Lake Caldonazzo

Lake Caldonazzo is the largest lake falling completely within the boundaries of Trentino. The lake basin, which is dammed up by the alluvial fans of the River Fersina to the north and the River Centa to the south, is hollowed out of crystalline schists, while the tributaries bring in mainly limestone sediment. The banks all the way round slope gently down to the water. Towards the end of January, the lake often freezes over (but hardly ever completely). The most important perennial tributaries are the Mandola in Calceranica and a source near the rocky little hill on which the little church of San Cristoforo stands. The River Brenta flows out of the lake, indeed Caldonazzo and Levico’s lakes are considered its sources. In the 1970s eutrophication due to an excess of nutrients caused a die-off of fish and reduced transparency to less than one metre. To bring the lake back into its original natural balance, an effluent collection system was installed around the lake, which conveys effluent previously discharged into the lake to a treatment plant downstream; since 1975 five “Limno” aerators have been used to pump air deep down into the water; in 1998 the anoxic bottom water started to be removed by transferring it to the river flowing out of the lake. These interventions have led to a considerable biological improvement: today the colour of the water tends more towards blue than green (3rd degree on the Forel scale); the degree of water clarity is good (a Secchi disc can be seen at a depth of 7 metres, with maximum depths of up to 9); the population of European whitefish has increased greatly in size and lake trout have acclimatised. The local Fishermen’s Association has set up fish breeding campaigns with artificial spawning in hatcheries from parent fish swimming up the Mandola tributary. Lastly, a bylaw of 1988 implemented in 1992 made the reed beds at San Cristoforo – two of the most pristine along the lake’s shores – provincial biotopes.