River Ecology and Restoration - River Dynamics and Integrated River Management - page 642

resilience of an ecosystem, r e , may be measured with the ratio of the impairment of ecosystem after
disturbances, W over the time needed for the ecosystem to return its original form, T :

W

r

(10.1)

T

Recovery is the degree to which a system returns to its original condition after a disturbance. Natural
systems are able to recover and restore stability following disruptions and disturbances.
A system may change yet still remain stable and in a good condition. When a large stable system has
small, local changes within it, it is described as having mosaic stability. A good example would be a
riparian system greatly disturbed by a 100-year flood. If this were to occur in an area which was urbanized,
then it would become a dangerous gap in a habitat that is already rapidly decreasing and would separate
and isolate cause populations of rare amphibian species. However, that same system, in a less urbanized
area may not be harmful to amphibians but may just represent a mosaic of constantly shifting suitable
and unsuitable habitats in a naturally functioning, unconfined system. A landscape with mosaic stability
is not likely to need restoration, whilst one without would urgently need restoring.

10.2 Ecological Stresses

Ecological stresses are defined as the disturbances that bring changes to river ecosystems. The ecological
stresses are natural events or human-induced activities that occur separately or simultaneously. The
structure of a system and its capability of carrying out important ecological functions may be changed by
stresses, regardless of whether they act individually or in combination. One or more characteristics of a
stable system may be permanently changed by a causal chain of events produced by a stress present in a
river. For instance, land use change may cause changes of hydrological and hydraulic features of the river,
and these changes may cause chages in sediment transportation, habitat, and ecology (Wesche, 1985).
Disturbances are not all of equal frequency, duration, and intensity and they may occur anywhere
within the stream corridor and associated ecosystems. A large number of disturbances of different frequency,
duration, intensity, and location may be caused by one single disturbance. Once people understand the
evolution of what disturbances are stressing the system, and how the system reacts to those stresses,
people can decide which actions are needed to restore the function and structure of the stream corridor.
Disturbance occurs within variations of scale and time. Changes brought about by land use, for
example, may occur within a single year at the stream or reach scale (crop rotation), a decade within the
stream scale (urbanization), and even over decades within the landscape scale (long-term forest
management). Despite the fact that wildlife populations, such as the monarch butterfly, remain stable
over long periods of time, they may fluctuate greatly in short periods of time in a certain area. Similarly,
while weather fluctuates daily, geomorphic or climatic changes may occur over hundreds to thousands of
years.
Although it is not observed by humans, tectonic motion changes the landscape over periods of millions
of years. The slope of the land and the elevation of the earth surface are affected by tectonics, such as
earthquakes and mountain creating forces like folding and faulting. Streams may alter their cross section
or plan form in response to changes brought on by tectonics. Great changes in the patterns of vegetation,
soils, and runoff in a landscape are caused by the quantity, timing, and distribution of precipitation. As
runoff and sediment loads vary, the stream corridor may change.

10.2.1 Natural Stresses
Climatic change, desertification, floods, hurricanes, tornadoes, erosion and sedimentation, fire, lightning,
volcanic eruptions, earthquakes, landslides, temperature extremes, and drought are among the many