Limnology Project at Mahaweli Reservoirs:I. Some Physical Properties of Kotmale, Victoria and Randenigala Reservoirs

Abstract

The Physical properties of Kotmale, Victoria and Randenigala reservoirs were studied since 1987 upto now and this paper presents the data obtained during 1987. The transparency values changed with the reservoir and the season. The highest frequency percentage of Secchi depth observed at each reservoir were. Victoria 1.21-1.61m, Randenigala 2.51-3. Olm and Kotmale 2.10-2.30m, indicating highest values at Randenigala and lowest values at Victoria. Accordingly the mean euphotic limits were 5.00m at Victoria, 6.25m at Randenigaia and 5.lOm at Kotmale. The highest differences in temperature gradients were observed from surface to about 25m depths in all three reservoirs, in the deeper layers the temperature gradient is distinctly low. The highest conductivity values were observed closer to the dams of all three reservoirs indicating the accumulation of dissolved ions towards the dams. Range of 30-87pslcm at Kotmale, 85-139 pslcm at Victoria and 110-188 pslcm at Randenigala. The highest values were recordedfrom Randenigala. The highest conductivity values were correlated with the water rentenion levels of the reservoirs. Fluctuations in pH values with the seasons follow a similar pattern in all three reservoirs. The mean pH values of the three reservoirs were in between 6.0 and 8.7. In all three reservoirs, the pH gradient was higher above 10m depth whereas below this depth it remained more or less constant. The high dissolved Oxygen concentrations were closely linked with the euphotic zones of the three reservoirs. The mean Oxygen concentration calculated during 1987 investigation period from surface to 5m depth were 4.75-7.50 mg]; at Kotmale, 4.70-6.60 mgjl at Victoria and 4.70-6.65 mgfl at Randenigala reservoirs. Dissolved concentrations were very low below the depth of 20m in all three reservoirs. However there was no evidencefor the presence of deoxy genated layers in all three reservoirs.