Our research project investigates the role of riparian woodland in controlling stream water temperature through the provision of shade. Observations in recent summers show that stream water temperatures in open streams draining southern England can greatly exceed tolerance levels for salmon and trout. The situation is expected to get worse with average temperatures predicted to rise by between 2 and 5 ºC by the end of this century. The influence of riparian woodland in limiting rises in water terperature is being investigated at several sites in England and Scotland.
Research objectives
Investigate water temperture in lowland forest streams of Southern England and the impact of thermal regime of freshwater fish communities
Investigate the role of riparian woodland in the protection of Freshwater Pearl Mussel
Findings and Recommendations
Some fish species such as salmonids (salmon and trout) are very sensitive to high water temperature, with possible effects on the timing of spawning, fish growth rates and even survival. Salmonid fish require temperatures of between 5 and 15 ºC for normal growth and temperatures above 24 ºC can be lethal. Temperatures in excess of 31ºC have been recorded in recent summers in small lowland streams in the New Forest, demonstrating that this tolerance limit is already being significantly breached in some small watercourses in southern England.
Riparian woodland features such as, exposed tree roots, fallen stems and large woody debris dams, create structural diversity in the instream habitat and maintain pockets of cool water below undercut river banks and deep pools. These provide important thermal reguia for salmonids.
Riparian woodland canopy may require management to maintain water temperatures within a favourable range for salmonid fish and other sensitive freshwater fauna.
The shade provided by riparian trees can significantly reduce peak summer water temperature. Planting trees on river banks may therefore have an increasingly important role to play in limiting the impact of climate warming on freshwater life. Riparian woodland creation is the key objective of the Keeping Rivers Cool project, which is currently being led by the Woodland Trust.
Our Involvement
A joint field study with Southampton University was set up in the New Forest to evaluate the cooling effect of riparian shade.
Twenty sites with variable levels of shade on the Dockens Water and Ober Water have been instrumented to characterise the thermal regime and assess the effects of shading on streamwater temperature and on fish populations, including fish survival, growth rates and behaviour.
The results will help to determine whether thermal stress poses a serious problem in these watercourses and if so, how riparian woodland management could help to protect the freshwater life from future rises in water temperature.
