[MCN] Saving streams and rivers for fish: How much shade needed?

Lance Olsen lance at wildrockies.org
Sat Sep 5 11:03:25 EDT 2015

Water Resources Research 27 May 2015

Seeing the landscape for the trees: Metrics to guide riparian
shade management in river catchments

Matthew F. Johnson and Robert L. Wilby

Key Points:
-Temperature over long stretches of river will not be affected by 
riparian shade
-Midreaches of headwater streams are most responsive to riparian shade
-To offset a 1C temperature rise, 1 km of trees is necessary in UK 
small streams


Rising water temperature (Tw) due to anthropogenic climate change may 
have serious conse- quences for river ecosystems. Conservation and/or 
expansion of riparian shade could counter warming and buy time for 
ecosystems to adapt. However, sensitivity of river reaches to direct 
solar radiation is highly heterogeneous in space and time, so 
benefits of shading are also expected to be site specific. We use a 
network of high-resolution temperature measurements from two upland 
rivers in the UK, in conjunction with topographic shade modeling, to 
assess the relative significance of landscape and riparian shade to 
the thermal behavior of river reaches. Trees occupy 7% of the study 
catchments (comparable with the UK national aver- age) yet shade 
covers 52% of the area and is concentrated along river corridors. 
Riparian shade is most ben- eficial for managing Tw at distances 5-20 
km downstream from the source of the rivers where discharge is 
modest, flow is dominated by near-surface hydrological pathways, 
there is a wide floodplain with little land- scape shade, and where 
cumulative solar exposure times are sufficient to affect Tw. For the 
rivers studied, we find that approximately 0.5 km of complete shade 
is necessary to off-set Tw by 1C during July (the month with peak Tw) 
at a headwater site; whereas 1.1 km of shade is required 25 km 
downstream. Further research is needed to assess the integrated 
effect of future changes in air temperature, sunshine duration, 
direct solar radiation, and downward diffuse radiation on Tw to help 
tree planting schemes achieve intended outcomes.
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Nature 525, 25-27 (03 September 2015) doi:10.1038/525025a

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