Geoscience Reference
In-Depth Information
Table 3.3
The estimated extent of selected features occurring along rivers and streams in England and Wales, based on
RHS baseline surveys in 1995-1996 and 2007-2008. Figures represent percentage mean values
±
standard error,
except for 'other features' - percentage only.
Feature
1995-1996
2007-2008 (core sample only)
Modification
(percentage of river length)
Channel and/or bank reinforcement
11.3
±
0.4
9.3
±
0.3
Invasive non-native plants
(percentage of 500 m river lengths)
Himalayan balsam
15.0
±
0.6
19.7
±
0.6
Japanese knotweed
8.8
±
0.5
5.5
±
0.4
Giant hogweed
3.6
±
0.4
4.0
±
0.3
Pattern of riverside trees
(percentage of 500 m river lengths)
None
10.5
±
0.5
8.8
±
0.4
Isolated or scattered
17.9
±
0.7
14.8
±
0.6
Regularly spaced
1.6
±
0.2
1.7
±
0.2
Occasional clumps
14.4
±
0.6
18.5
±
0.7
Semi-continuous
27.6
±
0.8
31.1
±
0.9
Continuous
28.0
±
0.8
25.2
±
0.7
Shading
(percentage of 500 m river lengths)
Extensive (
≥
33%) shading of channel
36.5
±
0.8
41.1
±
0.8
Other features
(percentage of baseline sites)
Large woody debris (present or extensive)
49.0
42.8
Extensive silt substratum (
≥
4 spot-checks)
22.0
23.8
In-channel bars (present or extensive)
51.7
47.1
of exposed gravel features (Sear
et al
., 1999); and
(ii) low altitude, low gradient channels, where
exposed depositional features are likely to be much
less prevalent; these watercourses are also more
likely to have been extensively resectioned (Figure
3.2b) and have gravel or silt removed regularly to
maintain flood conveyance (Purseglove, 1988).
knotweed was also broadly similar, although
the last of these showed an unexpected decline
(Table 3.3). One notable exception was that
Himalayan balsam had become more widespread
and extensive by 2007-2008; its prevalence had
increased from 15.0% to almost 20.0% of 500 m
river lengths, with extensive growth (
33% of
500m river length occupied) increasing from 2%
to 5% (Figure 3.3; Figure 7.1b in Vaughan and
Ormerod, this volume).
An important factor when comparing riverside
tree distribution in the two baseline surveys was
the incidence of
Phytophthora
disease affecting
alders. First recorded in Britain in the early 1990s,
initially there were fears that the
Phytophthora
disease could kill large numbers of alders, with
potentially devastating consequences for river
landscapes and riparian ecology (Gibbs
et al
., 1999).
RHS has recorded specifically the occurrence of
diseased and healthy alders from 1996 onwards
(Environment Agency, 2003). The results show
that the incidence of diseased alders remained
≥
Changes since 1995-1996
The 3036 sites surveyed in 1995-1996 were not
re-visited, so change was assessed by overall
differences between those sites and the core sites
(i.e. the 1:250 000 river sample) surveyed in
2007-2008. This means that some figures in Table
3.3 will differ slightly from those in Tables 3.1 and
3.2 because the 2007-2008 survey data include
both the core and minor watercourse sites.
Overall, between 1995-1996 and 2007-2008,
there was little change in the extent of channel
reinforcement or the distribution of riverside trees.
The occurrence of large woody debris, in-channel
bars, channel silt, giant hogweed and Japanese
