Geoscience Reference
In-Depth Information
Table 3.2 Channel and bank modification at RHS baseline survey sites in England and Wales in 2007-2008, classified
by water width and altitude. Figures show percentage of sites in each Habitat Modification Class (1
=
near-natural
channel form; 5 = severely modified). * Some water width data values are missing. n/s: no sites.
Sites > 200 m altitude ( n
=
534)
Sites
200 m altitude ( n
=
4163)
Habitat Modification Class
Habitat Modification Class
Number of
sites
ater width (m)
1
2
3
4
5
1
2
3
4
5
0.0-2.0
37.3
22.5
14.1
18.3
7.8
5.1
8.4
9.5
20.9
56.1
2738
>
2.0-5.0
30.8
30.1
22.6
13.5
3.0
7.1
15.8
13.7
21.3
42.1
1048
> 5.0-10.0
29.6
33.3
16.7
18.5
1.9
11.8
19.0
12.0
21.0
36.2
512
>
10.0-20.0
35.7
35.7
28.6
n/s
n/s
10.1
22.9
13.2
19.4
34.4
241
> 20.0
n/s
n/s
n/s
n/s
n/s
12.7
20.3
17.1
18.3
31.6
158
Number of sites
186
138
90
89
31
284
517
465
868
2029
4697 *
it occurred. As expected, extensive reinforcement
was
Kern Hansen, 1979). In upland areas, a lack of
trees is usually the result of livestock grazing and
moorland management.
An estimated 13% of 500 m river lengths in
England and Wales were treeless in 2007-2008,
but 23% had continuous tree cover. Large tracts
of upland streams in moorland landscapes were
treeless, as were parts of lowland fen and grazing
marsh in southern and eastern England (Figure
3.2d). Although an estimated 79% of 500 m river
lengths had some tree shade, less than half (41%)
were extensively (i.e. 33% of the water surface
area) shaded by trees.
associated
with
major
conurbations
(Figure 3.2c).
A higher proportion of sites in the minor
watercourse sample (50.9%) was severely modified
than that in the core sample (40.5%) (Table
3.1). The extent of modification was greatest in
lowland areas (altitude 200 m); 56.1% of sites
on watercourses with a water width 2.0 m
(and 52.2% on sites 5.0 m wide) were severely
modified (Table 3.2). The overall pattern generally
confirms that relatively unmodified streams are
largely confined to more upland areas, with
extensive modification associated with lowland
streams, drainage channels and rivers (Raven et al .,
1988b).
Invasive non-native plants
Three invasive non-native plant species -
Himalayan balsam ( Impatiens glandulifera ),
Japanese knotweed ( Fallopia japonica ) d
giant hogweed ( Heracleum mantegazzianum )-
are recorded by RHS specifically to track the
spread of these invasive plants in the UK. All
three species can cause problems by displacing
native plant species in the growing season, with
Japanese knotweed and Himalayan balsam often
forming dense monostands (Dawson and Holland,
1999; Hulme and Bremner, 2006). Autumn
die-back of these plants, particularly Himlayan
balsam, can create extensive areas of bare soil
and thereby increase the risk of bank erosion
during winter spates (Dawson and Holland, 1999).
Riverside trees and shading
Trees such as alder ( Alnus glutinosa ) and willows
( Salix spp.) are an important river landscape
feature, particularly in lowland Britain. In some
intensively farmed floodplains, riparian trees are
virtually the only 'natural' landscape feature,
providing an important wildlife corridor (Haslam,
1991), while sparsely distributed or no riverside
trees can indicate modification work such as
channel straightening or dredging (Purseglove,
1988). Tree shading can also reduce the amount
of aquatic macrophyte growth in lowland rivers,
reducing the need for weed-cutting (Dawson and
 
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