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

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, could cause irreparable damage for future generations. 

To give some context, River Nile 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, river Nile would have 133 hydro dams. 

Nyabarongo waterway is a major lifeline for electricity generation to power up the country. As of 2020, 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. 

With the economy expanding rapidly over the past over 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. 

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. 

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, and the larger ones. 

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. 

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. 

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.  

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 back water 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 deposition of sediment. 

With the presence of a dam(s), it changes the river’s “water surface elevation” on a daily basis. This is caused by a process called “hydro peaking”, 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.