translates into many square kilometres of shore appearing or disappearing. That is why Chapala's shoreline moves, and there are towns that in some years have the water at their feet and in others have it far away.

It also explains the floating vegetation covering parts of the shore and the water's brown look: a shallow lake stirred by wind keeps sediment in suspension and gives water that never looks clear.

06 How to travel it

The first practical consequence is that Chapala is not a lake to look at from close up, but from a distance and from above. From the shore you see a sheet with no far side; from any of the hillsides around it you grasp its shape at once.

The second is that it pays to travel the whole system and not only the lake: the Lerma's entry in the east, the Santiago's exit in the northwest and the stretch where that river incises into the canyon north of Guadalajara. It is the same water in three states.

And the third reorders the whole trip. If at each place you ask where the water comes from and where it is going, the region stops being a city with a lake beside it and becomes what it is: a very wide stretch of river, a plateau and a six-hundred-metre cut through which all of it drains.