Environmental Engineering Reference
In-Depth Information
(a)
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
(b)
9
8
7
6
5
4
3
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
ln (chlorophyll) (mg m
-2
)
ln (chlorophyll) (mg m
-2
)
(d)
(c)
2
30
1
25
1908 -1942
1966 -2000
0
20
-1
15
-2
10
-3
5
-4
0
3
4
5
6
7
8
9
ABCDE F
ln (fish biomass) (g m
-2
)
Downstream
Upstream
Fig. 2.11
Interrelationships among biological parameters measured in a number of reaches of the Colorado River to
determine the ultimate causes of the declining distribution of Colorado pikeminnows. (a) Invertebrate biomass vs algal
biomass (chlorophyll
a
). (b) Prey fi sh biomass vs algal biomass. (c) Pikeminnow density vs prey fi sh biomass (from catch
rate per minute of electrofi shing). (d) Mean recurrence intervals in six reaches of the Colorado River (for which historical
data were available) of discharges necessary to remove silt and sand that would otherwise accumulate, during recent
(1966 -2000) and pre-regulation periods (1908-1942). Lines above the histograms show maximum recurrence intervals.
(From Begon et al., 2006, after Osmundson et al., 2002.)
maintain a 'healthy' river ecosystem: 30% is often used as a rule of thumb. Hydraulic
methods relate discharge to the geometry of stream channels (based on multiple
measurements of river cross-sections); river depth and width begin to decline
sharply at discharges less than a certain percentage of mean discharge (10% in some
rivers) and this infl ection point is sometimes used as a basis for setting a minimum
discharge. Finally, habitat assessment methods are based on discharges that meet
specifi ed ecological criteria, such as a critical amount of food-producing habitat for
particular fi sh species.
Managers need to beware the simplifi ed assumptions inherent in these various
approaches because, as you saw with the pikeminnows, the integrity of a river eco-
system may require something other than setting a minimum discharge, like infre-
quent but high fl ushing discharges. Simply maintaining a minimum fl ow may be a
poor substitute for what is actually required to be ecologically effective (Palmer
et al., 2005) - a change to patterns of water release from dams to move the river
back to a more natural hydrograph.
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