Environmental Monitoring

Its location downstream from major population centres in Zambia and Zimbabwe and from areas where the demographic, agricultural and industrial pressure is intense, justifies that special care be taken in monitoring, appropriate for controlling the quality of the water that serves as a habitat for the ecosystems generated after the lake filled, and which flows into the aquatic systems, and also the preservation of the retention and electricity generation infrastructures.

Environmental monitoring of the lake thus follows the principles established in the Norms of Operation of Dams (NEB) and takes into account the recommendations of ICOLD (International Commission on Large Dams), of the specialized agencies of the UN, and of the existing protocols within SADC (Southern African Development Community).

The Cahora Bassa lake monitoring plan is centred on the strategy of assessing the environmental and ecological conditions of the lake and contributing to its management in such a way as to control environmental problems related with water pollution and sedimentation throughout the entire extent of the lake (Songo - Zumbo) annually.

Noteworthy objectives:

• To determine water quality and its trends;
• To diagnose discharges of pollutants;
• To accompany the evolution of ecosystem performance;
• To raise alerts with due notice in case of need;
• To assess the evolution of sediments at the bottom of the lake.

Parameters observed

Habitually, the following parameters are observed in situ through a through a probe covering several aspects: temperature (°C), pH, electrical conductivity (μS/cm), turbidity, chlorophyll, dissolved oxygen, TSD. Apart from the parameters mentioned, other aspects and physical characteristics of the water are observed visually, such as aquatic blooms, and the colour and transparency of the water.

The samples of water collected are sent to the HCB Chemical Analysis Laboratory, to determine their complete additional physical and chemical characteristics: hardness, dry residue, mineralisation (anions and cations), as well as indicators of pollution, dissolved gases and aggressivity index.

Resources

Campaigns near or at a medium distance from the dam have been undertaken on the river with a Catamaran. Campaigns along the entire length of the lake have been carried out by helicopter, and from 2005 they were undertaken using an appropriate Catamaran with conditions and autonomy for long distance work.

Collection and Analysis of Water and Bottom Sediments

As from 2004, campaigns have been carried out annually to collect samples of water and of sediments from the lake bottom and from the banks of the main tributaries, with the aim of making a preliminary estimate of the rate of sedimentation and the levels of silting. These campaigns have provided an understanding of the evolution of sedimentation and of the indicators of the level of erosion, transport and deposition, as well as the temporal and spatial distribution of the sediments along the length of the lake.

Control of Weeds

The environmental monitoring and audit programme prioritises surveillance of the development of aquatic weeds. This began in 2000 with the first overflight by light plane, and continued as from 2004, with helicopter flights and boat trips.

It should be noted that in the rainy season, particularly from January to April, there regularly occurs a high inflow of floating colonies of plants up to the dam wall.

In order to combat these floating colonies, a line of floating retainers was installed which prevents the colonies from approaching the suction area of the intake points, thus avoiding any eventual damage in the hydraulic circuits of the generators, and the consequences that might flow from this. In addition, the plants are later removed with the help of boats. The prevailing winds, aligned with the axis of the lake, and blowing in the direction opposite to the flow of the river, strongly hinder their approach to the dam.

As a result of the reduction in the charge of pollutants because of the annual renewal of the mass of water, and because there are still no significant sources of pollution, there has been a stable containment of the zones of weed colonisation. The great depth of the lake, together with the steep banks, has prevented their easy germination up to about 20 km upstream of the dam, naturally excluding some inlets.