Why water in Lake Victoria sometimes changes colour and stinks

As the world prepares to celebrate World Rivers Day on September 25, sensitisation on pollution has picked up. Plans to attend COP 27 in Egypt to implement the Paris Agreement have also kicked off. As that happens, we in the water sector are already feeling the first-hand impacts of climate change and pollution. Hence, we are not only interested in this conversation, but are in the middle of it. Rivers and streams feed our lakes which are our primary sources of raw material and consequently source of revenue. The rising lake levels and the effects of pollutants are very well known to us.

In the recent past, we have noticed a phenomenon that has changed the colour of water and odour of the Lake from what we are used to. We talked to our counterparts in Homa Bay County who confirmed the same fears that the lake is too polluted this time of the year and has a distinctive stench. These effects, as we learnt, are mostly felt in the gulf areas.

We sought the opinion of experts who explained that the lake has a significant impact on regional climate. Climate variables such as the Intertropical Convergence Zone (ITCZ), El Nino/Southern Oscillation (ENSO), complex orographic forcing and Indian Ocean zonal temperature gradient anomalies impact its thermodynamics and hydrodynamics on diel, seasonal and yearly periods.

The ITCZ, which separates the northeast and southeast monsoons, passes across East Africa twice a year. First, in March-April-May and again in October-November-December. The ITCZ's incursion and retreat are responsible for the region's two primary wet and dry seasons. The rainy season from March to May is known as the 'long rains', whereas the rainy season from October to December is the 'short rains'. The water body is primarily regulated by precipitation on the lake's surface, catchment input, controlled outflow from a hydroelectric dam on the Nile and evaporation.

When the lake becomes isothermal (retains the same temperature), there is a season of deep vertical mixing. When mixing happens, water is depleted of oxygen at the lake's bottom and the broken down organic materials at the bottom rises to the top. When these upwelling episodes occur, fish suffocate and perish. The interaction of decomposing organic waste with water results in the dark colouration of water, which is occasionally accompanied by a bad stench.

While the mixing is caused by climatic occurrences, the consequences of the changing climate and human influences are becoming more catastrophic. For example, higher precipitation in the previous four years has resulted in a considerable rise in lake level. Increased surface runoff carried a large amount of organic material into the lake, which is currently decomposing at the bottom. This breakdown takes place around June and July due to the seasonal advent of the southeast trade winds, and for a short time towards the end of July, the main body of the lake becomes isothermal in terms of depth.

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As a water company operating on the shores of Lake Victoria, we have adapted to the changes in the lake, that include hyacinth challenges, and have introduced systems to deal with such risks. This gives us the confidence to continue drinking water directly from the taps in Kisumu City. Let's join hands and take care of the rivers and understand that everything that is thrown into rivers and streams ends up in our lakes and in the end will impact our lives.