• Water temperature in lowland forest streams of Southern England

Water temperature in the lowland forest streams of Southern England

Summary

Planting trees along stream banks could play an important part in protecting sensitive fish such as salmon and trout from rising temperatures as the climate warms.

Results from an ongoing study in the lowland streams of the New Forest suggest that riparian shade can prevent summer water temperatures exceeding dangerous limits.

Using The Ober Water and Dockens Water streams in the New Forest, we have been monitoring stream water temperatures since January 1995. We have also investigated how water temperature is influenced by riparian tree cover.

Findings

Water temperature fluctuates through the day in small, rain-fed streams such as those of the New Forest. The study found that riparian woodland had a marked effect on the water temperatures, with daily peaks in water temperature typically 5 degrees cooler in the shade than in open water.

Graph showing fluctuation of water temperature in rain-fed streams in the New Forest in July 2006. Values (degrees Celsius): Open pool= 14 to 30, shade pool= 14 to 22

This was enough to significantly reduce the number of days on which stream water temperature exceeded critical thresholds for trout.

Number of days per year that Tmax exceeds critical termal thresholds for brown trout in streams in the New Forest

(a) 19.1 degrees Celsius - maximum limit for growth Graph showing number of days per year that T(max) exceeds critical termal thresholds for brown trout in streams in the New Forest (a) 19.1 degrees Celsius - maximum limit for growth. Approximate values: open pool=70, shade pool=40-45, open riffle=50, shade riffle=under 10
(b) 24.7 degrees Celsius - incipient lethal limit Graph showing number of days per year that T(max) exceeds critical termal thresholds for brown trout in streams in the New Forest (b) 24.7 degrees Celsius - incipient lethal limit. Approximate values: open pool=9, shade pool=1, open riffle=10-15, shade riffle=0

Critically, the study indicates that achieving about 20 per cent canopy cover along at least 500 metres of small, rain-fed streams could be effective in preventing current summer maximum water temperatures from exceeding potentially life threatening levels for native cold-water fish.  However, higher proportions of riparian woodland are likely to be needed to address future climate warming.

Number of days per year that T max exceeds critical termal thresholds for brown trout in streams in the New Forest
(c) in relation to percentage riparian shade upstream of logger location Graphs showing number of days per year that T(max) exceeds critical termal thresholds for brown trout in streams in the New Forest (c) in relation to percentage riparian shade upstream of logger location

The results illustrate how the riparian shade can be managed to provide essential cool-water refugia in the streams for the benefit of fish such as salmon and trout.

Further research is being conducted to build on these findings, with the intention of enabling future riparian tree planting to be targeted in catchments known to support trout and salmon fisheries.

Reference

Broadmeadow, S., Jones, J., Langford, T., Shaw, P., & Nisbet, T. (2010). The influence of riparian shade on lowland stream water temperatures in southern England and their viability for brown trout. River Research and Applications DOI: 10.1002/rra.1354