Lake Banyoles, the main lake of Catalonia, is located in the northeast of Iberian Peninsula, near the town of Girona (100 km from Barcelona). The lake belongs administratively to the City Council of Banyoles. It is the largest lake in the complex karstic system of Banyoles, with a surface area of 1.12 km2 and a maximum depth of 46.4 m. It is the last remnant of a much bigger lacustrine system developed during Pliocene Quaternary periods, and was formed due to the dissolution of underground gypsum bed and the collapse of overlying layers.

Lake Banyoles has a complex bathymetry because it is an assemblage of six basins (polje), formed by different episodes of collapse and grouped in three major lobes. In some of these basins, water is incoming from the bottom through different underground sources (up to 12) and maintain marly and argillaceous basement materials in suspension, forming a discrete layer of high density (1,100 1,400 kg m-3). The temperature of this slurry is nearly constant through the year (18 19deg C) and higher than that of the overlying hypolimnetic water. Therefore the lake is continually heated by the incoming groundwater, which considerably decreases the heat budget. The bottom inflow is about 40,000 m3 day 1, accounting for more than 80% of the whole inflow to the lake. From the east side of the lake five channels, constructed during the Middle Ages to canalize water and feed nearby marshes, flow out.

The basins differ considerably in water quality, physical conditions and mixing dynamics. Whereas the basins C III, C IV are meromictic, the basins C I, C II, C V, and C VI are holomictic. All of them have only one period of stratification during summer. The hardness of the lake water is considerably high, due to carbonates and sulfates loaded by subterranean water. In late summer, the bottom water in the basins of North Lobe becomes completely anoxic, and accumulates sulfide due to sulphate reduction. This process has been increasing in recent years, evidencing the advance of cultural eutrophication caused by excess fertilizer application on crop fields in the lake's catchment area.

The lake is used as a source of potable water for the town of Banyoles with a population of about 13,000, located to the east of the lake outside its catchment area. Recreation, sightseeing and tourism are also the use of the lake of great importance. Rowing events of 1992 Olympic Games were held here, and works for improving rowing track and restoring reed belt are currently going on in the lake (Q, 7, 9, 10, 11, 14).