Lake Mezzola (in Lombard Lagh de Mezzoeula) is located north of Lake Como, from which it is separated by the last course of the Mera river which flows in the western edge of Pian di Spagna, the last plateau of Valtellina formed by the deltaic sediments of the 'Adda and Mera itself. About 20% of Lake Como's water comes from this lake.
The Mera's inlet flow rate is around 20 cubic meters per second, outflow of around 24, thanks to the contribution of the other tributaries which, despite having a relatively short course, bring a fair volume of water to the lake.
In 2004 the Ministry of Health classified Lake Mezzola as non-swimming. The regional environmental agency in 2016 classified the chemical state of the lake as "not good", due to high concentrations of chromium and other metals.
From the government water health portal, in 2020 the samples in two points of the lake (in front of the Corsican campsite and South Arsena) gave excellent results for bathing.