Why lakes in Rift Valley are rising

A marooned Kenya Wildlife Services office at the Lake Nakuru National Park in July this year. The park’s senior warden, Mr John Wambua, said the phenomena has also caused flamingos to migrate to neighbouring lakes of Bogoria, Elementaita and Naivasha’s Crater Lake. PHOTO| SULEIMAN MBATIAH

What you need to know:

  • All lakes in Kenya’s Rift Valley have risen since 2011, to levels not seen in the last 50 years.
  • In the current environment of increased stress, the lake levels would be expected to rise and remain high even with normal rainfall until a new steady state is reached. We should be at the peak of relative stress differential where the lake levels will stop rising.

There is no denying that the rise of lakes pose a great risk to our lifestyles, environment and economy.

Water rises alter society physically and socially. For instance, due to the rising of water in Rift Valley lakes, the famous hot springs and jets at Lake Bogoria have subsided. This has had a direct impact on the aesthetics of the area and visitors that fancy this iconic site. ciety physically and socially. For instance, due to the rising of water in Rift Valley lakes, the famous hot springs and jets at Lake Bogoria have subsided. This has had a direct impact on the aesthetics of the area and visitors that fancy this iconic site.

There have also been reports from Lake Nakuru National Park of the migration of the scenic flamingoes because the lake is no longer saline. Yet the birds have been a major attraction.

In fact, navigating the park today is almost impossible. This is likely to affect tourism and, consequently, revenue.

Besides, those living around the lake will always be affected by the rising waters. The media have been awash with stories of communities and schools affected by the rising Lake Baringo. Yet this is no mistake of theirs.

Detecting such a phenomenon is a sophisticated process. Only through a disaster monitoring system, like the one we are putting in place at the Geothermal Development Company, can someone detect the impending rise and alert potetial victims. That a monitoring system on the behaviour of the Rift Valley is critical to avert disaster cannot be gainsaid.

All lakes in Kenya’s Rift Valley have risen since 2011, to levels not seen in the last 50 years.

These include Naivasha, Elementaita, Nakuru, Bogoria, Baringo, and Logipi. Others that are not in the main Rift Valley include Lake Simbi (Nyanza Rift) in Homa Bay County and Lake Chala at the Coast.

Various government agencies have advanced different hypotheses to explain the high rise.

The Meteorological department has reported that the rainfall patterns in the rift and its catchment areas are normal while the Ministry of Environment, Water and Natural Resources has blamed the rise in levels to siltation of the lakes due to degradation of the catchment areas. The media have picked some of these flawed explanations without much scrutiny.

However, scientifically, the rise in the water level of the lakes in Rift Valley is due to effects of regional tectonics influenced by the movements of global earth’s plate tectonics.

This creates near-field and far-field compressional and tensional stress fields. The near-field stress regime or earth area, will respond to the far-field stress regime, but with a time lag. In our case, the near-field is the Rift Valley responding to the pull of the Indian Ocean which is a far-field body. Near-field means a localised area affected by earth activities. Far-field means an earth area that is far away.

The East African Rift System is a narrow zone that runs from Afar in the north to Mozambique in the south in which the African “Plate” is in the process of splitting into two new tectonic “plates”, called the Somali Plate and the Nubian Plate.

DRAINING WATER

The localised Kenya Rift sector is the eastern arm of the East African Rift System. The stress field in this sector is, therefore, largely classified as near-field tensional and is characterised by open normal faults that have dips into the Rift graben. Effects of the near-field tensional fracturing arising from low stress fields result in open fractures common in Naivasha and Nakuru areas. It is through these structures that water drains into the ground.

The Rift Valley in Kenya is opening at an average rate of 2-3mm per year, varying from periods of fast extension to those of low extension.

The change in the rate of extension is controlled by the far-field plate tectonic processes occurring in the mid Indian ridge in the east and sub-duction in the Mediterranean Sea in the north. Under normal stress regime and with normal rainfall, the lake levels would be expected to maintain near constant levels that is equilibrium when the rate of drainage and input are at par.

However, during high near-field tensional stress regime and normal rainfall, ground water seepage would be reduced due to decreased porosity associated with closing of pores and fractures. Then we will experience cases of drying up lakes.

Our sector of eastern Africa is under increased near-field stress related to changes in global stress patterns due to a decreased far-field extensional process. It is similar to being at the rear end, that is the near-field, in a convoy of cars on the highway maintaining the same distance and speed. When there is a drastic reduction of speed by the car in front (far-field), the distance (fracture openings) between the cars at the rear of the convoy (near-field) will be less because of the reaction time lag. With time, the drivers will adjust and keep the same distance and speed again (the lake levels will stay constant again).

In the current environment of increased stress, the lake levels would be expected to rise and remain high even with normal rainfall until a new steady state is reached. We should be at the peak of relative stress differential where the lake levels will stop rising. When the differential is negative lake levels will rise and when positive lake levels will go down and even dry as has happened before.

This is more of a geophysical phenomenon that we have to live with than rainfall, forests and siltation.

In order for the influence of regional and local tectonics on lake levels to be well understood, it would be important for Kenya to institute national monitoring programmes that include geodetic measurements, interferometry of Rift volcanoes, seismic monitoring, and hydrogeological monitoring.

At Geothermal Development Company, we are carrying out monitoring at the Menengai geothermal field and undertaking geohazard monitoring for the whole of Rift Valley.

Dr Simiyu is a seismologist and chief executive of Geothermal Development Company.