The role of riparian shade in controlling stream water temperature in a changing climate

WALKERS BY STREAM ON HINDHOPE LINN TRAIL. KIELDER FDObservations 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 (Climage change scenarios for the UK: UKCIP02 Scientific Report).

This research project investigates the role that riparian woodland could increasingly play in limiting rises in stream water temperature through the provision of shade.

Fish and water temperature

Fish, and salmon and trout (salmonids) in particular, are very sensitive to changing 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 31 ºC have been recorded in recent summers in lowland streams, demonstrating that this tolerance limit is already being significantly breached in smaller watercourses in southern England.

Riparian woodland and shade

The shade provided by riparian trees can significantly reduce peak summer temperatures within watercourses. Riparian woodland may therefore have an increasingly important role to play in limiting the effects of climate warming on freshwater life. Judicious management of riparian woodland offers a means of maintaining water temperatures within a favourable range for salmonid fish and other sensitive freshwater fauna.

New Forest Research Study

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.