Dams and Impounded Rivers - River Dynamics and Integrated River Management - page 397

decreasing density with decreasing temperature—from that point downwards. In summer the higher-
temperature, low-density, water layer 'floats' on top of the lower, cooler, water. In autumn and winter
however, the temperature of the surface layer reduces to 4 ć and the density reaches its maximum and
higher than that of the lower layer. The surface layer settles down and the lower layer overturns. The
phenomenon is called overturn, which occurs two times a year in autumn and spring. Overturn occurs in
the reservoirs in high latitude areas.
7.1.2.2 Chemical Stratification
The quality of natural river-water is controlled, predominantly, by the climatic and geological characteristics
of the drainage basin. Water storage in open reservoirs induces physical, chemical and biological changes
within the stored water. Natural river-water commonly contains four important cations (calcium,
magnesium, sodium, and potassium), while chloride, sulfate and bicarbonate are the predominant anions .
The relative importance of the different ions depends, in general terms, upon the geographical location
(Gibbs, 1970). Soil disturbance, drainage and vegetation clearance, have increased concentrations of NO 3 - ,
SO 4 2- , and Mg 2 and urbanization can lead to the addition in stream-flows of phosphate, nitrogen compounds,
and heavy metals.
The chemical composition of stream water that is released from a reservoir can be significantly different
from that of the inflows—though, in some cases, releases can have a chemical composition, which
reflects that of the inflows and any precipitation received. Chemical changes within reservoirs have been
attributed to a variety of factors that are typically associated with their flow-dynamics and biological
activity. Major, biologically induced, water-quality changes occur within thermally stratified reservoirs.
Phytoplankton often proliferate in the warm epilimnion, releasing oxygen and maintaining concentrations at
near-saturation levels for most of the year. Due to the settling of dead phytoplankton, and the presence of
heterotrophic bacteria, oxygen will be consumed in the hypolimnion, often to exhaustion. Thus, the process
of organic-matter decay becomes anaerobic; hydrogen sulphide gas is produced; carbon dioxide is released;
PH decreases and the solution of iron and manganese occurs from the bottom sediments. The quality of
the hypolimnial water becomes progressively worse, until the autumn overturn (Petts, 1984).
Some reservoirs may mature after as little as 3 or 4 years, but the trophic upsurge can take 6 years and
in some reservoirs a period of more than 20 years may be required for the development of a stable
water-quality pattern. A unique example of a reservoir maturing is given by Zhadin and Gerd (1963). The
Dnieper Reservoir, USSR, built in 1934, was destroyed in 1941. During the intervening years considerable
organic deposits, up to 4.0 m deep, accumulated and they subsequently were rapidly colonized by land
vegetation. In 1947, the rebuilt hydroelectric power-dam was completed, the reservoir drowned the organic
deposits and the decomposition of these deposits had an appreciable effect on the chemical and biological
conditions of the new reservoir. During summer, a distinct stratification occurred, with the surface
between 4.5 and 9.5ÛC warmer than the underlying layers and for the early years oxygen conditions were
unfavorable. Above the decaying, flooded vegetation, oxygen was totally absent and the amount of free
carbon dioxide approached 20 mg/L.
A dissolved-oxygen profile may demonstrate stratification even if the reservoir is not thermally
stratified. In the epilimnial layer water-mixing, by wind and wave action, combined with photosynthesis
by Algae, maintains dissolved oxygen (DO) concentrations at near-saturation. The rate of photosyntheseis
in the upper layers of a reservoir will be limited by the supply of nutrients—particularly nitrogen and
phosphorus- and by light-penetration, influenced partly by the algae themselves but also by suspended
solids. Within Cherokee Reservoir, USA (Fig. 7.4), high dissolved-oxygen concentrations of 10 mg/L are
related to dense concentrations of phytoplankton, of between 30 and 60 thousand per mL, near the surface.
Below 10 m, however, conditions are unsuitable for phytoplankton and DO concentrations are reduced to