Geoscience Reference
In-Depth Information
Himalayan balsam
Giant hogweed
Japanese knotweed
60
50
Present
Extensive
40
30
20
10
0
0 to 5.0m
>5 to 10m >10 to 20m
>20m
0 to 5.0m
>5 to 10m >10 to 20m
>20m
0 to 5.0m
>5 to 10m >10 to 20m
>20m
Figure 3.3
The percentage occurrence of Himalayan balsam, giant hogweed and Japanese knotweed at RHS baseline
sites of different water width, in 2007-2008. Extensive occurrence represents plants present along
≥
33 % of bank
length.
The prevalence of Himalayan balsam, Japanese
knotweed and giant hogweed in 2007-2008 was
13.0%, 3.7% and 3.4% of 500 m river lengths in
England and Wales respectively (Figure 3.2e, f).
Himalayan
status as required by the WFD. In 2007-2008
silt was extensive (i.e. recorded as predominant
channel substratum in
≥
4 spot-checks) in
23.8% of RHS sites, largely in the lowlands
of central southern and eastern England (Table
3.3; Figure 3.4). It is often difficult to assess
whether silting is a problem within an individual
500 m site - for example silt accumulations
can indicate natural readjustment (narrowing)
by lowland rivers to previous channel widening
and deepening (Brookes, 1992). However, RHS
surveyors identified 'extensive silting' as a specific
problem in 5.6% of baseline sites.
balsam
in
particular
tended
to
be
more
frequently
found
on
larger
watercourses
(Figure 3.3).
Large woody debris
Large woody debris, especially where it
accumulates to form debris dams in streams
and small rivers, is an important influence on
river channel morphology and dynamics (Gregory,
1992; Gurnell
et al
., 2002). It also provides
important temporary or semi-permanent habitat
for a range of invertebrates and creates shelter
for fish and invertebrates during high river flows
(Harper
et al
., 1995). In 2007-2008 large woody
debris was present in 42.8% of RHS sites (Table
3.3), but debris dams were extensive only in 0.7%
of sites.
In-channel bars
Exposed river sediments such as in-channel
bars provide valuable habitats for invertebrates,
including some national rarities (Bates
et al
.,
2005). In England and Wales most in-channel
bars are formed from gravel material. RHS
differentiates between mid-channel, side and point
bars depending on their location in the channel and
also whether they are vegetated (
Channel silting
Silt accumulates naturally in very sluggish or
impounded river reaches, mainly in the lowlands.
Elsewhere, excessive silt deposition caused by soil
erosion in adjacent fields can clog fish spawning
gravels, reduce light levels and increase flood
risk (Greig
et al
., 2005; Owens
et al
., 2005).
Excessive silting is also likely to impoverish
benthic macroinvertebrate communities (Dunbar
et al
.,
50% of surface
area) or unvegetated (Environment Agency, 2003).
The frequency, location and type of in-channel bars
can provide useful information about the fluvial
behaviour (Newson, 1997) and ecological patch
dynamics of rivers (Poole, 2002).
The distribution of unvegetated bars in
2007-2008 is shown in Figure 3.5. They were
generally widespread except in: (i) chalk areas,
where high baseflow and little seasonal variation in
discharge generally prevent extensive development
≥
2010a),
thereby
potentially
preventing
affected
watercourses
reaching
good
ecological
