Geoscience Reference
In-Depth Information
aquatic ecosystems in several ways. These include providing bank stability and so minim-
izing erosion, filtering sediment, and processing nutrients from the drainage basin, particu-
larly nitrogen. Fallen branches or trunks from riparian trees also create woody habitat areas
for many fish and smaller creatures.
From the ecological perspective, the unidirectional flow of a river is a unique situation.
Flowing water influences many aspects of the river environment, moving things and thus
helping to disperse organisms and transport nutrients. Flowing water affects the shape of
the channel and the nature of its bed, disturbing both on occasions of strong flow, maintain-
ing a dynamic habitat for fluvial plants and animals. Rivers also deliver water, energy, sed-
iment, and organic matter to marine ecosystems. This flow is overwhelmingly in one direc-
tion, but not entirely so. Some fish swim against the flow, migrating upstream to spawn, for
instance. Fish that migrate from the sea into fresh water for breeding, so-called 'anadrom-
ous' species, such as salmon, are prime examples. Salmon attain most of their body mass
feeding at sea, so when they die in a river after spawning their carcasses make an important
contribution of nutrients and energy to both aquatic and adjacent terrestrial ecosystems.
The flow of water has almost inevitably produced an emphasis on spatial complexity in
studies of river ecology, but variations in stream flow over time are also important. The
quantity, timing, and variability of a river's flow create a mosaic of habitats to which fluvi-
al organisms have adapted. The ecology of rivers in regions with a Mediterranean climate,
for example, is attuned to substantial seasonal variability in flow because most of the rain
falls in winter (often 80% or more in three months). A cool, wet season is followed by a
warm, dry season which produces a rhythm of flooding and drying in the rivers, although
the intensity of each season can vary markedly from year to year.
There's no doubting the numerous fundamental ways in which a river's biology is depend-
ent upon its physical setting, particularly in terms of climate, geology, and topography.
Nevertheless, these relationships also work in reverse. The biological components of rivers
also act to shape the physical environment, particularly at more local scales. Beavers
provide a good illustration of the ways in which the physical structure of rivers can be
changed profoundly by large mammals. They cut wood and construct dams, trapping sed-
iments and organic material, modifying nutrient cycles, and ultimately influencing many
other communities of plants and animals.
Finally, it is worth emphasizing again the many ways in which the ecology of rivers has ef-
fects far beyond the channel itself. In the same way that a river plays a key role in shaping
the landscape it moves through, its flow provides important services to many of the plants
and animals that inhabit that terrain. The most obvious of these is as a source of water and
