Dams and Impounded Rivers - River Dynamics and Integrated River Management - page 402

of mixing the water from an anoxic hypolimnion; but more significant was the rapid rise in dissolved-
oxygen concentrations at depth. After 7 weeks, well oxygenated water was observed throughout the
profile; the time-lag in relation to temperature probably resulted from large organic accumulation in the
hypolimnion. The following year, pumping began before stratification had become established, although
a 7ÛC temperature difference existed between the surface and bottom waters after only 2 days of pumping,
an isothermal profile was developed and the dissolved oxygen concentration was stabilized.
Fig. 7.6 Profiles of temperature, dissolved oxygen, PH, and biochemical oxygen for Ham Lake, Oklahoma, the USA
before (July 12-14) and after destratification (after Garton et al., 1976)
7.1.5 Impacts on Ecology
Dams are one of the main reasons why one-fifth of the world's freshwater fish are now either endangered
or extinct. The percentage rises even higher in the countries which have been most heavily dammed—to
nearly 2/5 in the U.S. and 3/4 in Germany. Amphibians, molluscs, insects, waterfowl and other riverine and
wetland lifeforms are similarly imperiled ( World Resources Institute, 1994).
7.1.5.1 Phytoplankton
Impoundment of reservoirs favors the development of lentic plankton. Both zoo plankton and phyto-
plankton require a minimum retention-time to allow development. The development time may be longer
for the zooplankton than the phytoplankton (Brooks and Woodward, 1956). The quantity of lake plankton
tends to be inversely proportional to the speed of water-flow, so that a general transition may be observed
from a small, shallow reservoir receiving runoff from a large catchment area and having low primary
production, to a large deep reservoir with a relatively small catchment, and supporting high primary
production (Rzoska et al., 1955).
In all reservoirs, primary production is mainly derived from the activity of phytoplankton. Blue-green
algae often dominate in numerical terms but they are often outweighed in biomass by diatoms. Three
factors govern the contribution of lentic plankton to lotic systems: the rate of water replacement within
the reservoir (the retention time or flushing time), the seasonal pattern of lentic plankton development,
and the character of the outflows from the reservoir. Short retention-times are often associated with high