A free-flowing river occurs where natural aquatic ecosystem functions and services are largely unaffected by anthropogenic changes to fluvial connectivity allowing an unobstructed exchange of material, species and energy within the river system and surrounding landscapes beyond.
The longitudinal (river channel), lateral (floodplains), vertical (groundwater and atmosphere) and temporal components of fluvial connectivity can be compromised by infrastructure or impoundments in the river channel, along riparian zones, or in adjacent floodplains; hydrological alterations of river flow due to water abstractions or regulation; and/or, changes to water quality that lead to ecological barrier effects caused by pollution or alterations in water temperature.
For a river to be 'free,' it must be highly connected in four ways:
which refers to connectivity between upstream and downstream. Dams are the most common disrupter of longitudinal connectivity.
which refers to or the ability of a river to swell and shrink rise and fall naturally, and connect to its floodplains. This is disrupted when roads, buildings or other development (including agriculture when it is protected by levees or dikes) takes place on floodplains, limiting their ability to absorb the rivers' flows.
or the natural ability of river flows to change intermittently. For example, when a dam is built, it consistently holds a volume of water behind the structure and releases water in a way that does not match the timing of the river's natural flows.
which refers to the ability of a river to draw water from or contribute water to underground aquifers and the atmosphere. This can be interrupted by over-abstraction of groundwater and impermeable development on flood plains, among other causes.
To identify free-flowing rivers using global datasets, freshwater experts from around the world gathered information on 'pressure indicators,' each of which impacts at least one of the four areas of connectivity. These pressure indicators include:
Degree of Fragmentation maps dams and how they obstruct rivers' longitudinal connectivity. This is based on a database of major dams around the world; it does not include minor dams or infrastructure, as that data is not yet available in a consistent and comprehensive way across the entire globe.
Calculates how much a particular dam affects the annual average downstream flow, which relates to lateral, longitudinal, vertical and temporal connectivity. This too is based on global dam data.
Represents how lateral connectivity can be disrupted when roads are developed on floodplains or rivers pass through culverts, and is measured by a database of the world's roads called GRIP.
Represents how lateral connectivity can be affected by cities and other infrastructure developed in flood plains. It is measured by satellite imagery of nightlight intensity.
Consumptive water use is measured by water abstraction and consumption data available through WaterGAP and represents how lateral, vertical and temporal connectivity can be disrupted taking out too much waterby changing groundwater aquifers.
Represents the amount of sediment captured by the reservoir impoundment. Sediment capture can trigger a cascade of impacts on river systems and processes far downstream, and reduce sediment delivery for floodplains and coastal deltas alike.
Only one third of the world’s longest rivers (over 1,000 kilometres in length) remain free-flowing, and these are restricted to remote regions in the Arctic and the Amazon and Congo basins. Additionally, only 23% of the longest rivers still flow uninterrupted to the ocean. In southeast Asia, very few long rivers remain free-flowing, with the Irrawaddy and Salween as notable exceptions.
Dams and reservoirs are the leading contributors to connectivity loss in global river reaches. The results of this study and the new method of quantifying riverine connectivity and mapping free-flowing rivers provide a foundation for concerted global and national strategies to maintain or restore free-flowing rivers.