ECN site code: L10
ECN monitoring by: Scottish Environment Protection Agency

The Loch Lomond basin is of glacial origin, formed by an ice sheet moving southward from the Ben Lui area and depositing eroded material in the southern-most part by Balloch, thus ensuring that the loch was freshwater loch rather than marine. It is the largest (by surface area) body of freshwater in Britain, with a surface area of 71km2. The natural catchment area is ten times greater, at 781km2.

The two main feeder rivers are the River Falloch at the northern-most point, with a mean flow of 6.8 cumecs, and the River Endrick entering on the south-eastern side of the loch, with a mean flow of 7.8 cumecs. They have markedly different catchments - the Falloch's is mountainous with a catchment area of 80km2, whilst the Endrick's is a typical lowland rural catchment of 220km2. There are distinct differences in the chemistry of the two rivers, reflecting the differences in the geology of their catchments. The Highland Boundary fault cuts across the lower part of Loch Lomond, but there is also a narrow physical restriction halfway down the length of the loch. For these reasons, the water chemistry and topography are quite different between the so-called Northern and Southern Basins and, as a result, there are two ECN sampling sites, one from each basin.