Loch Lomond

Loch Lomond (; ) is a freshwater Scottish loch which crosses the Highland Boundary Fault (HBF), often considered the boundary between the lowlands of Central Scotland and the Highlands. Tom Weir. ''The Scottish Lochs''. pp. 33-43. Published by Constable and Company, 1980. Traditionally forming part of the boundary between the counties of Stirlingshire and Dunbartonshire, Loch Lomond is split between the council areas of Stirling, Argyll and Bute and West Dunbartonshire. Its southern shores are about northwest of the centre of Glasgow, Scotland's largest city. The Loch forms part of the Loch Lomond and The Trossachs National Park which was established in 2002. From a limnological perspective, Loch Lomond is classified as a dimictic lake, meaning it typically undergoes two mixing periods each year. This occurs in the spring and autumn when the water column becomes uniformly mixed due to temperature-driven density changes

Loch Lomond is long and between wide, with a surface area of . It is the largest lake in Great Britain by surface area; in the United Kingdom, it is surpassed only by Lough Neagh and Lough Erne in Northern Ireland. In the British Isles as a whole there are several larger loughs in the Republic of Ireland. The loch has a maximum depth of about in the deeper northern portion, although the southern part of the loch rarely exceeds in depth. The total volume of Loch Lomond is , making it the second largest lake in Great Britain, after Loch Ness, by water volume.Due to its considerable depth and latitudinal location, Loch Lomond exhibits thermal stratification during the summer months, with a distinct epilimnion, metalimnion, and hypolimnion forming in deeper areas. These stratification patterns have important implications for nutrient cycling and aquatic ecology within the loch. During periods of stratification, a decrease in hypolimnetic oxygen can occur in the deeper northern basin, which can affect the species distribution patterns.

Loch Lomond is considered oligotrophic (north of HBF) to mesotrophic (south of HBF), with relatively low to moderate nutrient levels and generally good water quality, although localised pressures such as agriculture and tourism can contribute to nutrient enrichment.

The loch contains many islands, including Inchmurrin, the largest fresh-water island in the British Isles. Loch Lomond is a popular leisure destination and is featured in the song " The Bonnie Banks o' Loch Lomond". The loch is surrounded by hills, including Ben Lomond on the eastern shore, which is in height and the most southerly of the Scottish Munro peaks. A 2005 poll of '' Radio Times'' readers voted Loch Lomond as the sixth greatest natural wonder in Britain.

Etymology

Loch Lomond takes its name from the highest peak in the area, Ben Lomond, , meaning ‘beacon mountain’ i.e. a site for warning beacons. However, the region in which the loch is located is anglicised as Lennox and Loch Lomond "was of old ''Loch Leamhna''" after the River Leven which means ‘elm water’ in Gaelic.

Formation

The depression in which Loch Lomond lies was carved out by glaciers during the final stages of the last ice age, during a return to glacial conditions known as the Loch Lomond Readvance between 20,000 and 10,000 years ago. The hydrology and structure of the loch are strongly influenced by its glacial origins and underlying geology. Formed during the last glaciation, the loch occupies a classic glacial trough, carved by ice movement from the Ben Lui region. The loch lies on the HBF, and the difference between the Highland and Lowland geology is reflected in the shape and character of the loch: in the north the glaciers dug a deep channel in the Highland schist, removing up to 600 m of bedrock to create a narrow, fjord-like finger lake.This area is underlain by ancient Precambrian and Cambrian schists and quartzites, which contributed to the formation of the deep and narrow northern basin, reaching depths of up to 190 metres . Further south the glaciers were able to spread across the softer Lowland sandstone, leading to a wider body of water that is rarely more than 30 m deep. The southern part of the loch features younger Devonian sandstones and volcanic rocks, especially near the Campsie Fells, resulting in a broad, shallow basin. In the period following the Loch Lomond Readvance the sea level rose, and for several periods Loch Lomond was connected to the sea, with shorelines identified at 13, 12 and 9 metres above sea level (the current loch lies at 8 m above sea level).

The change in rock type can be clearly seen at points around the loch, as it runs across the islands of Inchmurrin, Creinch, Torrinch and Inchcailloch and over the ridge of Conic Hill. The loch's shape and many of its islands follow the line of the HBF separates the ancient metamorphic rocks of the Highlands from the younger sedimentary rocks of the Lowlands. To the south lie green fields and cultivated land; to the north, mountains.

Catchment

Loch Lomond has a catchment area of approximately 781 square kilometres. The primary inflows include the River Falloch in the north, the Endrick Water in the southeast, and the Fruin Water, along with numerous smaller streams. The loch drains southward via its sole outflow, the River Leven, which eventually joins the River Clyde at Dumbarton. With a relatively short water residence time of approximately 1.9 years, Loch Lomond is a dynamic freshwater system shaped by both climatic and geological forces.

Rainfall varies significantly across the catchment, with the mountainous western Highlands receiving over 3,000 mm annually, while the southern lowlands average around 1,500 mm. This variation contributes to differences in river discharge and water chemistry throughout the loch. The loch's water quality is generally high, with low nutrient levels and good ecological status, although areas such as Luss Bay can experience temporary declines following heavy rainfall due to diffuse pollution from surrounding agriculture and forestry.

The contrasting sub-catchments of the River Falloch and River Endrick have a notable impact on water chemistry within the loch. The River Falloch drains a mountainous, base-poor catchment north of the HBF, characterised by steep slopes, high rainfall (averaging 2,842 mm per year), and acidic, nutrient-poor soils. This results in oligotrophic inflows with low nutrient and organic content, supporting lower productivity in the northern basin. In contrast, the River Endrick originates in a more fertile, base-rich catchment south of the HBF. It flows through improved grasslands and farmland, contributing higher nutrient loads and organic matter. This mesotrophic inflow raises productivity and nutrient concentrations in the southern basin, which also receives lower rainfall (around 1,500 mm per year).

Vegetation within the catchment is varied and reflects the topographic and geological diversity. In the north, upland moorlands and remnants of Caledonian pine forest dominate, while the south supports more managed landscapes, including agricultural land, riparian woodlands, and patches of deciduous forest. Native species such as Scots pine, birch, and oak are common, and the area supports a mosaic of heathland and bog habitats. Many areas are designated as conservation sites, including Special Areas of Conservation (SACs) and Sites of Special Scientific Interest (SSSIs), recognising their biodiversity and ecological importance.

Loch Lomond is also geologically unique due to its glacial landforms and subsurface features. The loch’s basin contains sedimentary layers that have accumulated since the last ice age, including the recently identified Inchmurrin Formation, formed by landslides after deglaciation. In some areas, sediment deposits reach depths of up to 127 metres. Between the major geological zones lies the Highland Border Complex, a narrow band of mixed metamorphic and sedimentary rocks representing an ancient ocean basin.

Human pressures such as diffuse agricultural pollution, bank modifications, and the spread of invasive species pose ongoing challenges. Restoration efforts focus on improving water quality and restoring degraded peatlands to enhance natural water retention and carbon storage. Luss Bay and other parts of the loch are designated bathing waters monitored by the Scottish Environment Protection Agency (SEPA), although swimming is not recommended after heavy rainfall due to runoff contamination.

Overall, Loch Lomond's hydrology is shaped by a complex interplay of glacial history, geological contrasts, climatic variation, and human influence. These factors create a highly diverse and dynamic catchment that supports a wide range of ecological, recreational, and conservation functions.

Islands

The loch contains thirty or more other islands, depending on the water level. Several of them are large by the standards of British bodies of freshwater. Inchmurrin, for example, is the largest island in a body of freshwater in the British Isles. Many of the islands are the remains of harder rocks that withstood the passing of the glaciers; however, as in Loch Tay, several of the islands appear to be crannogs, artificial islands built in prehistoric periods.

English travel writer, H.V. Morton wrote:

Writing 150 years earlier than Morton, Samuel Johnson had however been less impressed by Loch Lomond's islands, writing:

Flora and fauna

The Scottish dock (''Rumex aquaticus''), sometimes called the Loch Lomond dock, is in Britain unique to the shores of Loch Lomond, being found mostly on around Balmaha on the eastern shore of the loch. It was first discovered growing there in 1936 (else it grows eastwards through Europe and Asia all the way to Japan).

Powan are one of the commonest fish species in the loch, which has more species of fish than any other loch in Scotland, including lamprey, lampern, brook trout, perch, loach, common roach and flounder. The river lamprey of Loch Lomond display an unusual behavioural trait not seen elsewhere in Britain: unlike other populations, in which young hatch in rivers before migrating to the sea, the river lamprey here remain in freshwater all their lives, hatching in the Endrick Water and migrating into the loch as adults.

The surrounding hills are home to species such as black grouse, ptarmigan, golden eagles, pine martens, red deer and mountain hares. Many species of wading birds and water vole inhabit the loch shore. During the winter months large numbers of geese migrate to Loch Lomond, including over 1% of the entire global population of Greenland white-fronted geese (around 200 individuals), and up to 3,000 greylag geese.

In January 2023 RSPB Scotland released a family of beavers into the southeastern area of the loch under licence from NatureScot. The beaver family, consisting of an adult pair and their five offspring, were translocated from a site in Tayside, where beaver activity was having a negative impact that could not be mitigated.

One of the loch's islands, Inchconnachan, is home to a colony of red-necked wallabies.

Invasive species pose a significant threat to the biodiversity and ecological balance of Loch Lomond and its surrounding habitats. Several invasive non-native species have established themselves in the area, particularly along riverbanks and loch shores, where they outcompete native plants and disrupt local ecosystems. The most problematic invasive plant species in the Loch Lomond catchment include Giant Hogweed (Heracleum mantegazzianum), Japanese Knotweed (Fallopia japonica), Himalayan Balsam (Impatiens glandulifera), and American Skunk Cabbage (Lysichiton americanus). These species are targeted for control by local conservation organisations, as their rapid spread and dense growth can threaten native flora and fauna, destabilise riverbanks, and reduce habitat quality for wildlife. Efforts to manage and eradicate these invasives are ongoing, with volunteers and park authorities working to monitor, map, and treat affected areas to protect the loch's unique biodiversity.

Conservation designations

As well as forming part of the Loch Lomond and the Trossachs National Park, Loch Lomond holds multiple other conservation designations. 428  ha of land in the southeast, including five of the islands, is designated as national nature reserve: the Loch Lomond National Nature Reserve. Seven islands and much of the shoreline form a Special Area of Conservation (SAC), the ''Loch Lomond Woods''. This designation overlaps partially with the national nature reserve, and is protected due to the presence of Atlantic oak woodlands and a population of otters. Four islands and a section of the shoreline are designated as a Special Protection Area due to their importance for breeding capercaillie and visiting Greenland white-fronted geese: this designation overlaps partially with both the national nature reserve and the SAC. Loch Lomond is also a designated Ramsar site