Dams and Impounded Rivers - River Dynamics and Integrated River Management - page 403

turbulence, a mixed water-body, and a lack of thermal stratification—such that, if rates of water
movement through a lake exceed a few millimeters per second, little plankton will develop (Hynes, 1970).
In the Pitlochry Reservoir, UK, the retention-time is never more than 4 days and only occasional planktonic
organisms were found during a 4-year period of observation (Brooks and Woodward, 1956). In general,
sediment concentration in the reservoir limits the concentration of phytoplankton and zooplankton.
In the first few years of impoundment, the amount of nitrogen and phosphorus in the reservoir water
increase, as a result of the decay and mineralization of organic matter flooded by the lake, which may
lead to blooms of blue-green algae (cyanobacteria). The plankton population of reservoirs constructed on
a mountain stream or turbid lowland river, may require a long period of time to develop, whereas on a
clean lowland stream, the process could be rapid. Some species will adapt to the lake conditions
preferentially. It is these lentic species, which have significance for stimulating secondary production
through invertebrates to fish etc., not only within the lake, but also within the river channel downstream.
The impoundment of the River Niger in Nigeria, for example, has more than doubled the annual peak
phytoplankton density to about 2500 algae/mL, and tripled the zooplankton peak within the river
downstream (El-Zarka and El-Din, 1973).
Algal blooms occur in reservoirs if the concentrations of nitrogen and phosphorus in the lake water are
high and high temperatures and sunshine are maintained for a long time. Hergenrader (1980) examined
the eutrophication process as it occurred in a system of new reservoirs—the Salt Valley Reservoirs,
Nebraska, USA. The intensively-cultivated prairie soils supply runoff to the reservoirs with high nutrient
concentrations, and so promote eutrophication. All the reservoirs are shallow (maximum depth 10 m),
and have large surface areas, which allow wind-induced mixing that prevents thermal stratification and
provide a relatively homogeneous heat and nutrient distribution—conditions highly suited for vigorous
algal growth. One of the reservoirs, dominated by high levels of inorganic turbidity, developed only a
poor phytoplankton population within which diatoms persisted, whereas three 'clear' reservoirs produced
dense standing-crops of phytoplankton and their surface-releases led to a continued increase in the
nutrient pool, with a consequent increase in phytoplankton growth.
7.1.5.2 Vegetation Reaction
Beds of aquatic macrophytes play a significant role within river ecosystems (Turner and Karpiscak,
1980). Below deep-release dams, discharges may be oxygenated by plants during the day, and weed-beds
provide a diverse microhabitat. Macroinvertebrates can have density values up to fifty times as great as
in areas lacking macrophytes (Décamps et al., 1979). However, the development of aquatic plants is not
always favorable. The lack of regular flushing by floods on the Zambezi, for example, has encouraged
the development of dense aquatic macrophytes to the detriment of living-space for Hippopotamus
amphinius , crocodiles, and wildfowl (Attwell, 1970). Similarly, stabilized flows on the Tuolumne River,
California, U.S., have contributed to favorable conditions for luxurious growths of Eichhornia crassopes
(Water-hyacinth) in some years, which can impair, and totally block the salmonid migration in extreme
cases (Fraser, 1972).
Floating plants are of little importance in temperate latitudes, but in the tropics, where turbid water
inhibits the growth of submerged species, floating plants are often important (Hynes, 1970). The elimination
of high discharges to flush the problem species has allowed the extensive development of Eichornia
crassipes and Salvinia molesta in both Africa (Jackson and Rogers, 1976) and Australia (Walker, 1979).
Infestations of the floating plants caused deoxygenation of the water, clogged irrigation and water-supply
intakes, provided breeding grounds for mosquitoes, and interfered with recreation.
Under natural conditions, flood discharges periodically submerge portions of the river-bank or floodplain.
These flows rearrange sedimentary deposits, and can be key events in maintaining riparian habitats. An