Garda needs 26.8 years to renew its water

Lake Garda takes a theoretical 26.8¹ years to renew its water, against 4.1 years for Lake Maggiore and 4.2 for Lake Iseo, so what enters Garda stays there for decades.

26.8 years against 4.1 and 4.2

The theoretical renewal time of Garda is 26.8¹ years. Maggiore does it in 4.1 and Iseo in 4.2.

That is more than 6 times slower than either neighbor, and it is the single most consequential number about the lake. A pollutant that goes into Maggiore is largely gone within a presidential term. The same substance in Garda is a problem for a generation.

It also cuts the other way, which is less often said. Garda's inertia makes it slow to degrade and slow to recover, so improvements take as long to show up as damage does.

49 cubic kilometers is the reason

Garda holds around 49 cubic kilometers of water over a surface of 369.98² square kilometers, and the Mincio carries a mean 58.4² cubic meters per second out of it.

Divide a very large volume by a modest outflow and you get 26.8¹ years. There is no mystery in the mechanism: the lake is deep, wide and drained through a single river.

The catchment feeding it covers 2,260² square kilometers and rises to 3,556² meters, with the Sarca as the main inflow. Everything that happens in that basin arrives here and then stays.

346 meters, or 350: 2 official answers

The Comunità del Garda, the body of the lakeside comuni and provinces, publishes a maximum depth of 346² meters. The first OLL report of 2004, produced by the Lombardy Region with ARPA, the Fondazione Lombardia per l'Ambiente and CNR-IRSA, gives 350² meters.

We record 346² m, because the source is more recent and closer to the lake, and we record the disagreement rather than hiding it. It has not been resolved.

The other dimensions are not in dispute: 51.6 km² long, up to 17.2 km² wide, 158.4 km² of shoreline, mean depth 133.30² meters, surface at 65 meters above sea level.

In 2023 Maggiore only mixed to 60 meters

The winter mixing of Lake Maggiore reached 60 meters in 2023, staying inside the top 100³ meters for the tenth consecutive year, and oxygen in the deep water hit the lowest values in the historical series.

Winter mixing is how a deep lake breathes: cold surface water sinks and carries oxygen down. When the surface no longer gets cold enough, the bottom is cut off from the atmosphere.

Ten consecutive years is not a bad winter. It is a regime, and it is the measured local face of a warming climate on water bodies that people swim in.

What a 26.8-year lake means for a visit

Nothing about the renewal time changes your afternoon: Garda's 158.4 km² of shoreline are swum, sailed and cycled exactly as before.

What it changes is how you read the arguments you will hear locally, about sewage plants, about lake levels in a dry spring, about agricultural withdrawal for the Mincio. In a lake that turns over every 4 years those are seasonal disputes. In a lake that takes 26.8¹, they are decisions with a 30-year tail.

That is the honest reason to put a limnological number on a travel page. It is not a warning about the water you are about to swim in. It is the timescale on which this particular lake answers for what is done to it.

Where these numbers come from, and what is missing

The morphometry here comes from a single primary source, the Comunità del Garda, which is why the depth is recorded as 346² meters with the 2004 alternative flagged beside it.

We could not obtain independent confirmation. The CNR-IRSA limnological database resolves to a working address but the connection times out, so the second reading that would settle the 346² against 350² was not available.

The 2023 Maggiore figure is an annual measurement and will be superseded by the next monitoring report. The 26.8¹ years, by contrast, is a structural property of a lake 346² meters deep with 1 outflow, and it will still be true when everything else on this page has been revised.

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