1 BACKGROUND Free-flowing rivers (FFRs) are the freshwater equivalent of wilderness areas. They are amongst the most ecologically important freshwater habitats and many are critical to both people and nature. A river is “free-flowing” if the connectivity of the river is maintained along all river reaches within the river from its source to outlet. This means that water, species, energy and sediment can move both up and downstream, as well as into its connected floodplain and riparian areas. A free-flowing river is one where the natural aquatic ecosystem functions and services are largely unaffected by changes in the fluvial connectivity (Grill et al., 2019). Fluvial connectivity encompasses the following components: a) longitudinal (river channel), b) lateral (floodplains), c) vertical (groundwater and atmosphere), and d) temporal (intermittency). Connectivity can be compromised by physical infrastructure in the river channel, along riparian zones or in adjacent floodplains, by hydrological alterations of river flow due to water abstractions or regulation, and by changes to water quality that lead to ecological barrier effects caused by pollution or alterations in water temperature. Figure 1 describes how river connectivity acts through space and time, and how obstructions can fragment or regulate rivers, and consequently affect the river connectivity.
LONGITUDINALLY which refers to connectivity between upstream and downstream. Dams are the most common distrupter of longitudinal connectivity.
LATERALLY which refers to the ability of a river to swell and shrink, rise and fall naturally, and connect to its floodplains. This is distrupted when roads, buildings or other development (including agriculture when it is protected by leaves or dikes) takes place on floodplains, limiting their ability to absorb the river’s flows.
TEMPORALLY or the natural ability of river flows to change intermittently. For example, when a dam is built, it consistenly 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.
VERTICALLY 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 overabstraction of groundwater and impermeable development on flood plains, among other causes. Figure 1: The four dimensions of connectivity and how they get obstructed (source: http://www.free-flowing-rivers.org).
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