Geoscience Reference
In-Depth Information
variable. It also provides a useful context for
interpreting the distribution and population
changes of species which inhabit or use rivers,
such as otters (
Lutra lutra
) (Environment Agency,
2011), benthic macroinvertebrates (Vaughan
and Ormerod, this volume) and riverside birds
(Vaughan
et al
., 2007).
- or using known near-natural river reaches
elsewhere (preferably in the same biogeographical
region) as surrogate reference condition reaches
(Raven
et al
., 2010). This is particularly true for
lowland rivers where little or no field validation
of reference conditions within individual countries
is possible because of past modification: it is
also true for braided rivers that have suffered
dramatic losses in the European Alps, caused
by channelization and embankment over the
past 200 years (Pi egay
et al
., 2009). The lack of
reference condition sites has major consequences
for river restoration design (Newson and Large,
2006) and also defining and inter-calibrating
the reference conditions needed for biological
quality elements for the WFD (Furse
et al
., 2006)
- the latter because channel modification can
affect aquatic macroinvertebrate community
composition (Dunbar
et al
., 2010c). Finally, the
extent of modified lowland headwater streams
could potentially reduce ecological resilience by
reducing the ability for rapid recolonization by
biological communities in disturbed river reaches
further downstream.
Near-natural and 'reference'
conditions
The 2007-2008 results confirmed that a very small
proportion
of
sites
on
small
(
≤
2.0
m
water
≤
width) lowland streams (
200 m altitude) had
a near-natural channel form (5.1%), compared
with those streams of similar size in upland (
200
m altitude) areas (37.3%); for all watercourses
≤
5.0 m water width, the figures were 5.6%
and 35.4% respectively (Table 3.2). However,
a large proportion of upland streams have few
or no bankside trees because of sheep grazing
or moorland management; consequently, the
riparian habitat cannot be considered near-natural,
particularly in the context of 'reference condition'
state defined by the WFD (Council of the European
Communities, 2000) or the European guidance
standard for hydromorphological modification
(British Standards Institution, 2010; Boon
et al
.,
2010). This pattern has vindicated the RHS
'benchmarking exercise', carried out to locate and
survey the very best examples of near-natural
streams and rivers in the UK and mainland Europe
(Raven
et al.
, 2010). Benchmarking has confirmed
that near-natural river and catchment land-use
conditions are virtually confined to short stretches
of more remote headwater streams in protected
landscape and wildlife areas - often in catchment
areas
Sampling strategy
The site-based sampling strategy for the RHS
field method was designed to meet the need for
a time-limited, cost-effective way of describing
the physical character of the river network.
Consequently, the advantage of being able to
generate and analyse a large sample-based dataset
for national assessment and surveillance purposes
is offset by a limited ability to discriminate
features needed for specialist ecological purposes
or to provide all the diagnostic clues needed
for detailed fluvial morphological studies of
individual rivers (Orr
et al
., 2008; Sear
et al
.,
2008). Nevertheless, interpretation of RHS data
depends on a good understanding of the fluvial
morphological processes operating at various scales
(Newson
et al.
, 1998) and the ecological response to
various river management activities (Raven
et al
.,
1998a).
Other morphological survey techniques involve
surveyors walking much longer reaches of
river and making qualitative judgements about
10 km
2
and therefore too small to qualify
for WFD classification.
The scarcity and restricted location of streams
and rivers with near-natural channel form
and natural riparian land-use has important
consequences for calibrating habitat quality scoring
systems such as HQA and the river typology
criteria used by some survey methods (e.g.
LAWA, 2000). For instance, it means that expert
opinion has to be used to judge what constitutes
near-natural
and
reference
condition
habitat
