Water temperature monitoring- shaded vs unshaded rivers

Monitoring is an important part of the Ribble Rivers Trust’s work for a variety of reasons. It helps us further our understanding of river systems, develop our understanding of the problems facing rivers and potential solutions, and to monitor whether the work we have done is having a positive impact on our rivers.

One of the activities we’re undertaking as part of Ribble Life Together is to monitor water temperatures, specifically the differences in temperature between shaded and unshaded reaches.

This is important because although we know that the water in unshaded rivers is higher in temperature, we do not know the extent of the temperature difference. With climate change expected to lead to overall warmer air temperatures and warmer, drier summers it seems prudent to gather data which will help us to see how these changes might impact our rivers.

Additionally, many species, in particular salmon and trout, find it difficult to survive in warmer water. Sudden changes in temperature can lead to mass fish kills and change the fish’s behaviour, making them more vulnerable to predators or secondary threats such as pollution. This is because warmer air has a reduced oxygen carrying capacity.

The shaded section of Bashall brook chosen for monitoring

In order to investigate the differences in temperature we have selected  several sites within the catchment which have comparable shaded and non-shaded sections along similar stretches of river. Here we will use water temperature probes to measure water temperature and an additional probe to measure air temperature. One of the sites selected was Bashall Brook.

The upstream site is at the bottom of a predominantly tree lined section of Bashall Brook, here the trees protect the river from temperature rises. In comparison, the second site, which is 1km downstream, has no tree cover.

The unshaded section of Bashall brook chosen for monitoring

Our loggers collected data over the space of several weeks and the results, as expected, showed that the unshaded stretch of water reached far higher temperatures than the shaded section. Over the monitoring period the shaded stretch reached a maximum of 18°C, whereas the unshaded stretch reached 27°C, which was much more reflective of air temperatures and showed that the warming effect can occur over just 1km of unshaded river.

In the long term we hope that by planting riparian woodland we can help to keep the rivers cool and provide shade for rivers and fish, as well as providing shaded areas for livestock, birds, and mammals, and prevent these sudden spikes in water temperature.

Graph showing the results of the shaded water (blue), unshaded water, (black), and air (purple) temperature monitoring. This helps to show the link between air temperature and unshaded rivers, and how much warmer, and more reactive the unshaded rivers are compared with shaded rivers.