Lake Kivu is shared by Rwanda and DR Congo with the former occupying 42% and the latter 58%. The lake is the sixth largest and third deepest of the seven African Great Lakes. It covers a surface area of 2,370 sq. km, stretches for 97km long, 48km wide, has a shoreline of 860 km and holds a water volume of 560 cubic km.
The lake has an average to maximum depth of 240-485 meters making it the third deepest lake in Africa after Lake Tanganyika and Lake Malawi. Lake Kivu is a freshwater rift valley lake that was formed due to a combination of both volcanic and tectonic activities.
During the formation of the Virunga Mountains along with the rise of the Congo Nile Ridge lava flows blocked outflows from Kivu causing the water levels to rise and form the lake during the Pleistocene Era about 1.5 million years ago. Lake Kivu drains southwards into Rusizi River which eventually flows into Tanganyika.
Lake Kivu is situated at a relatively high altitude of 1,462 meters above sea level in the western arm of the Great East African Rift valley south of the Nyiragongo and Nyamuragira Africa’s most active volcanoes. Due to the rich volcanic fertile soils, the lake basin is ideal for human settlement and agriculture.
There are over 2 million people living in the basin that depend on the lake’s natural resources including fish, water, agriculture, transport, energy production, tourism, and rich biodiversity including 31 fish species; six mammal species; 80 birds; six reptiles, fifty two invertebrates species and five species of amphibians according to African Great Lakes Information Platform.
Some of the major towns and cities around Lake Kivu in DR Congo include Goma, Kalehe, Bukavu, Kabare, and Sake and Rusizi, Rubavu, and Karongi in Rwanda. Despite of the fact that Lake Kivu is an invaluable resource, it is also a threat to human life and the environment given that the lake’s deep layers below 260 meters contain high dissolved amounts of 60 km3 of methane (CH 4) mixed with toxic hydrogen sulfide and over 300 km3 (2.6 giga tones) carbon dioxide (CO 2) gases where they’re trapped under pressure.
According to UNESCO Geoscience study 2023, these gases can explode through lumnic eruption (lake overturn) a geological process through which dissolved gasses such as carbon dioxide or methane suddenly separate from the water and rise to the surface from the depths of the lake, forming a gas cloud capable of suffocating wildlife, livestock, crops and humans.
Lake Kivu and volcanicity
Lake Kivu is 12 km (7.5 miles) south of Mount Nyiragongo and 25 km (16 miles) Mount Nyamuragira which are active volcanoes. Nyiragongo volcano has erupted over 44 times since 19th century with the most recent violent eruptions occurring in 2002 and 2021. As a result of the plate tectonics and active volcanic activity, there are numerous geothermal hot springs around Lake Kivu including Nyamyumba, Rubavu, Bugarama hot springs through which gases enter into the lake leading to accumulation of methane and carbon dioxide.
Evidence from the Rwandan Environmental Management Authority confirmed that lava flows from 2021 Nyiragongo eruption flowed beneath Goma city and Lake Kivu through underground fissures. Such continuing volcanic eruptions as well as landslides, earthquakes and cool rainwater can triggered limnic eruptions in Lake Kivu.
However, a lake can also become unstable on its own when bacteria present in the sediments on the bottom of the lake decompose and ferment organic matter, raising the concentration of gases. For instance, Lake Nyos in Cameroon overturned in 1986 suffocating over 1,700 people and 3,000 livestock.
Given that Lake Kivu is almost three times larger than Lake Nyos when it explodes it could cause immense damage. In addition to the threat of limnic eruptions, there are also toxic gases that that are emitted from the continuous volcanic eruptions also pose short and long term health problems to the people.
Nyiragongo volcano has a permanent lava lake which releases large amounts of toxic gases including over 5,357 tons of Sulfur dioxide (SO2) and carbon dioxide into the atmosphere daily which is allegedly causing respiratory illnesses including eye irritations and chronic cough among people.
Impact of volcanicity
Methane, carbon dioxide, sulfur dioxide are among the primary greenhouse gases responsible for trapping heat in the atmosphere which contributes to global warming. Sulfur dioxide mixes with other components like corrosive fluoride, chloride and nitrogen oxides to form acidic rain. There has been volcanic acidic rainfall in the areas around Lake Kivu and Virunga Mountains.
Acid rain is harmful to biodiversity given that it can acidify soil and contaminate freshwater which reduces their ability to support plant growth. Implications from this include reduced agricultural yields and the ability of forests to absorb greenhouse gas emissions. Lake Kivu and the active Virunga volcanic system represent not only a geological threat to the local population but also a looming environmental risk with broader implications for climate change.
Extraction of methane from Lake Kivu
Rwandan established the KivuWatt power station in 2016 which extracts the gas and generates 26 megawatts of electricity to the national grid. The country aims to generate over 100 MW to provide clean energy to over 40,000 households. The station uses the latest technology of floating barges to pump and extract dissolved methane from the profundal zone and then separated from water to generate renewable energy.
This is projected to reduce the concentration of dissolved methane gas given that the lighter, degassed water is pumped back into the lake. Over time, the lake water is becoming less dense. The UNESCO study done before and after the Nyiragongo eruption of 2021 points out the there’s a significant change in the population of micro algae known as phytoplankton found at the depth of 60 meters.
Species of phytoplankton that had been abundant prior to the eruption were less so afterwards and species that had been less present before the eruption were more abundant afterwards. The research team intends to carry out more in-depth studies to understand whether such changes are linked to volcanic eruptions, shifts in atmospheric dynamics, or the ongoing methane extraction efforts in Lake Kivu.