ENVIS Technical Report: 25

The geographical distribution, morphometry and water quality of lakes within large calderas (> 2 km in diameter) were evaluated by Gary L. Larson ⁶⁶ (1989) through a review of the literature and maps. Eighty-eight lakes in 75 calderas were located in 31 volcanic sub regions. As a group, the lakes varied greatly in elevation, surface area, maximum depth, and shoreline development. The 'average' surface area was 16.9 sq. km, surface elevation 873 m, depth 151.1 m, and shoreline development 1.35. Water quality ranged from ultraoligotrophic to highly eutrophic. Water clarity in some lakes was among the highest recorded for freshwater systems, but there were indications of possible declining clarity in some cases. Secondary volcanic activity, such as primary (hydrothermal) water and eruptions, has been associated with deteriorated water quality conditions in some lakes. The study suggested that, as a group, the water quality characteristics exhibited by caldera lakes are not only a function of their size, shape, and watersheds and climate, but also a function of secondary volcanic activity as well.

The International Joint Commission (IJC) established by the 1909 Boundary Waters Treaty between Canada and the United States stands to cooperatively resolve disputes between the two countries, including water and air pollution, lake levels, power generation, and other issues of mutual concern has been involved in assessing and tracking aquatic ecosystem health in the Great Lakes. As the concern for pollution increased, the IJC was given the responsibility to assist in the implementation of the Great Lakes Water Quality Agreement (GLWQA), to restore and maintain the chemical, physical, and biological integrity of the waters (Canada & the United States, 1972). In 1987, the International Joint Commission's Great Lakes Water Quality Board (GLWQB) recommended and adopted, in principle, a set of quantitative and qualitative listing/delisting criteria for each of the 14 use impairments.

John H. Hartig and Michael A. Zarull ⁶⁷(1992) established a consistent set of criteria that can be uniformly applied throughout the Great Lakes basin to assist in making recommendations on listing and delisting Great Lakes Areas of Concern, which are intended to provide a 'set of rules’ that are consistent with the GLWQA and can be uniformly applied throughout the Great Lakes basin.

Huang, Yiping, Zhu, Min ⁶⁸ (1996) discussed basic strategies to protect water quality and prevent eutrophication in relation to lake Tailhu, the third largest freshwater lake in the most advanced economic zone in China, with a surface area of 2338 sq. km, in Changjiang River Delta,. Water quality monitoring was done during 1986 to 1993, the dynamic variations of water quality and eutrophication trends were analysed, showing obvious spatial and temporal variations. Great quantities of pollutants had been discharged into the lake, due to which its nutrient content had increased continuously, and phytoplankton blooms occurred in some areas.

Many rivers were being dammed owing to the irregular distribution of water supply in Africa in an effort to aid development. Volta, along with Nasser, Kariba, Kainji is one such large, man-made reservoir occupying over 7000 sq km of the total area of Ghana. Viner A.B ⁶⁹ (1969) discusses the general limnology of lake Volta and provides comparison of the lake's hydrological regime to other tropical lakes, based on investigations in January 1965 about six months after the closure of the dam. There was an initial brief boom of productivity followed by extreme deoxygenation with high turbidity, low light penetration and no thermocline. There was fish mortality, and the conditions common upon the immediate impoundment of water.