Restoration of rivers and floodplains - Restoration Ecology - page 144

mean that rivers and their floodplains are always
adjusting to both local changes that affect the annual
variability of flood frequency and size of flow events,
and global variation that can result in major shifts
in river planform pattern and transform their natural
geomorphological state. The precise mechanisms of geo-
morphological change in rivers cannot be explained
within the confines of this chapter but many textbooks
already cover these issues (see for example Callow &
Petts 1992, Knighton 1994, Rosgen 1996, Gregory 1997,
Thorne et al. 1997, Wolfert 2001). Nevertheless, the
importance of understanding geomorphic processes
cannot be overestimated. It can both hold the key to
explaining the changes that have occurred in our
riverine landscape as a result of anthropogenic inter-
vention and help to predict how specific restoration
methods will enhance degraded rivers.

ice age that ended some 10,000 years ago. This
formed not only many of the rivers themselves but
also the valleys and entire catchments. In northern
Europe, the ice pack covered much of the landscape,
rounding off mountains and scouring out wide U-
shaped valleys, far larger than could have been
shaped by their current watercourses. This large-scale
process had impacts on both flora and fauna and areas
once below the ice now have less genetic diversity than
those that survived further to the south, since, in this
case, they relied on colonization by plants and ani-
mals able to travel large distances. Where the ice failed
to advance, new rivers developed and old rivers were
displaced to drain the meltwater. The River Thames
moved miles further south, discharging into a new estu-
ary where London is now located. The River Skjern
valley in Denmark, which drains 11% of Jutland's soil,
was gouged out by torrents of meltwater and, even
beyond the immediate influence of the ice, deposition
of eroded rock and sediment occurred, creating lakes
and deltas, thus helping to shape the course of rivers
and streams.
The late medieval period, generally known as the
medieval warm period, resulted in another significant
shift in the riverine regimes of Europe. This period

11.2.1 Natural impacts

Although rivers and floodplains have constantly been
shaped by changes in climate over time (see Table 11.1)
the riverine topography we have inherited today owes
many of its features to the impacts of the last major

Table 11.1 Historic climate change and associated channel processes. Adapted from Gregory and Lewin (1987).

Year BP Climate Precipitation Temperature Vegetation Hydrology Channel processes

Pre-15,000
15-10,000

Glacial
Early part
of the late
Glacial

Snow
Snow

Low
Ameliorating:
cold winters

None
Grass/sedge
succeeded by
birch
woodland
Herb-rich
grassland

Summer flood
High peak
discharge, but
decreasing as
forest spread
Runoff reducing,
but high
sediment supply
Runoff lower
than today

Multi-thread
Multi-thread

11-10,000 Late Glacial Mainly drier Extreme cold Fluvial deposition,

unstable

10,000 - 4000 Holocene Rain Rising Mixed oak

forest with
some clearance

Fluvial erosion
dominant with
single-thread
meandering
channels
Lowland cut-offs
and floodplain
accretion of
sediment

4000-present Holocene

cont.

Rain Reducing but

fluctuating

Deforestation Seasonally

fluctuating
discharge