Integrated River Management - River Dynamics and Integrated River Management - page 725

riparian wetlands are sites of high biodiversity that depend on flows from the river. Extending the duration
of river water remaining on the continent may provide aquatic bio-communities larger and better habitats
and provide humans longer time for river uses, e.g., flood water utilization and water surface recreation.
There are two strategies for this purpose: (1) extending the river course; and (2) reducing the flow velocity
(Wang et al., 2007).
Fig. 11.9 Large boulders create high resistance and protect the bed from erosion, which make the stream achieve
super-equilibrium (See color figure at the end of this topic)
Low velocity has important ecological implications. The stream and riparian waters are habitats for
various species that depend on water flows. Most of aquatic animals live in lentic or low flow velocity
waters. A suitability index, SI , is defined as the suitability of physiochemical conditions of the habitat for
species to live and spawn. SI 1 represents the best conditions and SI = 0 means the worst conditions. In
fact, SI 0 implies that the species prefer to not live in the habitat. Wang et al. (2007) studied the
suitability index for most important vertebrate species in rivers and presented the distributions of the
suitability index as a function of velocity for 36 fish species. Table 11.1 lists the values of critical
velocities of U c 1 , U c 2 , U c 3 , of which U c 1 is the critical velocity when SI increases to 1; U c 2 is the critical
velocity when SI begins to reduce from 1; and U c 3 is the critical velocity when SI reduces to zero. The
value of SI equals to 1 for velocities between U c 1 and U c 2 .
The velocity range of SI for adult, fry, and juvenile fishes and spawning is different for different
species. Figure 11.10 shows the statistical distributions of the three critical velocities for adult, fry, and
juvenile fishes and spawning. The vertical axis is the percentage of the statistical species at which the
critical velocity is smaller than U c . The striped shaded area is the range for SI 1, or, the best range of
velocity for the species to live and spawn. As shown in Fig. 11.10(a) and (b), 100% of the species have
an SI value less than 1 and about 90% of the species prefer not to live in areas with velocity higher than 2
m/s for adults, and 100% of the species have an SI value less than 1 and about 85% of the species cannot
spawn if the velocity is higher than 2 m/s. Figure 11.10(d) shows that 100% of the species have an SI
value equal to zero for juvenile fishes if the velocity is higher than 2 m/s.
If the velocity is very low algal blooms may occur in rivers with eutrophication. According to field
investigation very few algal bloom events occur if the flow velocity is higher than 0.3 m/s. Therefore, a
flow velocity in the range of 0.3-2 m/s is the best for river ecology.