February 21, 2022

Accounting for Rwanda’s Soil Loss: A Journey Along Nyabarongo River

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Upstream from the Nyabarongo is River Rukarara. As seen from the photo, the river is clear as it remains less contaminated with sediment. However, as it moves downstream towards River Mwogo, it becomes more dark due to heavy erosion along its path

Potential environmental and social impacts of the Nyabarongo I dam, according to the environmental impact assessment highlighted above, also include sediment transport and erosion, relocation of populations, impact on rare and endangered species, loss of livelihood and passage of migratory fish species in the Hydropower plant.

As Nyabarongo river reaches the reservoir, The Chronicles team observed new streams also heavily polluted from mines up the mountains, that pour onto Nyabarongo. About 5km upstream from the hydro dam, there is an unbearable sight of sediment piled up and exposed due to the low level of the reservoir. It is a clear impact of the dry season for the past months. 

The Chronicles asked for permission to visit these hydro dams as part of our probe. The government parastatal, Energy Utility Corporation Limited (EUCL) which manages Nyabarongo I, and Prime Energy Ltd, a private company that owns the three on River Rukarara, both did not respond to our official letters. 

At Nyabarongo I hydro dam, we found one gate, which is on the right side of the three gates, gushing out river water. Local residents said that the gate had been open, releasing water for at least the past two years. 

“We heard that the gate was damaged before COVID-19 and technicians have not yet come. So the river flows through 24 hours. We are not able to cross it now.” Mukandutiye Aniziya, a mother of two said. 

It perhaps explains why the reservoir’s water level has reduced so much; it  is down by about a meter or more, with the reservoir’s side exposed. The water’s marking is clearly visible. 

With river water pumped out at such a huge speed, there is no doubt about Nyabarongo’s impact downstream. 

One particular site is testament. At the confluence between Rivers Akanyaru and Nyabarongo to form River Akagera, the conjoining spot has grown by at least 50 meters in the past 10 years, according to the residents. 

The banks of River Nyabarongo, as it meanders through the wetland towards its meeting point with River Akanyaru, has grown by at least two meters on either side during the same period, say the residents. 

Lake Rweru is brown too 

HOW TO VIEW: Hold cursar down on the White Vertical Line in the center and move to either left or right. The visualisations are based on images generated using Google Earth Timelapse

Another testament as to the amount of soil or sediment that River Nyabarongo is ferrying, is the fact that it is about two meters deep with mud at the bottom. We found people crossing by canoe from communities on either side of Akanyaru river. The canoe operator had a long bamboo stick that helped him steer the boat. We asked him to insert  this stick  in the middle of the river for us to calculate its depth.  It measured more than 4 meters. 

Our next leg of Nyabarongo journey took us to the Rweru sector on Lake Rweru. From the horizon, the lake appears brown. We arrived at the Rweru fish landing site. The team could not reach the exact entry spot of Akagera as it enters the lake;  it is located deep inside a vast wetland. 

On the banks of the lakes, the water is brownish. It is infested with water hyacinth, a dangerous plant that has ravaged lakes and rivers in the great lakes region. Local residents and fishermen we found at the site said the hyacinth is brought by the Akagera river. They use their fishing boats to remove some of it on a daily basis. 

We were told that the river had in the past three to four years broken its historic route; biggest portion of it passed on the side of Lake Rweru and continued down to Lake Victoria. Today, the river pours entirely into Lake Rweru, and then exits to continue its journey. That piece of land that separated it from the lake, is gone. As a result, not only is the lake completely brown, but is all mud at the bottom, according to local fishermen. 

To confirm the changes such as the browning of the water  due to silt, the expansion of River Akanyaru at entry and size of the lake, The Chronicles team consulted satellite imagery. The results are shocking, to say the least. 

In 1984, when the first satellite imagery began to be compiled, at least 70-80 percent of the lake was blue. It suggests that even with the dark-brown Nyabarongo/Akagera rivers, the river waters were not enough to impact Lake Rweru. The lake’s brown color has grown quickly in the past 10 years. 

The scientist’s view

Nyabarongo I Hydropower plant

Everything about the dams on River Nyabarongo is not gloomy. The hydro dams have had some noticeable positives. A four-member team of researchers including one from Yale University and another from Florida International University in the USA undertook a  study on Sediment pile up on Nyabarongo river’s upper catchment areas.  

“The effect of the [hydro] dam was seen in our water quality sampling to temporarily improve the clarity of the water, because of sediment settling in the reservoir,” said Dr Amartya Saha, from the team, in email exchange with The Chronicles

He explained further: “Dams are not directly linked to soil erosion; erosion happens in the watershed (valley slopes) above the river, while a dam is on a river.”

Dr Amartya Saha acknowledges the fact that reservoirs trap sediments that settle down once the waters stop flowing, and so the water downstream is likely to be less turbid.

He says small dams can be useful for “water storage” and micro hydro generation, but “large dams are destructive”. 

Government’s response 

The government agency directly concerned with management of waterways in the country  said the government would not have put up the hydropower dams on the river if they were a danger.

“Feasibility studies are done before a hydropower plant is constructed. In the case of Nyabarongo, the problem is erosion which we must deal with,” said Prime Ngabonziza, Director General of the Rwanda Water Resources Board (RWB). 

He added: “Actually, if it is determined that more dams are a good thing, more may be built. There is no way the government can go for dams if environmental impact assessments show otherwise.”

As example of how government interventions have worked to turn a brown river to clearwater, the water Board touts River Secoko one of Nyabarongo’s major tributaries. It flows from the mountains of Ngororero district in central-western region and goes via Rutsiro, down to Karongi where it meets with Nyabarongo. 

For years, Secoko had been causing devastation especially in Rutsiro. A 2016 USAID-funded study found River Secoko, over the period January-April, to be carrying “extremely high sediment loads into the Nyabarongo, with heavy deposits on the river bed and banks”. 

In March 2019, the government of Rwanda  launched a massive Rwf 16 billion ($16m) program to implement different projects to reverse the erosion along R. Secoko route and catchments. It was to cover 10,000 hectares by forestation, terracing and improved agricultural practices. 

The government 2018-2024 restoration plan for the upper section of Nyabarongo, identified R. Secoko as urgent. One of the activities was to rehabilitate and prevent creation of gullies covering 26 ha with 5.6 km of check dams. These types of dams come in different forms, but all are structures built on a route of a river to reduce its speed and hold back soil. 

Today, river Secoko has changed significantly. It’s flowing water is clearer, appearing to have less silt carried downstream to Nyabarongo. 

To deal with soil erosion across the whole country especially the mountainous regions, the water Board director general Ngabonziza said estimates put the cost at at least Rwf 500billion ($500m), to cover programs over the next 15-30 years.

The way forward 

HOW TO VIEW: Hold cursar down on the White Vertical Line in the center and move to either left or right. The visualisations are based on images generated using Google Earth Timelapse

For existing hydro dams, scientists believe the facilities, especially the large Nyabarongo I dam, need new structures far upstream on the tributary rivers that hold back sediment (soil) and let through just water to flow to the dam’s reservoir. The other solution highlighted is building a bypass canal such that sediment doesn’t pile up on the walls of the hydro power plant thereby requiring regular flushing. 

A 2005 study by South Africa’s University of Stellenbosch proposed that dams already on this river need sediment diversion, sediment bypassing, and off-channel storage reservoir. 

As for planned new hydropower dams, the study proposed that dam sites should be selected in regions with relatively low sediment yields. The researchers also put forward what was described as “storage operated (minimisation of spillage) reservoir”, which is basically setting up a very large reservoir that can hold sediment for up to 50 years.  

If the existing hydropower dams are exacerbating such damage along River Nyabarongo, it remains to be seen what will happen after completion of the 43.5 MW Nyabarongo II hydropower dam due in 2025 at a cost of $214m. There is also Rukarara VI dam with a 10MW capacity that is currently under construction.  

This article was produced in partnership with InfoNile with funding frm JRS Biodiversity Foundation.

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