Riparian-zone restoration is the ecological restoration of riparian-zone habitats of streams, rivers, springs, lakes, floodplains, and other hydrologic ecologies. A riparian zone or riparian area is the interface between land and a river or stream. Riparian is also the proper nomenclature for one of the fifteen terrestrial biomes of the earth; the habitats of plant and animal communities along the margins and river banks are called riparian vegetation, characterized by aquatic plants and animals that favor them. Riparian zones are significant in ecology, environmental management, and civil engineering because of their role in soil conservation, their habitat biodiversity, and the influence they have on fauna and aquatic ecosystems, including grassland, woodland, wetland or sub-surface features such as water tables. In some regions the terms riparian woodland, riparian forest, riparian buffer zone, or riparian strip are used to characterize a riparian zone. The perceived need for riparian-zone restoration has come about because riparian zones have been altered and/or degraded throughout much of the world by the activities of mankind affecting natural geologic forces. The unique biodiversity of riparian ecosystems and the potential benefits that natural, vegetated riparian have to offer in preventing erosion, maintaining water quality that ranges from being decent to completely healthy, providing habitat and wildlife corridors, and maintaining the health of in-stream biota (aquatic organisms) has led to a surge of restoration activities aimed at riparian ecosystems in the last few decades. Restoration efforts are typically guided by an ecological understanding of riparian-zone processes and knowledge of the causes of degradation. They are often interdependent with stream restoration projects.
Causes of riparian-zone degradation Riparian-zone disturbance falls into two main categories: hydrologic modifications that indirectly impact riparian communities through changes in stream morphology and hydrologic processes, and habitat alterations that result in direct modification of riparian communities through land clearing or disturbance.
Hydrologic modifications
Dams and diversions
Dams are built on rivers primarily to store water for human use, generate hydroelectric power, and/or control flooding. Natural riparian ecosystems upstream of dams can be destroyed when newly created reservoirs inundate riparian habitat. Dams can also cause substantial changes in downstream riparian communities by altering the magnitude, frequency, and timing of flood events and reducing the amount of sediment and nutrients delivered from upstream. Diverting water from stream channels for agricultural, industrial, and human use reduces the volume of water flowing downstream, and can have similar effects. In a natural riparian system, periodic flooding can remove sections of riparian vegetation. This leaves portions of the floodplain available for regeneration and effectively “resets” the successional timeline. Frequent disturbance naturally favors many early-successional (pioneer) riparian species. Many studies show that a reduction in flooding due to dams and diversions can allow community succession to progress beyond a typical stage, causing changes in community structure. Changing flood regimes can be especially problematic when exotic species are favored by altered conditions. For example, dam regulation changes floodplain hydrology in the southwest US by impeding annual flooding cycles. This modification has been implicated in the dominance of saltcedar (Tamarix chinensis) over the native cottonwood (Populus deltoides). Cottonwoods were found to be competitively superior to saltcedar when flooding allowed seeds of both species to cogerminate. However, the lack of flooding caused by altered hydrology creates more favorable conditions for the germination of saltcedar over cottonwoods.
Groundwater withdrawals Riparian zones are characterized by a distinct community of plant species that are physiologically adapted to a greater amount of freshwater than upland species. In addition to having frequent direct contact with surface water through periodic rises in stream water levels and flooding, riparian zones are also characterized by their proximity to groundwater. Particularly in arid regions, shallow groundwater, seeps, and springs provide a more constant source of water to riparian vegetation than occasional flooding. By reducing the availability of water, groundwater withdrawals can impact the health of riparian vegetation. For example, Fremont cottonwood (Populus fremontii), and San Joaquin willow (Salix gooddingii), common riparian species in Arizona, were found to have more dead branches and experienced greater mortality with decreasing groundwater levels. Plant community composition can change dramatically over a gradient of groundwater depth: plants that can only survive in wetland conditions can be replaced by plants that are tolerant of drier conditions as groundwater levels are reduced, causing habitat community shifts and in some cases complete loss of riparian species. Studies have also shown that decreases in groundwater levels may favor the invasion and persistence of certain exotic invasive species such as Saltcedar (Tamarix chinensis), which do not appear to show the same degree of physiologic water stress as native species when subjected to lower groundwater levels.
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