Climate Change: Defining Reference Conditions and Restoring Freshwater Ecosystems - Climate Change Impacts on Freshwater Ecosystems - page 222

socio-economic processes, conservation and restoration strategies will be more
effective if treated as part of the landscape development rather than as isolated
entities (Holling & Meffe 1996; Meir et al . 2004). One example of this involves
the interaction of catchment forest and stream structure as illustrated in the case
study of the next section.
Use of large woody debris in stream restoration
The term 'stream (or river) restoration' is used for a wide variety of project
objectives, ranging from conventional bio-engineering to the restoration of natural
processes aiming to generate natural in-stream structures and a natural channel
pattern (Kondolf 1996). In the United States, stream restoration is defined as 'the
return of an ecosystem to a close approximation of its condition prior to disturbance'
(National Research Council USA 1992). However, in many parts of the world,
irreversible changes of the natural setting have occurred and hence the re-creation
of a previous historical state is impossible (Kauffman et al . 1997; Brown 2002).
This is especially true for a densely populated cultural landscape in Europe and
other parts of the Old World, where streams have been altered by human activity
since the Mesolithic period. Therefore, the equilibrium state of a stream, which
potentially would develop under the present natural setting without further human
intrusion, is frequently used in Central Europe as a reference and target condition
in stream restoration projects (Deutscher Verband für Wasserwirtschaft und
Kulturbau 1996). The present natural setting may differ from the historical natural
setting as a result of irreversible changes of the natural setting, for example as a
result of sediment deposition in floodplains since the Late Holocene caused by
anthropogenic deforestation and increased erosion (late Holocene alluvium),
mineralization of organic soils, large-scale excavation (surface mining) and landfills,
subsidence caused by mining and recent climate change.
The restoration of hydromorphologically degraded rivers has become a widely
accepted social objective and scientific interest in stream restoration has been
steadily increasing over the last two decades (Shields et al . 2003; Bernhardt et al .
2005). In densely populated areas such as Central Europe, a large proportion of
rivers are heavily degraded; thus, there is a strong demand for simple and cost-
effective restoration measures. Large wood (defined as logs with a diameter
> 0.1 m and a length > 1 m according to Gregory et al . 2003) is an important
component of stream ecosystems in temperate forested ecoregions. It influences
stream hydrology, hydraulics, sediment budget, morphology and biota (e.g.
Gregory et al . 2003). Considering these beneficial effects, even in a densely
populated region like Central Europe, up to one third of the streams could
potentially be improved by restoration with wood (Kail & Hering 2005). After
evaluating the use of large wood in 50 stream restoration projects, Kail et al .
(2007) concluded that (i) potential effects of wood placement must be evaluated
within a catchment- and reach-scale context; (ii) wood measures are most
successful if they mimic natural wood; (iii) effects of wood structures on stream
morphology are strongly dependent on conditions such as stream size and
hydrology; (iv) wood placement has positive effects on several fish species; and (v)
most projects revealed a rapid improvement of their hydromorphological status.