Why the water level of L. Victoria is not the same at different places

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

10 May 2020

George Njeru was intrigued by a report that appeared in the Daily Nation on 27 April 2020 about the of water in Lake Victoria. It stated that the at Kisumu, Jinja and Mwanza were at 1,132.11m, 1,135.8m and 1,134.28m above sea level, respectively. George was left wondering why the three locations have different heights of the same water body.

The answer lies in the size of the lake – it is the second largest fresh water lake in the world. The distance from Kisumu to Bukoba in a straight line is about 350km. This is approximately 100km farther than from Kisumu to Nairobi!

Lest some one comes to challenge that statement by saying that they know the distance from Kisumu to Nairobi is more than 350km, let me reiterate that I am referring to the straight-line distance, not the road distance. From Kisumu to Nairobi is about 250km.

On the north-south direction, the distance on a straight line from Mwanza to Junja is about 300km. In short, Lake Victoria is immense. For that reason, we should not expect its surface to behave in the same way as water in a basin!

Wind blowing over the surface will push the water from one shore to another. The outflow into the White Nile at Jinja also affects the water level at that area. However, the greatest effect is from tidal motions.

Just like the sea, large inland water bodies do experience tidal motions. Consequently, when the tide is high on one side, it will be low on another.

Indeed, because of these effects, even the sea level itself is not the same all over the world! For instance, the Atlantic side of the Panama Canal is about 20cm lower than the Pacific side.

For that reason, experts carry out sea level measurements at different location and over a 19-year period and use that data to get a Global Mean Sea Level. Why 19 years; why not 20? The answer is that 19 years is the duration after which the phases of the moon occur on the same days as the previous cycle.

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On a slightly different matter, but still to do with water levels, the spill ways at the hydroelectric dams along River Tana were opened last week resulting in flooding in downstream areas, especially Garissa and Bura. At about the same time, the Nairobi Water Company shut down its main treatment plant at Ng’ethu for two days.

Now; Ng’ethu gets water from Rivers Chania and Thika (Ndakaini dam); both rivers join in Thika town and empty into the Tana at Masinga dam. Nairobi fetches about 400 million litres daily from these rivers, so the question arises: was the shut down of Ng’ethu treatment works responsible for the over flow in the Tana?

In other words, should we blame the Nairobi Water Company for the flooding in Garissa and Bura? I will discuss that next week; in the meantime, why are the FIVE dams on River Tana called the “SEVEN Forks”?