The state of Scotland’s fresh waters
Scotland’s fresh waters are a diverse and vital resource, but tackling issues like sewage pollution, climate change, and habitat loss is crucial to ensure their health, from dams and water quality to river restoration and rewilding.
The nature of the resource
Scotland has a rich diversity of freshwater ecosystems ranging from shallow lochs, meandering rivers, and canals in the lowlands of the Central belt to deep lochs, reservoirs (usually for hydropower) and tumbling streams in the Highlands (Maitland et al., 1994). These vary greatly in their nutrient levels (from highly oligotrophic to eutrophic), and their biodiversity and conservation value. They also vary in the ways in which they have been exploited: as sources of food, mechanical or electrical power, means of transport or of heating/cooling, or for recreational activities. These uses have varied spatially and temporally, leading to a dynamic picture in which the status of Scotland’s fresh waters has been continually changing. In this article I will attempt to summarise the current status of the rivers, lochs and canals of Scotland and their future prospects in an era marked by changes in climate, in human activities, and in the pressures on our natural environment.
While the topography of mainland Scotland is commonly divided into three major zones (the Highlands, the Central belt and the Southern Uplands), it is more useful to consider five regions when considering the country’s hydrology. Two of these (the West Highlands and SW Scotland) have catchments draining to the west coast, while the remaining three (NE Highlands, Grampian, and Forth & Tweed) have catchments draining to the east. The presence of mountains close to the west coast results in the west-draining rivers being on average shorter and of steeper gradient, and having smaller catchments, than those draining into the North Sea; the Grampian region has both the longest river in Scotland (the Spey) and that with the greatest discharge (the Tay) (Smith and Lyle, 1994). Conversely, Grampian has the lowest percentage of land area covered by standing waters (i.e. lochs, lochans and reservoirs), while the region with the highest percentage on the Scottish mainland is the N.E. Highlands. However, here the waters are often small and very shallow lochans that form part of the Flow Country. If one considers volume rather than surface area, then the region with the greatest volume of standing water in Scotland is the West Highlands, which has a significant number of deep lochs (Lyle and Smith, 1994).
Scotland has a modest canal network, built in the late 17th century (partly with the aid of surveys done by the engineer James Watt) and early 18th century. Three of the five canals were built as short cuts: the Caledonian and Forth & Clyde canals allowed boats to travel between the east and west coasts without needing to navigate the treacherous Pentland Firth, while the Crinan canal avoided the need for boats travelling between the Clyde and the Atlantic to detour around the Kintyre peninsula. The Union and Monklands canals were built as extensions to the Forth & Clyde canals, primarily for the distribution of coal and other goods into the increasingly urbanised Central Belt. The decline of commercial traffic on the canals in the 20th century led to those in the Central Belt falling into disuse and becoming partially filled in, but a major renovation project resulted in the reopening of the Union and Forth & Clyde canals in the early 21st century, mostly for recreational purposes.
Impacts of dams and weirs
One of the major human impacts on Scotland’s fresh waters has been the building of weirs and dams to divert or hold back water. The first water-powered mills installed on Scotland’s rivers were to grind flour, but these were later joined by textile mills. Both involved installing weirs to divert the flow of water through the mill. While the mills are long gone, the weirs are often still present and are now recognised as a major barrier to the movement of migratory species such as fish. Even if the fish are capable of jumping over the obstacle their rate of progress is slowed (which has implications if they encounter multiple such barriers on their migration), and fish aggregating at the obstacle are more vulnerable to predators. This is a global problem (Belletti et al., 2020) – and it has even been suggested that the decline in European salmon populations was not due to overfishing or pollution during the 20th century, but happened much earlier in medieval times due to the installation of weirs associated with mills that reduced the ability of fish to reach their spawning grounds (Lenders et al., 2016). Removing these obsolete weirs is costly, complicated and time-consuming; it often involves the breaking up and removal of large volumes of concrete from the middle of a fast-flowing river, and the consequences for flood risk and habitat alteration both upstream and downstream of the weir need to be carefully considered. However, it has proved to be a very effective means of increasing the availability of spawning and nursery habitats for migratory fish populations such as salmon and sea trout, since the removal of a single barrier such as a weir in the downstream reaches of a river can make available tens of kilometres of prime river habitat (de Leaniz, 2008). The EU’s Water Framework Directive, with its requirement for ‘good ecological status’ (which includes the potential for migratory species to move freely), has provided a useful impetus for the removal of unnecessary barriers, but there are still many on Scottish rivers that could usefully be removed.
The topography and climate of Scotland, with its fast-flowing rivers and lack of a dry season (!), means that it is in the fortunate position of being able to generate significant amounts of renewable energy through hydropower schemes. Major hydroelectric power schemes were developed over the course of the last century, mostly in the Highlands; these generally involve the creation of large dammed reservoirs in the upper parts of a catchment, occasionally linked in sequence so that the same water can flow through more than one turbine. Perhaps the most complex is that on the River Conon catchment in the NE Highlands, which involves 8 reservoirs, 9 dams and 6 power stations, linked by a network of aqueducts and tunnels to make the maximum use of the water. However, any development that involves damming rivers inevitably comes at a cost to the ecology of that catchment, since the water flows are highly regulated (which changes the nature of the stream bed) and the dams are usually largely impassable to aquatic organisms. The salmon population in the Conon catchment has been maintained only by trapping as many of the returning adults as possible below the dams and using them to generate fertilised eggs that are held in a hatchery over winter and then distributed through the upper reaches of the catchment prior to hatching (McLennan et al., 2019). However, populations of less valuable migratory species (or those that require the greater instream disturbance of unregulated rivers) are not so lucky – this is one of the inevitable compromises involved in generating electricity through renewable sources. In recent decades there has been an increased interest in micro-hydropower projects, where relatively small amounts of electricity are generated, for local consumption, from small water discharges (i.e. involving either a stream or a small fraction of the total discharge of a river). These are less intrusive and are likely to have less of an ecological impact, since they do not involve creating complete barriers across rivers, but they may still change ecological conditions at a local level. An alternative approach is to generate electricity using pumped storage hydro schemes, in which water is pumped up to a storage reservoir at times when electricity generation is in surplus (e.g. at night) and then allowed to flow down through a turbine when electricity is needed. Such a scheme has worked very successfully for many years at Cruachan on Loch Awe, and there is a need for more such systems in order to buffer the fluctuations inherent in wind and solar-powered electricity generation. However, while several sites have been proposed and some have got as far as receiving Government approval (e.g. at Coire Glas near Spean Bridge and Glenmuckloch near Kirkconnel), the high capital costs and some vocal public opposition (perhaps misplaced) have so far prevented any from being built. This logjam needs to be resolved if Scotland’s aspirations to be a net exporter of green electricity are to be realised.
Impacts of changes in water quality and nutrient levels
Many water bodies in Scotland are presumed to have been of high water quality until the industrial revolution and the development of heavy industry in the Central Belt, which led to the heavy pollution of a number of important river systems such as the Clyde and the Forth. This wiped out populations of sensitive species, but the decline of this industry and greater regulation of discharges during the second half of the 20th century has often led to ecological recovery. A remarkable example of this is the return (with no human assistance) of Atlantic salmon to the Clyde: there were no salmon in the river for around 100 years, until one was caught by a (presumably astonished) angler in the city centre in 1983. The population increased steadily over subsequent decades and became self-sustaining, the distribution of salmon in Central Scotland is now more limited by the remaining impassable barriers than by any issues with water quality (Doughty and Gardiner, 2003). However, there is currently great concern over the continued decline in the size (rather than distribution) of salmon populations across Europe, including Scotland. This is despite the ending of most of the commercial fishing pressure and the imposition of conservation measures such as catch-and-release angling (Thorstad et al., 2021). It is recognised that much of the problem lies in poorer marine survival, likely due to climate change and altered ecosystems in the northern Atlantic (Thorstad et al., 2021). This is not something that can easily be fixed, and so attention in Scotland has therefore been focussed on restoring the freshwater environment so that the spawning success and early survival of the fish can be maximised, in an attempt to offset the greater losses at sea (Bernthal et al., 2022; Soulsby et al., 2024).
The biological productivity of fresh waters is strongly related to the levels of key nutrients, notably phosphorus (P) and nitrogen (N). Concentrations of these vary widely across Scotland: upland streams in the Highlands often have extremely low levels of both P and N, making them highly oligotrophic and nutrient-limited (Jarvie et al., 2018). This is partly because of the underlying geology, but also as a result of land use: the loss of trees and bushes from the riparian zone, and the prevention of any recovery due to overgrazing by sheep and deer, has removed a source of nutrients in the form of leaf litter (and also streamside invertebrates that fall into the water) (Bernthal et al., 2022). This can limit the productivity of the upper parts of catchments as a result of this ‘cultural oligotrophication’. Conversely, inputs of nutrients further downstream from agricultural runoff and sewage effluent have resulted in reduced water quality and eutrophication. Spills of untreated sewage into rivers are a problem which is increasing due to the higher incidence of storms (see next section) and underinvestment in the infrastructure needed to cope with them; the greater flows of waste water that occur during storms can result in treatment plants being unable to cope, so that they deliberately discharge untreated water into the river system. The problem of nutrient overloading is most acute in lowland water bodies, especially those near urban areas and areas of high agricultural intensity (e.g. lowland Grampian) (Anon., 2024). This can result in rivers and (especially) lochs having a risk of algal blooms (Bailey-Watts, 1994). Some of the larger catchments would thus benefit from receiving nutrients in their headwaters yet have nutrients removed from the lower reaches – a difficult balancing act (Bernthal et al., 2022). The overall quality of fresh waters in Scotland has improved over the last two decades, at least in terms of the number of reported environmental pollution incidents, but there are still problems of poor water quality in urban areas; the current trend is hard to determine (and comparisons with England impossible) due to a reduction in the level of monitoring by Scottish Water and the Scottish Environmental Protection Agency (Anon., 2024).
Climate change is affecting Scotland’s fresh waters
Climate change is already causing changes to Scottish fresh waters. The general climatic change (both recent and forecast) in Scotland is a trend towards warmer temperatures and greater annual rainfall, with the biggest increases in precipitation in the autumn and winter months, and a greater likelihood of extreme weather. This is leading to a number of trends in river flows and loch/reservoir water levels, such as an increased mean discharge in autumn and winter, a greater incidence of storms, spates and floods, and conversely a reduced likelihood of freshets due to snow melt in the spring (Werritty, 2002). Meanwhile shallow water bodies such as lochans and streams are likely to experience a higher risk of summer heat waves, exacerbated by low water levels, and even main-stem rivers are at risk of experiencing temperatures that are stressful to many coldwater-adapted species (Jackson et al., 2021). One such species is the Freshwater pearl mussel Margaritifera margaritifera. It is a remarkably long-lived species (individuals living to be over 100) but requires cool and unpolluted waters, and is impacted by silting of rivers due to agricultural or forestry run-off. It has gone extinct in many Scottish rivers over the last century, and while fully protected from pearl fishing since 1998, many populations are ageing due to no recent recruitment, so the long-term future of the species in Scotland is fragile (Cosgrove et al., 2016).
Changes to the climate are already causing multiple problems in Scottish fresh waters. From a human perspective, the management of water levels in reservoirs becomes more complex, and there is an increased flood risk to properties and sensitive agricultural land. Recent incidents of high stream temperatures are already implicated in the mortality of young fish, even in northern Scotland (Bernthal et al., 2023). Summer drought can also cause the drying up of peat bogs and lochans, with the loss of sensitive bog species and the release of stored carbon. Scotland has lost many areas of bog and mire through drainage for agricultural or forestry use, and it is important that we retain (and where possible restore) water levels in the remaining areas. These areas are of international conservation significance: the Flow Country was recently designated a UNESCO World Heritage Site – the first peatland in the world to be awarded that distinction, and the first location in Scotland to be given World Heritage Site status purely for its natural (rather than cultural) value.
A separate cause of environmental change is the spread of invasive non-native species (INNS). The invasive plants particularly associated with Scottish fresh waters are Himalayan balsam Impatiens glandulifera (found along river banks), Giant hogweed Heracleum mantegazzianum (damp ground including the riparian zone) and American skunk cabbage Lysichiton americanus (damp woodland and pond margins). All three have the potential to form dense plant cover, excluding native species. Meanwhile Signal crayfish Pacifastacus leniusculus are slowly spreading northwards, and are found in both flowing and still water, while a number of non-native fish species including the Ruffe Gymnocephalus cernua are well established in some water bodies, notably Loch Lomond (Winfield et al., 2011). The American mink Neovison vison spread across many water bodies in Scotland. All of these animals have been found to have an adverse effect on native species, either through competition or predation. Humans were responsible for deliberately introducing most of these INNS, and have found it almost impossible to subsequently remove them (a rare exception being the very successful programme of removing American mink from large areas of northern Scotland (Bryce et al., 2011)). We really need to be more careful about moving species around the globe – for instance, even in an era of greater awareness of INNS there are still very few restrictions on which non-food and non-forestry plant species can be brought into the UK to satisfy the demands of gardeners.
River restoration and ‘rewilding’
In recent years there has been much interest in restoring river ecosystems that have been degraded through excessive canalisation, removal of riparian vegetation or natural substates and installation of barriers and hard banks (Dadson et al., 2017; Wohl et al., 2015). Restoration can have many benefits apart from simply increasing biodiversity. ‘Natural’ rivers are generally thought to reduce the risk of damaging flooding, by allowing water during spate conditions to spill onto floodplains in the upper parts of a catchment rather than flood urbanised areas further downstream. While there are still places where a ‘hard engineering’ approach of concrete flood defences is deemed necessary, there is an increasing acceptance that a more natural approach may have greater long-term resilience. An exemplar of this is the Eddleston Water Project, where interventions such as riparian tree planting, flow-attenuation ponds and re-meandered sections of the river were made across this catchment in the Scottish Borders and their effectiveness monitored – in terms of both flow regulation and benefits for biodiversity (Spray et al., 2022). The development of a riparian corridor of trees can help restore nutrients to denuded upland streams (Bernthal et al., 2022) and can also reduce the impact of damaging heatwaves by providing shade (Dugdale et al., 2024; Jackson et al., 2021); planting riparian trees also potentially stabilises the banks, so reducing soil erosion, and captures carbon. However, this tree planting needs to be done in an appropriate manner. Commercial forestry practices, which typically involve the digging of drainage ditches and major earthworks both when planting and clear-felling, can result in the washing of sediments into water courses, leading to silting of bottom substrates and loss of key invertebrate taxa (Best, 1994), with adverse effects on fish populations (Egglishaw et al., 1986). Planting of alien conifers can also cause water acidification (Best, 1994), and so the ideal practice is to only plant native species and to do so by hand.
Meanwhile the reintroduction of beavers Castor fibor to Scotland is causing changes to river and wetland habitats through their role as ecosystem engineers. While beavers can cause localised problems, their impact is generally beneficial, as they create a greater complexity of habitats that supports a higher biodiversity (Law et al., 2017; Willby et al., 2018). However, the enthusiasm for ‘rewilding’ of rivers is not without its critics, who point out that some interventions, carried out without any scientific evaluation of their effectiveness, can potentially cause more harm than good (Soulsby et al., 2024).
Recreational use of Scotland’s fresh waters
Scotland’s rivers, lochs and canals provide many wonderful recreational opportunities, including angling, kayaking, sailing, wild swimming, and jet skiing. The impact of these activities on the natural environment is relatively minor, either because they are regulated (e.g. angling) or because they are concentrated in a few locations (e.g. Lochs Insh, Lomond and Morlich, or canoeing at Grandtully on the River Tay). Even within one heavily-exploited site the use is patchy: on a sunny weekend the southern end of Loch Lomond can seem highly congested with recreational craft, but their use is regulated (in terms of boat registrations, speed and area restrictions) so that the northern end of the same loch is almost deserted (Adams et al., 1992; Dickinson, 2000).
In summary, Scotland’s fresh waters provide a varied and valuable resource that has been used in many different ways over time. However, they need to be managed carefully. One of the greatest current concerns is the sewage levels in some rivers, and the fact that this is barely being monitored (much less so than in England or Wales (Anon., 2024)), which might explain why sewage in Scotland’s water bodies has received so much less media attention than the equivalent problem in the rest of Britain. While there have been localised improvements in aquatic habitats and a greater appreciation of the benefits that a restored riverine habitat can provide (spurred by the Riverwoods initiative), the decline in Scotland’s salmon populations (categorised by the IUCN as ‘endangered’ since 2023) is an indication of the threats to aquatic biodiversity resulting from both climate change and habitat loss. We need to do more if these vital ecosystems are to survive.
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