Accounting for Rwanda’s soil loss: A journey along Nyabarongo River.

Accounting for Rwanda’s soil loss: A journey along Nyabarongo River.

By Fred Mwasa and Sylidio Sebuharara 

On a river stretching just 297km, there are already four hydropower dams. A fifth dam is under construction; building of a sixth much larger dam is slated to start soon. While the hydro plants are producing much needed electricity, they are leaving a trail of environmental degradation, which if not dealt with today, will cause irreparable damage for future generations. 

To give some context, the Nile River is 6,695km, with 25 hydropower dams as of 2019. It is at least 23 times longer than the River Nyabarongo. If we are to take this variation to provide an understanding of how many dams are on river Nyabarongo, the Nile River would have 133 hydro dams. 

Nyabarongo waterway is a major lifeline for electricity generation to power up the country. As of 2020, the overall installed capacity for the whole of Rwanda of power was about 224.6 MW from different sources including methane gas and solar. Hydropower makes up approximately 46.8% of the total installed capacity, the biggest percentage of it coming from river Nyabarongo and its tributaries. 12.8% is generated by the largest power dam so far, Nyabarongo I. 

Nyabarongo Dam

With the economy expanding rapidly over the past 20 years, the government desperately needs cheap electricity to maintain the growth rates. Rwanda targets to have 512 MW installed power generation capacity by 2023/24. 

However, experts point out that even with this target, it’s still far too small compared to what the country is able to produce. Rwanda can generate electricity economically with local resources including, hydropower, peat, lake methane gas, and geothermal energy estimated to total around 1,613 MW. The country is therefore utilizing less than 10% of its local electricity potential. 

Rwanda’s flowing streams, capable of generating power, are not many, leaving River Nyabarongo as the route for hydropower dams. 

Reservoir of Nyabarongo I Hydropower Dam

River Nyabarongo, before it becomes this appellation, begins with two major rivers; Mwogo River and Mbirurume River, both located in south-central Rwanda. Rukarara River, springing from the Rubyiro and the Nyarubugoyi rivers, is what forms Mwogo river. Mbirurume River, for its part, flows from the mountains in Gisovu, receiving various other streams. 

Rukarara River bridge and signpost

Rivers Mwogo and Mbirurume confluence at a location between Nyamagabe, Muhanga, and Ruhango districts . From this point on, is what becomes River Nyabarongo. As the river meanders north, and then south, it ends with River Akanyaru at another confluence location between Kamonyi, Nyarugenge, and Bugesera districts. From here, it turns to River Akagera, which heads on to Lake Rweru shared by  Rwanda and Burundi. 

With these various rivers, subsequent Rwandan governments, up until the late 2000s, preferred constructing hydropower dams. The first was commissioned in 1957, the next coming two years later. By 1994, there were only five hydropower dams, with the biggest ones being Ruzizi II and Mukungwa I, each producing 12MW, as well as Ntaruka at 11.25MW. 

However, as of 2021, the country had added on another 24 hydropower dams. Among these hydro dams, four of them are on River Nyabarongo.

Such a significant number of dams on the same river has exacerbated a major problem that Rwanda has been facing for decades: soil erosion, and thereby massive soil loss. 

A 2016 study modeled the extent of soil loss in the River Nyabarongo basin, showing that the total annual estimated soil loss was 409 million tons with a mean erosion rate of 490 t·ha (tons per hectare). 

Linking dams to soil loss

Independent reviews show that River Nyabarongo is experiencing heavy pollution from landslides, mining, encroachment, unsustainable agriculture, and domestic and industrial wastes. These factors are made worse by the mountainous terrain, marked by steep slopes along the route of the river. As a result, River Nyabarongo has had a dark-brown color, throughout its journey for decades. 

However, one issue that has not been explored widely, is the role the hydropower dams are playing in worsening the rate of soil loss in the Nyabarongo river basin. 

Investigation by The Chronicles shows that the hydropower dams are making an already existing problem worse. Our findings show the dams on River Nyabarongo have caused it to more than double in width, eat up huge chunks of arable land, made the river shallow due to huge amounts of soil it carries, and increased its flow speed. 

Nyabarongo I Dam

The Chronicles team visited the environs of all the four dams on River Nyabarongo. We also took routes along the river, traveling more than 300km through Karongi, Ngororero, Kamonyi, and Bugesera districts, as we observed the level of damage that has been left behind, and will continue to happen. During our ground travel, we spoke to local residents in areas worst affected. 

We observed that Nyabarongo turns brown from both its major sources: Rivers Mwogo and Mbirurume. At the confluence of these two rivers, Mwogo has distinct features, which make it much darker than Mbirurume. The team noted that River Mwogo, which comes from River Rukarara that has three hydropower dams (Rukarara I, Rukarara II, and Cyimbili/Rukarara V) is much deeper, reaching two meters in depth. But even with this depth, the river also has vast amounts of mud at the bottom, which means a person could sink further. 

Mwogo-Mbirurume connect to form river Nyabarongo

The Chronicles team also identified that River Mwogo is calmer than Mbirurume, flowing quietly. This could be as a result of no rocks at the bottom, but just mud composed of the soil it carries from upstream. On the other hand, Mbirurume was ferrying a lot more sand.

It was clear, River Mwogo was more polluted with silt from upstream which was pushed harder by the rushing river waters helped by the three dams. 

As The Chronicles team moved further downstream, we passed alongside large valleys that have been tilled for centuries. The devastation of hydro dams is particularly impactful in the valley located in Mubuga cell, Murambi sector. We found men seated in bars drinking local brew. They said they had nothing to do because much of their land had been eaten up by the expanding River Nyabarongo since Nyabarongo II hydropower dam, a 28MW facility, was built in 2008.  

Rukarara I Dam

The communities upstream began experiencing floods as a result of the dam in 2012.  In a valley located in Mubuga cell, Murambi sector, the residents told us that since the dam construction, they can only grow food crops, especially rice, for just a single season. Before the dam, they had two planting seasons annually. 

“Right now, you found us drinking alcohol. We have no work to do,” said Telesphore Nyandwi, as he narrated: “The land you see in the valley has been mixed with sand brought by Nyabarongo [River ] so very little grows there.”

Pascal Habenimana, 49,  a resident of the area, said: “Everything has changed. It was a small stream 20 years ago; it’s now a wide river after expanding almost two meters on either side.” 

In April, every year, which is when the rainy season is heaviest, this valley is completely flooded as the reservoir on the dam pushes backwater upstream and more river water comes in from the mountains. Before the dam, the residents here had banana plantations along the valley’s side. Today, a small portion remains, which residents say may be no more in the coming years. 

During our probe, it was difficult to come by scientific research on the impact of the hydropower dams on River Nyabarongo. We came across an impact assessment report for the massive Nyabarongo I hydropower dam, released in April 2020. 

Part of it reads: “The simplest hazard is excessive sedimentation within the water reservoir resulting from flooding. For the reason that the reservoir is located between mountains, all through heavy rainy seasons, the flood incorporates thousands of tons of soil within the water reservoir.” The challenge with this is that it, “reduces the usable extent of water and if it maintains it, is able to have a major effect on plant production,” the report argues.   

To understand how these hydropower dams are changing the nature of Nyabarongo river, we talked to some senior water and hydro dams scientists and researchers in Rwanda. They requested us not to name them due to what they termed as the sensitivity of this topic. 

The researchers said separately that the dams on Nyabarongo are an obvious problem, as it is with dams elsewhere, causing what they termed “erosion cycle”. Here is how it essentially happens: the natural process for moving sediment on rivers is that it has a meander-bend with a flow concentrated on the outside of the river. This is a natural place of erosion on any stream or river. The inside of the bend is a natural site for the deposition of sediment. 

The presence of a dam(s), changes the river’s “water surface elevation” on a daily basis. This is caused by a process called “hydropeaking”, whereby a dam’s gates are locked with no water going through, and then opened to pump it downstream so that the reservoir doesn’t overflow. Those daily fluctuations from a hydro operation impact heavily on the stream/river banks, causing soil on its bottom side to be washed away. 

As this ‘erosion cycle’ continues, the overhead part of the river bank collapses into the river and is washed away. This repeats itself over and over, over years – leading to the widening of the river.  

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. 

Nyabarongo Reservoir level

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. 

Akanyaru-Nyabarongo form Akagera. The conjoining spot has grown by at least 50 meters in the past 10 years

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 

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. 

Boat crossing River Akanyaru around the confluence

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; the 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. 

Soil erosion near Nyabarongo I dam

To confirm the changes such as the browning of the water due to silt, the expansion of River Akanyaru at entry, and the 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

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 an email exchange with The Chronicles

“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.”

researchers from Yale University AND Florida International University

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”. 

Rukarara II Dam

Government’s response 

The government agency directly concerned with the 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 an example of how government interventions have worked to turn a brown river into clearwater, the water Board touts River Secoko one of Nyabarongo’s major tributaries. It flows from the mountains of Ngororero district in the 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 River 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 River Secoko as urgent. One of the activities was to rehabilitate and prevent the 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 the soil. 

Today, river Secoko has changed significantly. Its 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 least Rwf 500billion ($500m), to cover programs over the next 15-30 years.

The way forward 

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 hydropower plant thereby requiring regular flushing. 

A 2005 study by the 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 a “storage operated (minimisation of spillage) reservoir”, which is basically setting up a very large reservoir that can hold sediment for up to 50 years.  

Rukarara VI Mushishito Dam

If the existing hydropower dams are exacerbating such damage along River Nyabarongo, it remains to be seen what will happen after the 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 from JRS Biodiversity Foundation.

This story was first published on The Chronicles.

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