A National Geographic short video highlights a man's interest in providing energy alternatives to harvesting trees for cooking and heating in Uganda. While the focus of the narrative is poverty reduction - it could just as easily gone in the direction of protection of water resources. The video mentions the incredible deforestation actively denuding Uganda - as it has happened in Rwanda, Ethiopia, and so many other places within the Nile River Basin, particularly in the last 30-40 years. One major and relevant Grand Ethiopian Renaissance Dam concern is the operability of the dam after only some years of sediment trapping. The Blue Nile carries incredible amounts of sediment suspended in the water - this same sediment is what was responsible for fertilizing Egypt's successful farming efforts for millennia before the High Aswan Dam cut that sediment flow off 100%. Deforestation accelerates erosion rates. Keeping trees in place then looks to be crucial in slowing the rate of erosion. This is important especially for downstream neighbors.
What is sediment and what does it do?
Sediment, in general terms, is eroded material made up of small fragments of earth, so small and light they can be transported in water, suspended in the flow for a period of time, usually due to how fast the water is moving. Sediment is heavy enough though, that it will fall out of the water when the water is slowed down or still. Places that cause sediment to deposit include wetlands, floodplains, deltas, and dams.
Wetlands, like the large inland complex in South Sudan called the Sudd or the Nile delta in Egypt, are important traps for sediment. There is a wetlands complex in the Mara River Basin where the SELVA team is working now. Water velocity is reduced when it comes into a wetland, usually because of the topography - wetlands are typically flat and wide. In general, if water has room to spread out, it doesn't rush forward as intensely. When water enters this space after being channeled in a river bed, it spreads out and the slower movement allows for the suspended material to settle to the bottom of the water column. This suspended material can hold nutrient-rich qualities that help plants grow. It can also be the basis for building new land. And because of fluctuations in water movement and sediment transport, as well as biological growth or population, these areas are in constant flux - land will build and submerge, islands will change shape. In the case of the Mara wetlands complex, the water prior to entering appears brown from such high sediment load, and clear where it leaves the wetlands, having left behind a significant amount of the matter the water was carrying. The nutrients and calm environment make these areas perfect nurseries for species such as birds, small mammals, and aquatic species.
The same sediment deposition is true in flat areas called floodplains. Floodplains are the areas that are found around a riverbed that stretch for some distance and are where water will reach when there is a flood event - either an annual cycle of precipitation, or an isolated event. Engineers try to create forecasts of where these plains extend, but often they can't account for large natural events - or political sidestepping - to guarantee the safety those who choose to live in a floodplain. There are plenty of stories in America alone about the mishaps caused by human engineered landscapes, particularly of rivers, that do not guarantee safety and security of communities encouraged to claim those spaces. Flood plains, even ancient ones, such as the Willamette Valley in Oregon, are incredibly productive farmland. The history of Egypt contests to this along the Nile - although deposition no longer occurs because of High Aswan Dam, the depth of years of deposition extends far into the earth.
Finally, there are dams. A dam is essentially a man-made wall in the middle of a waterway. The wall has particular spots where there are openings that allow water through, concentrating the energy of that water into those openings. In the case of Renaissance Dam, those openings will contain turbines to turn and, through a process, generate electricity. But the water slows down when it hits a wall, before it finds its way to those openings. Some of the energy is lost and in turn, the suspended material is dropped. The deposition of this sediment behind a dam can cause a loss in the functionality of channeling high energy water into those openings - thus causing the turbines that generate the electricity to slow down or stop working altogether. Sediment loading behind Roseries Dam in Sudan has been such a problem to dam functioning that the dam had to be elevated by 10 meters in 2014. This has also been a perennial problem in the Inga Dams on the Congo River. It costs money to dredge the sediment out from behind a dam - money and an energy-intense vacuuming process - then where to put this sludgy substance? Sediment is not only earth, it is whatever is on or in the earth too - any sort of chemicals, heavy metals, other pollutants - when you have a pile of this stuff that is concentrated, it may not be good for human health.
All of this is to say that deforesting is the major culprit in sediment load in rivers and sediment is good for some purposes, such as farming and formation of wetlands and deltas, and destructive in other cases, such as irrigation canals and dams. When you remove trees from the landscape to put in large agriculture or because the trees themselves are a product that can be traded for cash or used for fuel - you risk altering that landscape significantly - the land and the water. In some cases, such as the Dalmatian Coast, the trees are cut, the soil washes away, and then there is no replacing it. The topsoil is gone and nothing can grow. To this day the need for wood to supply material to build Venetian shipping fleets leaves its mark along the Balkan coastline. Aggressive development or just uncoordinated efforts can lead to some unintended, but not unforeseen, consequences. In the Nile River basin, there is still no coordinated development effort - no ongoing communication between basin countries about how to best manage the shared resource. This may stem in part from the historic domination of the Nile River by one country, Egypt, but that domination is changing due to the natural shifts in political and economic landscapes in East Africa as countries come into their own and regional populations boom and demand more resources. Perhaps the Tri-Governmental cooperation on the Grand Ethiopian Renaissance Dam is a step toward a Nile governance that is cooperative and has an aim of coordinated development efforts. Environmental management would fit in nicely with such a new direction.