Artificial barriers downstream from the site

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Artificial barriers downstream from the site

03. River fragmentation

General description

River fragmentation is caused by discontinuity in any of the river’s three spatial dimensions: longitudinal, lateral and vertical. Such discontinuities disrupt hydrological connectivity (Pringle, 2003), interrupt the transfer of water, mineral sediment, organic matter and organisms within and between elements of the river system, and thus impact on the river’s biotic and physical components (Bunn and Arthington, 2002). Longitudinal fragmentation may be produced directly by the presence of dams or artificial barriers, but it may also be produced indirectly by certain conditions caused by HYMO processes and water quality degradation. Hydrological connectivity is water-mediated. For example, reduction of flows, especially of base flow, during some periods may disconnect habitats and species’ populations. Anoxic water conditions along stream reaches, or thermal discharges may also act as barriers for riverine aquatic organisms. Lateral fragmentation is caused by the presence of lateral barriers such as levees and dikes that disconnect river ecosystems from their floodplains by preventing overbank flooding. The integrity of both riparian and aquatic ecosystems is thought to be dependent, in part, upon exchanges of energy and matter between the main river channel and adjacent floodplain surface and the patches present within and between them during periods of flooding (Amoros and Roux, 1988; Junk et al., 1989, Junk and Wantzen, 2004). Disturbances or flow regulations that eliminate or reduce flood flow magnitude, or lateral barriers that limit the extent of inundation of the floodplain, disrupt connectivity between river and floodplain. In addition, certain indirect effects of pressures, through their correponding HYMO proccesses, may cause floodplain isolation. For example, restriction in sediment supply to a river may induce river bed incision, which in turn reduces hydrological connectivity between river and floodplain. Processes such as channel bed incision or riparian and floodplain accretion, which both disrupt river-floodplain connectivity are frequently found in disturbed rivers. Vertical fragmentation can be produced by physical processes that reduce river bed permeability such as siltation of the riverbed surface and clogging of pore spaces within stream bed gravels (Hancock 2002). This may result from increased delivery of fine sediment to the river as a result of for example changes in land use or agricultural practices, as well from reduced flow energy, for example due to flow regulation. In either case, the balance between sediment supply and sediment transport is disrupted, leading to the accumulation of fine sediments within the river bed (Kondolf and Wilcock, 1996). In addition, physical modification of river channels, such as straightening and simplifying channel form (Kondolf et al., 2006) may reduce water depth and retention within the channel, adversely affecting vertical connectivity. Henceforth, this review will only focus on the pressures involved in longitudinal fragmentation, as lateral fragmentation will be incorporated in pressures related to channelization (embankments and levees) and vertical fragmentation will be incorporated in pressures related to substrate siltation and clogging, and riparian soil sealing and compaction.

Effect/Impact on (including literature citations)

  • HYMO (general and specified per HYMO element)
  • physico - chemical parameters
  • Biota (general and specified per Biological quality elements)

Case studies where this pressure is present

Possible restoration, rehabilitation and mitigation measures

Useful references

Other relevant information

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