Loss of vertical connectivity

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Loss of vertical connectivity

04. Morphological alterations

General description

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[1]). 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[2]). In addition, physical modification of river channels, such as straightening and simplifying channel form (Kondolf et al., 2006[3]) may reduce water depth and retention within the channel, adversely affecting vertical connectivity.

Riparian and floodplain soils may lose their infiltration capacity (vertical connectivity with groundwater) as a result of urban development, road and pavement construction, the weight of vehicle traffic, soil trampling, and recreational activities.

Effect/Impact on (including literature citations)

Colmation of the upper layers of the channel bed and riparian and floodplain soils by fine sediment particles can hinder exchange processes between surface water and groundwater (Brunke and Gonser 1997). Natural colmation processes usually occur through the siltation of fine material during low flow episodes; whereas spate or exfiltration episodes reopen the interstices and reverse the process. Increased current velocities can flush fine material out of the surface layers of the river bed, but bedload movement is needed to reopen deeper interstices (Brunke and Gonser 1997). Due to the position of the surface bed layers between surface water and groundwater, the balanced alternation between colmation and scour can be disturbed by impacts affecting both surface and groundwater habitats (Hancock 2002), which can cause permanent colmation. The most common causes of colmation are organic and fine sediment inputs to the river channel, river channel engineering, and increased filtration through the channel margins during water extraction for drinking, industrial and irrigation water (Petts, 1988). External colmation can result from increased sewage loading that promotes the development of dense algal mats, or causes sedimentation of an organic layer on the river bed. Internal colmation is caused by the infiltration / intrusion of fine particulate organic or inorganic matter into the cavities within the bed sediments (Schalchli, 1993, after Brunke and Gonser 1997). Land use practices which increase seston and sediment loading are directly responsible for the extent of the unbalanced colmation processes (Karr & Schlosser, 1978; Platts et al., 1989, after Brunke and Gonser 1997). By preventing the communication between surface and groundwater, cascading effects in ecosystem structure and function may occur (Brunke and Gonser 1997). For instance, siltation of the interstices reduces the shelter for invertebrates, and thus the impacts of natural and anthropogenic disturbances, such as urban stormwater runoff, are magnified. Sealed interstices cannot function as nurseries for the benthos. Colmation can diminish or prevent the reproductive success of fish spawning on gravel. On the other hand, a clogged bed may act as an intrusion barrier that prevents the contamination of groundwater by polluted surface water (Younger et al., 1993; Komatina, 1994, after Brunke and Gonser 1997). Colmation might not be the only cause of the loss of vertical connectivity. The water velocity near the most superficial sediment layers determines the dominant flow direction between the surface and groundwater. If the temporal distribution of water velocities change, more subtle disturbances in the flow between both systems can be noticed. For instance, Curry et al. (1994) found that discharge fluctuations caused by hydroelectric power generation alter the mixing relationships between surface water and groundwater in the hyporrheic zone. And this could have severe impacts on the reproductive success of gravel spawning fish. This effect was found to occur naturally in some reaches along an upland salmon spawning catchment by Malcolm et al. (2005). They found that at sites dominated by surface water, hyporrheic DO remained high throughout and rates of embryo survival were correspondingly high. Restoration actions have been conducted and benefits in both the hyporrheos and the groundwater table near the river have been reported. After restoration programs, Sarriquet et al. (2007) detected an increase in vertical exchanges of water between surface and interstitial habitats, with an increase in the depth of hypoxia. Golz et al. (1991, after Brunke and Gonser 1997) reported that the mechanical opening of a clogged section of the Rhine’s stream bed near a drinking water bank filtration site induced a 1 m rise in the groundwater table near the river, but after a few weeks the opened section had become sealed again. Therefore, the viability of such restoration works may be associated with catchment management designed to reduce fine sediment inputs to the river (Sarriquet et al. 2007). Loss of vertical connectivity not only affects interactions between channel and adjacent terrain but also between the land surface and underlying and aquifers. In urbanised areas, previously permeable surfaces are replaced by paved impermeable surfaces (Schick et al. 1999). This may cause a general decline in the water table and a deterioration in groundwater quality with urbanization (Jat et al. 2009).

Case studies where this pressure is present

Possible restoration, rehabilitation and mitigation measures

Useful references

Hancock, P.J., 2002. Human impacts on the stream–groundwater exchange zone. Environmental Management 29: 763–781. Kondolf, G. M., and P. R. Wilcock 1996. The flushing flow problem: defining and evaluating objectives. Water Resources Research 32: 2589-2599. 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/

Other relevant information

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