Current and Projected River Temperatures 

River temperature has a major influence on ecosystem health, with extreme conditions risking damage to biodiversity. In unsuitable temperatures, biogeochemical cycles, water oxygen content (Loerke et al, 2023), and organism lifecycles are all negatively affected (Pohle et al, 2019). With continuing climate change, river warming could become highly detrimental in Scotland as the country supports a wide array of cool-water species (Scottish Government, 2021). River warming could also reduce the economic potential of Scottish rivers, with whisky distilleries and salmon anglers relying on suitably cool waters (Loerke et al, 2023). 

Current Temperatures:
The estimated maximum river temperatures on the warmest and coldest days of 2015-16 are shown in figure 1 (Jackson et al, 2018). The North and West of the country experience a wider range of river temperatures than elsewhere, ranging between 24.5°C and -0.5°C around Caithness and Sutherland. In contrast, along the East Grampian coastline river temperatures vary between about 20°C - 3.5°C. Looking more closely at Aberdeenshire’s Dee catchment in figure 2, inland rivers are warmer in the summer, while more coastal rivers are warmer in the winter, due to the regulatory temperature effect of the ocean on coastal locations. 

An Ongoing Trend:
Scotland’s rivers have been warming over the last century. The Spey for example has been on an upward temperature trend for atleast the last 100 years, reaching a rate of warming of 0.02 K / year between 1961 and 2015 (Pohle et al, 2019). This trend can be identified across the world, with global average decadal streamflow temperatures increasing at a rate of 0.16°C from 1960-2014 (Loerke  et al, 2023). 

Future Projections:
While many factors influence river temperature, air temperature is thought to be the primary driver (Loerke et al, 2023). Air temperature is predicted to increase across the country, having risen by 1.25°C since the mid-late 20th century (RMetS, 2024). Thus, being so closely connected, river temperatures are quickly following suit. To read more about these increasing air temperatures, please see the sheet 'Scotland's Projected Temperature'. 

Under a high emissions scenario, Loerke et al (2023) predict all of Scotland will experience increases in river temperature, with the most extreme change occurring in the north and northwest. In these areas, river temperatures may rise by over 4°C by 2051-2080. Figure 3 demonstrates their findings based on annual and summer readings, with each assessed catchment predicting rising temperatures. These findings are backed by Jackson et al (2018) in data published on the NMPI interactive map (Scottish Government, 2017), shown in figure 4. This map again suggests that northerly and northwest regions will experience the greatest increase in river temperature, rising by 0.8°C should air temperature rise by 1°C. 

The data from Loerke et al (2023) suggests summer river temperatures will soon creep dangerously close to the ecological thresholds of certain species, such as the 23°C threshold for Atlantic Salmon wherein juveniles may abandon these habitats (Scottish Government, 2021).