Geoscience Reference
In-Depth Information
Figure 3.5
The distribution of
unvegetated in-channel bars at RHS
baseline sites in 2007-2008.
Crown
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The pattern of very little overall change in
the physical character of rivers in England and
Wales between 1995-1996 and 2007-2008 was
also not unexpected. This is because major river
channel works associated with land drainage had
been completed by the early 1980s (Brookes
et al
.,
1983; Purseglove, 1988). River restoration work
has been on a very small scale (Mainstone and
Holmes, 2010; Aberg and Tapsell, this volume)
and was undetectable at the RHS site sampling
density used in the baseline surveys. For a similar
reason the local impact of new suburban and road
development (particularly 'out-of-town' shopping
centres and industrial estates on floodplains) during
1997-2008 was also not detectable at the England
and Wales scale.
Channel modification of minor watercourses
(which were under-represented in the 1995-1996
baseline survey) appears to be broadly similar to
the core sample (Table 3.1). The predominance of
severely modified small watercourses in lowland
areas (
≤
200 m altitude; Table 3.2) probably reflects
the presence of artificial drainage channels in these
parts. The RHS results from a separate survey of
headwater streams in England and Wales suggest
that there was no significant difference in HMS
scores between repeat surveys of the same sites
in 1998 and 2007 (Dunbar
et al
., 2010b). Mean
HQA scores increased by an average 5.6 in England
(
n
47), due
mainly to an increase in gravel bars, riverside trees
and side-channels. However, great care is needed
in interpreting the increase in mean HQA scores
because of the small number of sites involved.
Knowing that physical habitat remained
largely unchanged is valuable as it allows better
interpretation of other environmental trends, such
as river water quality, by removing a measurable
=
150) and by 6.8 in Wales (
n
=
