Summer Temperatures of Our Local Rivers Revealed

Jon Bass

River temperature is a key factor influencing the life below the surface in our rivers. Temperature can directly affect growth rates in fish, plants and algae, whilst indirectly affecting decomposition rates of nutrients and oxygen levels. The cool water flowing up from the ground to feed out chalk streams results in relatively warm winter and cool summer temperatures. However, different factors come into play downstream which can alter the relatively stable temperature regime of these unique rivers, including air temperature and a whole host of human influences.

In general, river temperature trends follow prevailing weather conditions and relate to site location within the catchment and reach-specific characteristics. Particular local features such as diversion into small channels, tree-shading, and the amount of river weed can all play their part, and together with unseasonable flood events can modify annual patterns. Some of our most iconic chalk stream fauna, such as the salmon and trout, require cool, stable river temperatures to thrive, but with summers becoming increasingly hot, it's important we work together to increase the resilience of our rivers to the effects of climate change. 

Tracking dynamic changes in river flora and fauna has become more sophisticated, however major government cuts in monitoring resources create uncertainties as to the current state of river reaches. Step forward 'Citizen Science' delivered by NGOs. It is useful to explore historical and new environmental data in conjunction with temperature data recorded every 15 minutes by Wessex Rivers Trust. These data are stored at relatively low cost and downloaded each autumn from about 20 sites, including the Avon, Test, Itchen Allen, Dun, Anton and Meon.

Highlights of summer 2020
Figure 1. A short sequence of exceptionally warm days in June 2020 saw daily river temperature climb briefly. An example of changes in daily river temperature are provided from Fullerton on the River Test.

June had a brief and intense heatwave towards the end of the month drove river temperatures to unseasonably high levels for a few days (Figure 1). At some monitoring sites logger placement was delayed by a few days in early June 2020 so for those sites the mean temperature for June is unknown.

July normally provides the highest monthly average temperature at all sites plus annual peak river temperatures, but this was not the case in 2020.

August included a very warm week with successive daily warming to record annual peaks at most of our river monitoring sites. This pushed both average and maximum values above the figures recorded in July.

September was typical in terms of river temperatures observed over the last 10 years across our monitoring sites.

The overview of average monthly river temperature at all sites from June to September (Figure 2) displays no clear trends over the period 2011-2020, but it is worth noting the early years had data from fewer sites. Highest annual temperatures were recorded in July except where stated.

Figure 2. Average monthly river temperature at all sites from June to September. Note a variable number of sites were monitored and the display excludes months when there were fewer than 8 monitoring sites.

Our river temperature data sets are freely available on request, and we welcome exploration and invite collaboration of other organisations and individuals.