Focussing on process in stream restoration

Posted on September 21, 2011


Many stream restoration schemes rely on the  assumption that interventions such as adding woody structure to river channels will increase ecological diversity.  However, the mixed success of such projects suggests that to be most effective, stream restoration activities should be guided by well-defined aims based on an understanding of key ecophysical processes. In a follow-up assessment of restoration works on the River Spree, Germany, two projects were compared, one in urban Berlin and one at a more rural, upstream location.  In the urban setting, an artificial shallow bay and a wave-protected shallow bank had been constructed, while at the upstream location two oxbows had been modified to re-establish former meanders in the river channel.  At the urban rehabilitation sites, especially the shallow bay, densities of fish were 20-30 times higher than in adjacent unrestored areas.  The species were mainly young roach and perch, but current-loving forms such as ide, gudgeon and asp were also present.  At the rural upstream location however, the remeandering work had done nothing to reverse an historic trend toward the loss of current-loving and gravel-spawning species.  These findings suggested that restoration had relieved an important ecological constraint at the urban sites, but not the rural location.  It’s been argued that in space-limited urban waterways with reasonable water quality, the most significant constraint on fish diversity is the lack of shallow, wave-protected nursery areas, and in this context the Berlin restoration work provided shallow protected refuge areas for recruiting fish.  In contrast, in rivers without the pressures of urbanisation and boat traffic, suitable spawning habitat is more likely to be a more critical requirement.  At the rural location the motivation for restoration had been more aesthetic (a desire for a stable, meandering channel) than functional (a desire to improve habitat).  Rather than leading to an increase in coarse substrate, flow variability or habitat diversity, the oxbow remeandering reduced the slope and velocity of the stream and produced a less attractive environment for current-loving and gravel-spawning fish.  The authors join a growing number who point out that dynamically migrating streams tend to be ecological rich, and who  question the assumption that a stable meandering channel is an ideal target for stream restoration.

References: 

Kondolf, G. M. 2006. River restoration and meanders. Ecology and Society 11(2), 42. [online] URL: http:// http://www.ecologyandsociety.org/vol11/iss2/art42/

Wolter, C.  2010.  Functional vs scenic restoration – challenges to improve fish and fisheries in urban waters.  Fisheries Management and Ecology 17, 176–185.

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