Of glacial origin, Loch Lomond is probably the best known Scottish lake and is the most southerly of the five Scottish lakes considered in this report. Its main axis is north to south and maximum length of 36.4 km. The northern portion of the lake is long (22.3 km) and narrow (average just over one kilometer), but south of Ross Point it opens out and reaches a maximum width of 8.8 km. This part of lake contains many islands. The surface area of the lake is 71.1 km² - larger than any other British standing water, the nearest rivals being Loch Ness (56.4 km²) and Loch Awe (38.5 km2). The maximum depth is 189.9 m (the third deepest in Scotland) but the mean depth is only 37.0 m, because of the influence of the large shallow southern basin. The volume of Loch Lomond is 2,627,900,000 m³, greater than any other lakes except Loch Ness. There are two distinct basins deeper than 30 m in the lake, one in the south of the Douglas Water delta and one in the north and these are known to have quite distinct characteristics.

Loch Lomond's catchment is characterized by a relatively low mean altitude and gentle slopes with a high percentage of arable ground and base-rich rocks. There are far more roads than in any of the other catchments and relatively high population. The extent of arable ground and base-rich rocks gives a good indication of the potential natural richness of the lake waters draining from them. The lake, or certainly its south basin, seems likely to be the richest, comparing with the other four Scotland lakes. The lake exhibits a dimictic circulation and is rated oligotrophic in respect of chemistry, phytoplankton and macrophytes, but mesotrophic in terms of invertebrates and fish (1, 2, 4, 10).