Free-Flowing Rivers As Nature-Based Solutions

The Zambezi River carrying sediment downstream. WWF

This is the fifth in a seven-part series that explores the application of Nature-based Solutions (NbS) to climate and nature challenges, with a focus on water management in Africa for climate adaptation (and drawn from the report Waterways to Resilience). This post goes deep on a specific type of NbS: rivers that deliver sediment to build up and maintain downstream deltas and mangrove forests that can provide buffers to coastal storms, floods and erosion. Free-flowing rivers are generally not included among lists of Nature-based Solutions (NbS). However, rivers that carry a natural sediment load are essential to the functioning of many of the more commonly listed NbS interventions, such as floodplains, coastal marshes and mangroves. That is because rivers carry sediment, and a key function of many NbS interventions is their ability to turn river-borne sediment into land. For example, deltas are landforms created by the sediment deposited when rivers meet the ocean. Over time, those sediments become compacted and are eroded by waves and storms and so, to remain stable, deltas require ongoing replenishment from the sediment delivered by a river (Figure 1). Figure 1. Rivers carry sediment from the mountains and deposit silt and sand in downstream ... [+] floodplains and deltas. WWF When a dam is built on a river, the reservoir behind it can trap the river's sediment supply, leaving much less sediment available downstream to be carried to the delta. Globally, reservoirs capture approximately 25 per cent of the sediment that rivers carry each year and this reservoir capture is a primary reason that deltas are sinking and shrinking around the world, with rising sea levels exacerbating this problem (Figure 2). For example, dams on the Nile trap more than 95 per cent of the river's sediment and the Nile Delta – home to nearly 40 million people and responsible for most of Egypt's agricultural production – is rapidly eroding. Widespread losses of deltas would have global implications as they are home to 500 million people and produce 4 per cent of the world's food from just 0.5 per cent of the land surface.

Figure 2. On the left, the Srepok River, a tributary of the Mekong River, flows into a reservoir ... [+] created by the Lower Sesan II Dam (identified on the left of the photo). The Srepok was once one of the largest contributors of sediment to the Mekong Delta, but now that sediment is captured in the reservoir (note the muddy plume as the Srepok enters the reservoir on the right side of the photo). As the sediment supply to the Mekong Delta diminishes, including due to extensive sand mining, erosion rates have increased (photo on right).LEFT: NASA; RIGHT: MARC GOICHOT, WWF On deltas, management agencies often invest in a range of interventions, such as sea walls and rip rap to hold back salt water and reduce erosion. Managers are beginning to recognize the value of using river borne sediment as a NbS for protecting deltas. For example, the US Army Corps of Engineers is creating openings in levees to allow sediment-rich floodwaters to flow out into marshes to build their elevation and provide additional protection from rising sea level and storm surges along Louisiana's coast. In the Netherlands, WWF began working with a range of partners in 2015 to direct water into the Haringvliet, which had formerly been an estuary before being disconnected from the sea by floodwalls. Managing flows through the floodgates will allow more natural sediment deposition, helping to rebuild elevations in the estuary, counteracting subsidence and reducing risks of flooding and erosion. Furthermore, this reconnection will allow migratory fish, such as salmon and eel, to once again enter the Maas and Rhine rivers through the Haringvliet. The restoration will be partially funded through 'bankable projects' that leverage private capital.

Sediment deposition from rivers is also crucial to the functioning of mangroves. In systems where mangroves grow in areas influenced by rivers, their essential NbS functions – including sediment trapping, land building, and coastal protection – all depend on the supply of sediment from rivers. Globally, about 70 per cent of mangroves grow in areas directly influenced by rivers and their sediment supply (estuaries and deltas).

Understanding the sediment dynamics of where rivers meet the ocean, in estuaries and deltas, including sediment supply and how currents and tides distribute that sediment, will be key for planning and maintaining many coastal NbS projects. Because sediment supply is crucial to coastal protection and the viability of NbS projects, anything that affects sediment supply should be viewed in that context. Thus, upstream sand mining and reservoir capture should be viewed as factors that affect the viability of coasts and NbS coastal protection projects. Planning and management of both reservoirs and sediment mining should incorporate this perspective. If a dam is planned on a free-flowing river, the value of the river's sediment for maintaining downstream deltas and mangroves should be an important decision variable. While not often framed this way, the protection of a free-flowing river, and its sediment supply, could be among the most beneficial and impactful NbS interventions that can be implemented.

In Africa, there are 56 relatively large rivers (those with a discharge > 100 cubic meters per second at their mouth) that flow into mangrove forests along the coast (Figure 3). Of these, 39 (70%) remain free flowing (the others already have dams). Hydropower dams have been proposed on 8 (21%) of those free-flowing rivers. If those were to be built, then just over half (55%) of the large rivers that flow into mangrove forests of Africa would remain free flowing. Furthermore, nearly 80 per cent of the already dammed rivers that flow into mangroves have additional dam plans, which would further reduce the sediment supply delivered to mangroves and other coastal ecosystems.

The value of sediment for downstream deltas, mangroves and marshes – and the protection of people and economies along the coast – should be factored into decisions about dam development, particularly as the renewable energy revolution has resulted in such a dramatic drop in the cost of solar PV and much of Africa has abundant solar resources.Figure 3. Connectivity status of large rivers in Africa that flow into coastal mangroves, along with ... [+] existing dams and proposed hydropower dams. WWF