Restoration of rivers and floodplains - Restoration Ecology - page 145

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
result was more convective rainfall in the summer
months resulting in high-intensity but short-duration
rainfall. Evidence in England from the middle reaches
of the River Trent in the Midlands suggests that as
a consequence of this climatic change the character
of the rivers shifted from single-thread channels to a
braided system, then anastomosing (multi-thread) and
back to single thread during this period as the river
profiles adjusted to accommodate larger, more unpre-
dictable (flash) flood conditions (see Benito et al. 1998
for more details). South of the Alps, the impact on
rivers was equally complex. The higher temperatures
resulted in higher evaporation, lower annual discharge
and groundwater and less snowmelt. Conversely, high
sea temperatures increased the amount of convective
rainfall. The effects on the rivers and floodplains, there-
fore, depended on the local antecedent conditions, and
the infiltration rate of the substrate. In northern Italy
the rivers were unstable and a braided form dominated,
while in the south rivers tended towards single-thread
systems again where flows were often out of bank and
hence deposited large amounts of sand and silt on the
floodplain.
Since the medieval warm period, the rivers of
Europe have been subjected continually to minor
changes in climate. Perhaps the most notable of these
was the Little Ice Age that occurred between the 12th
and 17th centuries. Documentary evidence suggests that
the River Thames, in London, often froze during this
period although it is not completely clear if this was
related solely to climate change or exacerbated by the
hydrodynamic effects of the bridges. Nevertheless,
the effects of the Little Ice Age were not restricted to
the UK and records across the whole of Europe show
a similar pattern, with many lakes and rivers freez-
ing over and evidence of glacier expansion. Further-
more, records of this period imply that this was also a
period of dry weather and hence intermittent riverine
activity (Benito et al. 1998).
Today there is much discussion about future climate
change and the impact on biodiversity (see, for
example, Gitay et al. 2002). By extension, this has
implications for the river systems and their future flow
regimes. This in turn could have an adverse effect on
ecological status and hence any future restoration

initiatives should build in some design flexibility to
account for changing scenarios and appreciate that rivers
are very sensitive to change (see Downs & Thorne 1998).

11.2.2 Human impacts

Effects of climate and local weather, although instru-
mental in driving the form of Europe's rivers, have
gradually been matched by an increase in the degree
of human intervention, starting during the Holocene
period (within the last 10,000 years) with simple wood-
land clearance. Although some distinctive phases of
human activity that ultimately affect our river systems
can be identified since that period, it often remains
difficult to disentangle anthropogenic impacts from
more natural or non-human determinants. Take for
instance the impact of human interference through
deforestation compared to the natural decline in forest
cover as a result of periods of glacial activity. Imme-
diately it becomes apparent that trying to distinguish
the relative impacts of each of these is problematic.
Furthermore, especially in northern Europe, many
riverine areas are still recovering from the Little Ice
Age, while at the same time some have been widened
and deepened to drain the land or for navigation
purposes. It is important to recognize the additional
difficulties associated with a long period of human
intervention and constraints when evaluating degraded
rivers with a view to restoration. A pristine riverine
state to start with is difficult, if not impossible, to
identify and in many cases, as expressed by Macklin
and Lewin (1997), today's rivers are often 'climatically-
controlled but culturally blurred'.
Deforestation in particular has had a major impact
on river systems especially during the last 4000 years.
Along the Rhine, for example, estimates of forest
decline vary but broad figures suggest a 75% reduc-
tion; today there is only 150 km 2 of forest remaining
along the Rhine's corridor. Such massive decline in
forested areas resulted in an increase in sediment
availability. This often deposits on the river bed both
directly by means of runoff and indirectly via systems
of field and land drains. During the medieval period,
rivers throughout much of Europe became harvested
as a source of energy to drive mills for a range of
activities from cloth- to flour-making industries and
hence marked the beginning of a period of major