Environmental Engineering Reference
In-Depth Information
Table 17.1 Historic climate change and associated channel processes in Europe. Adapted from Gregory and Lewin (1987).
Years BP
Climate
Precipitation
Temperature
Vegetation
Hydrology
Channel
processes
15 000
Glacial
Snow
Low
None
Summer fl ood
Multithread
15 000-
10 000
Early part
of the
late
Glacial
Snow
Ameliorating
- cold
winters
Grass and sedge
succeeded by
birch
woodland
High peak
discharge, but
decreasing as
forest spread
Multithread
11 000-
10 000
Late
Glacial
Mainly drier
Extreme cold
Herb rich
grassland
Runoff reducing,
but high
sediment
supply
Fluvial
deposition
- unstable
10 000-
4000
Holocene
Rain
Rising
Mixed oak forest
with some
clearance
Run off lower
than today
Fluvial erosion
dominant with
singe thread
meandering
channels
4000-
present
Holocene
cont.
Rain
Reducing but
fl uctuating;
presently
rising
Deforestation
Seasonally
fl uctuating
discharge
Lowland
cut-offs and
fl oodplain
accretion of
sediment
Europe, the ice pack covered large areas, rounding off
mountains and scouring out wide ' U ' - shaped valleys
far larger than could have been shaped by water-
courses alone. This large-scale process had impacts on
both fl ora and fauna, and areas once below the ice
now have less genetic diversity than those that sur-
vived further to the south; they relied on colonization
by plants and animals 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 in England moved miles further
south, discharging into a new estuary where London
is now located. The valley of the River Skjerne in
Denmark, which drains 11% of the Jutland region's
soil, was gouged out by torrents of meltwater and,
even beyond the immediate infl uence of the ice, depo-
sition of eroded rock and sediment occurred, creating
lakes and deltas, thus helping to shape the course of
rivers and streams.
The Medieval Warm Period, which lasted for a few
centuries around 1000-1200 CE , resulted in another
signifi cant shift in the riverine regimes of Europe. This
period was represented by hotter, drier summers north
of the Alpine areas and, although the winters did not
differ substantially from today's climatic conditions,
the outcome was more convective rainfall in the
summer months, resulting in high-intensity but short-
duration rainfall events. As a consequence of this cli-
matic change, the character of the rivers in Middle
England shifted from a single-thread channel to a
braided system, then anastomosing (multithread)
systems, and fi nally back to single-thread channels as
river profi les adjusted to accommodate larger fl ash
fl oods, and generally more unpredictable fl ood condi-
tions (see Benito et al . 1998 for more details). In north-
ern Italy, however, the rivers remained unstable
whereas further south they tended towards single-
thread systems with frequent out - of - bank fl ows result-
ing in large deposits of sand and silt on fl oodplains.
Ultimately, the effects on the rivers and fl oodplains,
therefore, depended on a combination of precipitation,
local antecedent conditions and the infi ltration rate of
the substrate.
Since the Medieval Warm Period ended, the rivers of
Europe have been continually subjected to minor
changes in climate. Perhaps the most notable of these
was the Little Ice Age that occurred between the twelfth
and seventeenth centuries CE . Documentary evidence
suggests that the River Thames, in London, often froze
over during this period although it is not completely
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