Alteration of riparian vegetation

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Alteration of riparian vegetation

04. Morphological alterations

General description

Many different pressures impact on riparian vegetation driven by processes acting at local up to global scales from. In this particular context we are specifically referring to activities carried out immediately adjacent to the river (e.g. cultivation of crops) or in the riparian zone itself (e.g. logging, grazing and trampling, gravel and water extraction, and recreation).

Effect/Impact on (including literature citations)

The encroachment of agriculture has had a major effect on river margins. This is particularly true of lowland floodplain rivers, where river margin soils are often moist and rich. As a result, the edges of many rivers are directly in contact with agriculture and, as a consequence, riparian zones are fragmented and are often reduced to narrow strips or isolated trees on the river banks. Agricultural practices of tilling and harvesting prevent riparian vegetation regeneration and lead to degradation of the riparian seed bank. The rough canopy of natural riparian vegetation traps sediment from flood waters, leading to bank aggradation and extension. At the same time, the roots of riparian vegetation reinforce river banks, limiting their erosion during high flows, and enabling channel banks to extend into the channel and aggrade vertically. In particular, riparian vegetation retains and stabilizes fine sediments and narrows river channels to increase flow velocities. Both of these processes reduce fine sediment supply and settlement within the channel and thus its potential to infiltrate channel bed sediments causing interstitial siltation and clogging of the bed.

  • Logging & tree removal

The presence of riparian woodland is crucial to the structure, morphology and dynamics of river margins, since it interacts with flows of water and sediment to create and reinforce river margin landforms (Gurnell, 2013). As a result, clearance of riparian woodland can lead to simplification of river margins, channel widening, and in extreme cases a change in river planform from meandering to braiding. These fundamental morphological impacts affect the moisture regime of river margins; exchanges of water, suspended and dissolved material between the river and its riparian zone; as well as numerous biogeochemical and ecological processes (Gurnell and Petts, 2011). Removal of riparian trees has an immediate effect on river ecosystems by reducing shading and thus increasing stream temperatures and light penetration. Removal also decreases bank stability, inputs of litter and wood, and retention of nutrients and contaminants; reduces sediment trapping and increases bank and channel erosion; alters the quantity and character of dissolved organic carbon reaching streams; lowers retention of benthic organic matter owing to loss of direct input and retention structures; and alters trophic structure (Allan, 2004). Sabater et al. (2008) studied the effects of riparian vegetation removal on algal dynamics and stream nutrient retention efficiency by comparing NH4-N and PO4-P uptake lengths from a logged and an unlogged reach in a forested Mediterranean stream. Their study showed that the elimination of riparian vegetation altered in-stream ecological features that lead to changes in stream nutrient retention efficiency. Moreover, it emphasizes that alteration of the tight linkage between the stream channel and the adjacent riparian zone may directly and indirectly impact biogeochemical processes with implications for stream ecosystem functioning. In this context, the role of the riparian vegetation in filtering nutrients coming from agricultural watersheds, is well known, and underpins the use of buffer strips to prevent river eutrophication (Osborne & Kavacic, 1993). Removal of riparian vegetation inevitably leads to a severe reduction in the supply of wood to the aquatic system. Furthermore, large wood is often deliberately removed from forested rivers for flood defense purposes. Large wood plays a complex and important role in aquatic ecosystems. It affects flow hydraulics, sediment dynamics and sorting, channel morphology and stability, physical habitat composition, dynamics and diversity, and nutrient cycling (Gurnell et al., 1995), with effects varying with channel size and planform, and with riparian tree species (Gurnell et al., 2002; Gurnell, 2013). Loss of large wood debris in a stream alters flow hydraulics, causing a simplification of channel bed sediments and habitats, a reduction in organic matter retention, and often a reduction in bed and bank stability. Diez et al. (2001) identified large wood as the main hydromorphic element in river channels in forested basins.

  • Transformation into farming lands

Riparian vegetation acts with flow, sediment and topography to influence channel form, instream habitat, nutrient dynamics, and temperature and flow patterns. Therefore, removal of upland and riparian vegetation through farming and urbanization disrupts land-water linkages leading to reductions in water quality, simplification of stream channels, less stable thermal and flow regimes, and ultimately, reduced biological integrity (Snyder et al. 2003). However, removal or modification of natural riparian vegetation where trees are not naturally present, may not result in such deep-seated and long-lasting effects because agriculture in such areas usually consists of grazing (Williamson et al. 1992). Riparian ecological degradation and transformation to agricultural uses often leads to invasion by alien plants (Planty-Tabacchi et al., 1996). Plant invasions are increased directly or indirectly by many types of human-mediated disturbances to rivers and riparian zones (Richardson et al., 2007). Once introduced and established in a catchment, many alien plants can exploit opportunities provided both by natural flood events and by anthropogenic disturbances to which they are better attuned than native species (Planty-Tabacchi et al., 1996).

Conceptual framework of alteration of riparian vegetation effects on HYMO processes and variables (POM= Particulate Organic Matter; LWD= Large Woody Debris).

Case studies where this pressure is present

Possible restoration, rehabilitation and mitigation measures

Useful references

Other relevant information

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