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. 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)

River fragmentation is mainly assumed to be caused by barriers that interrupt the longitudinal gradient or by lateral dikes that disconnect the channel and floodplain. Thus the fragmentation effects of large dams is superimposed on their effects on the flow, sediment and physico-chemical regimes and is recognizable along the entire river continuum and laterally across floodplains (Ward and Stanford, 1983, 1995). However, reaches of river channel that are subjected to artificial drought or heavy pollution can also act as river fragmentation factors. Furthermore, riparian corridors are frequently fragmented by forestry and farming activities. River fragmentation is of wide significance, since most large rivers of the world are fragmented due to flow regulation schemes, and only a few river systems in the northern third of the world are free flowing (Dynesius & Nilsson 1994). The impact of dams and related flow regulation and fragmentation on riparian vegetation has been well-studied. Andersson et al (2000) found that riparian floristic continuity was reduced below dams, and Merritt et al. (2010) suggested that water dispersal of plant propagules may be reduced and the long-term species richness of the riparian community may decrease. However, there is no evidence that dams reduce the abundance and diversity of water-dispersed propagules by acting as barriers for plant dispersal (Jansson et al, 2005). The construction of weirs and other embankments on the lower Macintyre River floodplain has had lateral fragmentation effects by preventing water movement through a series of anabranch channels thereby reducing the availability of these floodplain patches by 55% (Thoms et al 2005), and thus reducing the potential dissolved organic carbon supply from some anabranch channels to the main channel by up to 98% (Thoms et al 2005). The ways in which dams and weirs act as physical barriers to the migration of fish and other biota have long been recognized (Kingsford 2000a,b). Barriers located near the river mouth have the greatest impact on fish with diadromous life histories while those located near the center of the river network have the most impact on fish with potadromous life histories (Cote et al. 2009). Sanches et al. (2006) showed a clear decline in densities and number of fish species caught after the closure of the Porto Primavera dam, Brazil. Also, larvae of migratory species, were restricted to the confluence of non-dammed tributaries, indicating that the closure of the dam had caused negative impacts on fish reproduction downstream of the dam. In a fish population modeling experiment Jager et al. (2001) found that increased fragmentation by dams produced a reduction in genetic diversity and an exponential decline in the likelihood of persistence, but no extinction threshold that would suggest a minimum viable length of river. They also found that migration patterns played a significant role in determining the viability of riverine fishes, with those populations with high downstream, and low upstream, migration rates showing a higher risk of extinction.

Conceptual framework of river fragmentation effects on HYMO processes and variables.

Case studies where this pressure is present

Possible restoration, rehabilitation and mitigation measures

Useful references

Amoros C. & Roux A.L. (1988) Interaction between water bodies within the floodplain of large rivers: function and development of connectivity. Münstersche Geographische Arbeiten 29: 125–130. Bunn, S.E. & Arthington, A.H., 2002. Basic Principles and Ecological Consequences of Altered Flow Regimes for Aquatic Biodiversity. Environmental Management 30: 492–507. Hancock, P.J., 2002. Human impacts on the stream–groundwater exchange zone. Environmental Management 29: 763–781. Kondolf, G. M., A. J. Boulton, S. O'Daniel, G. C. Poole, F. J. Rahel, E. H. Stanley, E. Wohl, A. Bång, J. Carlstrom, C. Cristoni, H. Huber, S. Koljonen, P. Louhi, and K. Nakamura. 2006. Process-based ecological river restoration: visualizing three-dimensional connectivity and dynamic vectors to recover lost linkages. Ecology and Society 11(2): 5. http://www.ecologyandsociety.org/vol11/iss2/art5/ Kondolf, G. M., and P. R. Wilcock 1996. The flushing flow problem: defining and evaluating objectives. Water Resources Research 32: 2589-2599. Junk, W.J., P.B. Bayley, R.E. Sparks, 1989. The flood pulse concept in river-floodplain systems. Canadian Journal of Fisheries and Aquatic Sciences 106: 110–127. Junk, W.J., Wantzen, K.M., 2004. The flood pulse concept: new aspects, approaches and applications–an update, in: Proceedings of the Second International Symposium on the Management of Large Rivers for Fisheries. pp. 117–140. Pringle, C., 2003. What is hydrologic connectivity and why is it ecologically important? Hydrological Processes 17: 2685–2689.

Other relevant information

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