Environmental Engineering Reference
In-Depth Information
17.1
INTRODUCTION
etation in the riparian interface on both sides of the
river (Osborne & Kovacic 1993). Naturally, rivers and
fl oodplains support a wide variety of interacting ter-
restrial and aquatic fl ora and fauna and are an essen-
tial component in creating pathways or corridors
between other habitats such as woodlands. For
example, fi sh show highest diversity within the main
channel, but many species need both the main fl ow-
ing river channel and backwaters to complete their
life cycle. Upland, boulder-strewn streams can, for
instance, provide spawning grounds for salmonids
which, once hatched, move downstream to slower
moving and larger sections of river systems where they
are dependent on the food supply provided by aquatic
and bankside invertebrates. In addition to the river,
many backwaters, cut-off channels (natural and
human-made) and pools can provide permanent
and semipermanent habitat for a range of wetland and
sheltering species (e.g. in seasonally connected back-
waters, damsel - and dragon - fl ies can occur, amphibi-
ans may migrate to fl oodplain areas to deposit eggs and
aquatic plant species richness increases when com-
pared to a river system that is disconnected from its
associated fl oodplain).
Like any
ecosystem
, rivers and fl oodplains contain
characteristic communities of species that are inter-
connected through shared habitat use, food resources
and refuge from predation. They also have a funda-
mental interdependency through transfers of organic
material, minerals and nutrients between and among
different trophic groups. Each type of organism has
natural functions within the ecosystem: for example,
green plants photosynthesize, some invertebrates
shred and break down coarse organic matter trans-
ported by the river from upstream, bacteria decompose
organic matter and so on (Menezes
et al
. 2010 ). If river
habitats of interacting species - so-called
keystone
species
in particular - are disconnected, then there
will be a disruption to normal ecological functioning
within the river. There are two basic categories of eco-
logical disconnection: (1) spatial - whereby specifi c
habitats and/or species become isolated, and (2) tem-
poral - where connections are broken at crucial times
of the year. Increasing isolation can have dramatic
negative effects on
biodiversity
within the river
system, and break down food web structure in particu-
lar (Shulman & Chase 2007). Once the river habitats
become degraded or disconnected, then restoration
may not provide the whole answer; the complexity
of the system and the interconnections must be
Natural river systems are dynamic features in terres-
trial landscapes that change continuously as a result
of their inherent physical conditions such as slope,
bedrock geology and the complexity of the drainage
network. Yet, their characteristics are also infl uenced
by external factors including climatic conditions, pre-
cipitation and human activities such as afforestation,
deforestation, urbanization, land drainage, pollutant
discharges and fl ow regulation; it is the combination of
the inherent river processes that occur over time
(Brunsden & Thornes 1979 ; Schumm 1979 ; Darby &
Sear 2008), and often uncontrollable external infl u-
ences, that ultimately determine the form of a water
course. Recognition of these two aspects - inherent
processes and external infl uences - is key towards sus-
taining a healthy river system, in which both natural
relationships are maintained, pollutants are absorbed,
wastes decompose and sediment is redistributed during
fl oods (Postel & Richter 2003).
From a geological point of view, the continent of
Europe, on which this chapter primarily focuses, is
relatively young and this has resulted in a large number
of river catchments that tend to be small, compared
with much of the rest of the world. Only about 70
rivers in Europe have catchment areas exceeding
10 000 km
2
. Of these the largest is the Volga
(1 360 000 km
2
; compared with 3 349 000 km
2
for the
Nile) and this river, together with the Danube and the
Dnieper, drain a quarter of continental Europe. Fur-
thermore, the rivers of Europe account for only 7% of
the world's rivers, with a total annual discharge of
approximately 3100 km
3
of fresh water into the sea
each year (Kristensen & Hansen 1994). Although this
is a relatively small amount (8%) of the total discharge
worldwide, the overall impact is not insignifi cant. The
temperate humid climate together with a high percent-
age of extremely erodible limestone means that the
amount of dissolved solids in European rivers is consid-
erable, and its impact is exacerbated by a heavily popu-
lated continent and associated agriculture, resulting in
an intense concentration of minerals within the rivers.
Rivers and fl oodplains have internal physical, chem-
ical and biological properties that are fundamental in
determining the types, abundance and spatial distribu-
tion of organisms that occur there. But they are also
affected by the landscapes within which they are
located. Land type and land use determine water
quality, but it can also be altered by the action of veg-
