Ecological Connectivity of Rivers

A river has ecological connectivity when water, sediment, nutrients and organisms can move through its channel and between the channel, floodplain and riverbed. A barrage is a gated structure across a river that raises upstream water levels and regulates passage or diversion of water; it can interrupt these connections physically and by changing the flow regime.

The connections that barrages interrupt

Connectivity depends on both an open pathway and suitable flows at the right time. Barrages can therefore disrupt river ecology even when some water continues downstream.

• Longitudinal connectivity links upstream and downstream reaches. A barrage can obstruct fish migration, while impoundment and diversion alter the downstream transport of sediment and organic matter.
• Lateral connectivity links the channel with floodplains, wetlands and side channels. Where regulation reduces flood peaks, these habitats may be inundated less frequently or for shorter periods.
• Vertical connectivity links river water with water within and beneath the riverbed. Changes in water levels and deposition of fine sediment can alter this exchange.
• Connectivity also has a seasonal rhythm: the timing, duration and frequency of high and low flows influence migration, reproduction and access to habitats.

How altered flows affect river ecosystems

The ecological effect depends on barrage design, gate operation, diversion volumes and the river’s natural flow regime. Several barrages along a river can create cumulative fragmentation.

• Water diversion can leave downstream reaches with reduced habitat availability, restricting the depth, velocity and wetted area required by aquatic organisms.
• An upstream backwater creates slower-water conditions, changing habitats used by species adapted to flowing water.
• Migration barriers can separate fish from spawning and feeding grounds, while altered seasonal flows can weaken reproductive cues.
• Sediment deposition upstream and altered sediment delivery downstream can change channel form, riverbed substrates and spawning habitats.
• Reduced floodplain inundation can limit access to nursery habitats and exchanges of nutrients and organic matter.
• Lower downstream flows can reduce pollutant dilution, aggravating water-quality stress where sewage or other pollutants enter the river.

Maintaining connectivity under flow regulation

Environmental flows should preserve ecologically important features of the flow regime, rather than provide only a constant minimum discharge.

• Barrage operation should account for seasonal flows, floodplain connections and the needs of downstream habitats.
• Suitable fish-passage measures can improve movement, but their effectiveness depends on species, design and operating conditions.
• Sediment management and coordinated operation of successive barrages should accompany flow releases; restoring water quantity alone does not remove every barrier.