riparian

Riparian Zones - Ray Holland

Riparian zones are ecosystems with tremendous biodiversity. As transitional zones connecting aquatic and terrestrial ecosystems, they include wetland adapted as well as terrestrial plants, aquatic migratory birds and upland birds, many species of mammals, amphibians, and microorganisms as well as periodic visits from riparian obligate terrestrial animals. These ecosystems which are so vital to earth’s biodiversity and provide ecosystem services to humans are continuing to disappear even now that we understand their importance.
 * Abstract **

The riparian zone is the land along a stream from the stream edge upland including the area in which vegetation may be affected by the high water table or is frequently inundated by flooding. This transitional zone is a unique ecosystem containing a diversity of both aquatic and terrestrial plants, animals, and other organisms (Kerem, 2012). The riparian zone links both perennial and intermittent streams and wetlands with terrestrial upland ecosystems. The extent of a riparian zone is determined by the specific hydrology and geomorphology of the stream. These transitional ecosystems with high species richness contain much of the biodiversity of many ecoregions. Small riparian ecotones often provide the habitat for a majority of the species in the area. Dryland rivers such as those flowing through the arid southwestern US have particularly important riparian habitat. In New Mexico, riparian areas make up about 1% of the land area, but over 75% of our species are dependent on riparian habitat for at least a portion of their life cycle. Riparian zones provide buffers along streams and rivers, reducing runoff and filtering out nutrients and nonpoint source pollutants. Riparian zones contain an abundant variety of plant species which support both upland dependent and aquatic dependent animals. Forested aquatic buffers provide wildlife corridors through developed areas. Many trans-continental migratory bird flyways follow rivers since the riparian areas provide the travelers with cover and available food. Many local upland birds find favorable nesting and feeding sites in riparian zones. Vegetated riparian zones provide streambank stability, reducing erosion and sedimentation. The vegetated areas contribute to river water quality by filtering out pesticides, herbicides and excess nutrients. During peak flow periods, the presence of an open floodplain allows the river to spread out, slowing velocities and reducing peak floods resulting in increased groundwater recharge and reduced runoff along with less channel incision in the higher velocity reaches and less deposition in the lowland plains.
 * Riparian zones – sources of biodiversity **

Anthropogenic alterations including reduced flows, elimination of sinuosity, river channelization, and bank stabilization disrupt the natural flow regime (Ward, 1998). The river system fails to function as a holistic unit if the river is cut off from the floodplain. Rivers require both longitudinal and lateral connectivity. According to the River Continuum Concept, the river’s biotic communities are structured according to available resources as the river proceeds downstream (Vannote, 1980). Biodiversity is limited in the headwaters by the limited primary production in the normally canopied upland streams and riparian zones. In the middle reaches, if connectivity to the floodplain and seasonally inundated wetlands is maintained, the river achieves its greatest level of productivity and biodiversity (Ward, 1998). This biodiversity is dependent on lateral connectivity. As a mid-order river meanders back and forth in its floodplain, almost limitless interactive pathways can be formed. A diversity of lotic and lentic ecotones are created. Braids, backwaters, and side-channels provide the abiotic diversity for the proliferation of biotic diversity found in swamps and wetlands. As long as the floodplain sees periodic flooding, this abundance of wetland vegetation, aquatic invertebrates, and river and riparian vertebrates is maintained. Riparian ecosystems include complex and varied habitats, ecologically diverse and high in species richness of plants, animals, and microorganisms. These communities are specifically adapted to changing conditions, continuing in periods of inundation as well as long periods with no river flooding. This seasonal hydrologic variation controls riparian biotic and abiotic conditions (Nilsson, 2012). Geomorphic changes resulting from peak flows create a variety of riparian habitat. Silt, leaf litter, sediment, and larger aggregate are redistributed. Nutrient cycling is enhanced. Seed distribution is facilitated (Middleton, 1999). Many and varied living communities are continuously being formed as the abiotic conditions continue to change. Biodiversity is dependent on these ever changing communities which occur in periodically inundated riverine-wetland ecosystems (Gumiero, 2013). Urbanization has greatly impacted extent and quality of riparian zones. Significant changes to the structure and function of riparian habitat result from urbanization. Altered flow regimes, pollution, nutrient runoff, exotic species invasions, and habitat degradation accompany urbanization. Cumulative area of river and stream riparian zones has been greatly decreased by development. Ecosystem services are diminished. Habitat quality is reduced. Wildlife corridors are eliminated. In recent years, significant efforts have been made to restore riparian habitat within urban areas in the US. Although isolated riparian communities are re-established, many riparian obligate species still do not have sufficient upland habitat within landscape scale urbanized regions (Oneal, 2009). Restoration of riparian habitat within urban areas continues to be of primary importance to maintain biodiversity. Livestock grazing is another threat to riparian areas. Grazing, especially overgrazing can impact entire watersheds, but the riparian zones are particularly sensitive to degradation. When not excluded, cattle tend to select the cooler, more moist, higher density vegetation areas near streams and ponds. Preferred species of vegetation are often overgrazed, resulting in a reduction in species richness and vegetation removal. Vegetation removal in a catchment allows greater runoff and more erosion leading to increased sedimentation. Less vegetation can result in less shade, which generally will lead to higher stream temperature. An increase of only a few degrees can eliminate trout and other cold water species from a stream reach. Warmer stream temperatures can bring about greater primary productivity, algal blooms, and lowered dissolved oxygen. When cattle eat the vegetation down to the ground and trample over the stream bank, the bank collapses allowing more erosion. Increased erosion and sedimentation impacts the entire river system.
 * Anthropogenic alterations **

These cascading detrimental impacts can be reduced and even reversed by excluding cattle from riparian areas. Healthy vegetation along stream banks provides bank strength and stability and reduces erosion. In one study of a stream in Utah, only four years after cattle were excluded, the composition of the riparian vegetation had already changed from grazing tolerant grasses to “hydrophytic graminoid and shrub species” (Hough-Snee, 2013) more typical of the native riparian community. Hough-Snee, et al and several other researchers suggest that even passive restoration techniques as simple as installing cattle exclosures is often all that is required for the stream and riparian ecosystems to recover from the multiple detrimental impacts of grazing. Depending on the past land use of the riparian area and current land use of adjacent land in the same catchment, other measures such as replanting of native vegetation may be required (Burger, 2010). But in many cases, if we just get the cattle out of riparian areas, ecosystem restoration may take place naturally.


 * Connectivity **

Floodplain connectivity has been found to be essential for the health of both rivers and the riparian zones. Wetlands which are periodically inundated, even if only for a small portion of the year, are among the more bio-diverse and productive natural ecosystems. The pulsing of the river, the flood pulse, is “the major force controlling biota in river-floodplains” (Junk, 1989). The river cannot be completely disconnected from its floodplain which Junk refers to as the aquatic/terrestrial transition zone. The transitional zone supports the river just as the river supports the floodplain. This interdependence is seen in nutrient cycling as well as in the food web. Most of the nutrient inputs into mid-order river channels come from smaller order upstream tributaries. Leaf litter and other allochthonous organic matter enters the stream system. Primary productivity, although often insignificant in the generally shaded forest upland streams, increases in the open river. Pulse flows distribute this nutrient rich water over the floodplain. Invertebrates thrive in the slower moving waters of ephemeral ponds, oxbows and backwaters. These invertebrate communities are a major food source for second order consumers from upland communities as well as fish and other vertebrate communities in the river channel. Regular periodic inundation allows this cycle to continue. River-wetland-floodplain systems which have been significantly altered by human activities often lack this important cycle.

In many rivers, few species of fish remain in the main channel throughout all stages of their lives. Riparian wetlands and backwaters provide spawning, nursery, and feeding habitat for many of these lateral migrators (Ward, 1998). Diversions and flow reductions, wetlands drainage, construction of levees, and floodplain reclamation has reduced or even eliminated floodplain connectivity in many reaches of many rivers.

Vertical connectivity is also being eliminated in the riparian zones of many human altered river systems. The hyporheic zone is at the interface of the river and the associated groundwater aquifer (Wetzel, 2001). A unique ecotone with unique biogeochemical conditions and interactions along with its own flora, fauna, and microorganisms exists in many hyporheic riparian areas. Because of dams and diversions, we have lowered the water table in certain reaches of many rivers, resulting in vertical disconnectivity in some riparian areas. As anthropogenic impacts alter hyporheic zones, we further reduce biodiversity.

The importance of connectivity between aquatic and terrestrial ecosystems continues to be studied. Increasing numbers of dams for water diversion and flood control alter and simplify the structure of rivers. Lowered flows result in channelization and reduced floodplain connectivity. One study was done in the middle Rio Grande, which is a highly regulated river system flowing through the desert of central New Mexico. The flow regime of the Rio Grande is so altered from the natural hydrograph that in some reaches channelization prevents overbanking onto the floodplain except on rare occasions. The aquatic/terrestrial transition zone provides much of the habitat heterogeneity in the middle Rio Grande. This varied habitat is responsible for the diverse biota in both the riparian zone and the nearby upland ecosystem. This study attempted to show the effect of channelization on the invertebrate communities in the river and in the floodplain (Kennedy, 2011). Isotope markers indicated that the transition zone has a macroinvertebrate community which is unique from the macroinvertebrates in the bosque. Reaches where channelization has occurred were compared to reaches where limited channel-floodplain connectivity still exists. Average densities of macroinvertebrates were 50% lower in the channelized reaches. Also taxonomic richness was demonstrated to be lower in the channelized reaches (Kennedy, 2011). River-riparian zone connectedness contributes to overall biodiversity in the river system.

Another study was done to determine the effects of channelization on the riparian habitats of headwater streams in the Midwestern US. Researchers found that the riparian zones of channelized streams were narrower than similar riparian areas in unchannelized streams (Seger, 2012). There is a greater abundance of woody vegetation and more canopy cover in rivers that have not been channelized. Channelization results in a decrease of macroinvertebrate diversity and in the abundance of pollution sensitive taxa. Channelization also reduced species richness of woody vegetation in the studied riparian zones (Seger, 2012).


 * Flow regime alterations **

Anthropogenic flow regime changes also impact seed distribution along the river and in the riparian zones. Vegetation in riparian areas is frequently affected by both natural and human induced disturbances (Vosse, 2008). Therefore these areas are highly susceptible to invasions of exotic species. In many river systems re-vegetation of disturbed areas occurs naturally since seeds and propagules of indigenous species are stored in the natural seed banks of riparian ecosystems. However, seasonal peak flows are required to distribute the seeds and propagules over the floodplain. Reseeding is seldom successful without pulse flows and either local rain or an extended period of inundation to distribute the seeds, initiate germination, and provide extended moist conditions as the seedlings are established. With floodplain vegetation declining in the lower Murray River in South Australia, an attempt was made to re-establish native vegetation from natural seed banks using environmental flows. The highly regulated Murray River has not been receiving its natural pulse flows in recent years. River managers made environmental releases timed with natural peak flows mimicking a short flood event. Seeds and propagules were distributed and some germinated. Success was very limited in this instance because there was not enough water committed to the project to make it through the required germination and seedling establishment process. Much was learned, however, about the importance of flow levels and timing to mimic the natural flow regime (Jensen, 2008).

We have discovered in recent years that as we divert a larger share of the flow of rivers, we are giving up several other ecosystem services provided by the river-wetlands ecosystems. Even if we are willing to accept environmental degradation as a cost of human water security, it appears that we are limiting the ability of our riparian systems to continue to provide the many ecosystem services which we have been receiving. Healthy river-wetlands systems provide humans with almost irreplaceable ecosystem services such as sediment retention, nutrient cycling, water filtration and the greatest source of biodiversity of any of earth’s ecosystems. According to Pahl-Wostl, et al, environmental flows for the benefit of riparian ecosystems is a “legitimate water use within an Integrated Water Resources Management (IWRM) context” (Pahl-Wostl, 2013). She defines environmental flows as “quantity, timing, and quality of water flows required to sustain freshwater and estuarine ecosystems and the human livelihoods and well-being that depend on these ecosystems” (Pahl-Wostl, 2013). The paper explains how that even if only for human well-being, it is imperative that we provide adequate flows to sustain healthy riparian ecosystems. We may even need to establish environmental flow requirements necessary to support healthy ecosystems. We need to look a little deeper than the minimum flows to provide for fish species. An analysis of the complex daily, seasonal, and annual flow requirements may be necessary. In many human altered river systems, implementation of environmental flow requirements may be needed for the continued provision of services by the riparian ecosystems.


 * Pulse Flows **

We now understand that pulse flows which connect rivers to their floodplains are essential for the health of the riparian communities on the floodplains. With dams for flood control, hydroelectric production, and water diversion on rivers worldwide, we are now experiencing “global scale ecological change” (Molles, 1998) resulting from channel-floodplain disconnection. A study was done on the middle Rio Grande in the 1990s to determine the impact of flooding on the ecology of the bosque, the riparian Cottonwood forest in the Rio Grande floodplain. Due to reduced flows and flood control levees, inundation of the floodplain in the study area had not occurred for about 50 years. The study site in the Bosque del Apache National Wildlife Refuge was in a Cottonwood forest ecosystem. The Cottonwoods were aging as flooding is required for germination and establishment of new Cottonwoods. The researchers wanted to determine how the absence of the flood pulse had affected the ecology of the forest floor. Two sites were selected, one as a control and one which was inundated by a flood pulse for the first time in over a half century. The flooding caused a reorganization of the biotic community of the forest floor. Microbial populations increased. Soil bacteria, decomposer fungi, and mycorrhyzal fungi increased and became more active. The arthropod community of the forest floor increased. The ecosystem was reorganized as a result of the flood pulse (Molles, 1998). It was estimated that recruitment of Cottonwoods and restoration of mammal populations would require several years, but could begin to occur with re-establishment of regular flooding of the riparian bosque. The researchers noted that riparian zones are “recognized as areas of high primary production, exceptional biodiversity, and rapid biogeochemical cycling.” (Molles, 1998) They acknowledged that it is unrealistic to attempt to completely restore river systems to their original state, but that managed flooding would allow the riparian ecosystem to reorganize and begin to restore itself.

As we understand the importance of high flow events and the need to maintain connectivity between river and riparian ecosystems in the floodplain, we are beginning to incorporate environmental flows into river management. It is now clear that, as it is important to provide e-flows for specific riparian ecosystems, we must eventually include e-flows in the management of entire river basins. The goal of natural or at least normative flow regimes will benefit our riparian ecosystems. Many of the challenges facing riparian ecosystems in our human altered river systems such as sustainability of biodiversity, modification of nutrient and sediment flux, and invasions by exotic species are reduced when we include e-flows into our hydrologic management decisions (Poff, 2013). Natural disturbances, such as extreme flow events, play an important role in regulation of geomorphic and ecological structure and function of river-floodplain systems. Environmental flows which mimic at least the critical components of the natural hydrograph allow riparian ecosystems to sustain themselves. Both permanent and ephemeral wetland systems are amazingly resilient and will often restore themselves if given the opportunity. An important component is to put water back in the system in a somewhat natural flow regime. We need to recognize the riparian ecosystem as a legitimate stakeholder whose water needs are considered when apportioning our often limited river flows (Poff, 2013).


 * Bibliography **

Har-Even, Hagai, Karem Ahuva, 2012. __Riparian Zones: Protection, Restoration, and Ecological Benefits__,Nova Science Publishers.

Nisson, Christer, et al. 2013. Boreal riparian vegetation under climate change. Ecosystem 16:401-410.

Midddleton, Beth. 1999. __Wetland Restoration, Flood Pulsing and Disturbance Dynamics__. John Wiley and Sons.

Gumiero, B, J Mant, T Hein, J Elso, B Boz. 2013. Linking the restoration of rivers and riparian zones/wetlands in Europe: Sharing knowledge through case studies. Ecological Engineering July.

Oneal, Amber S and John T Rotenberry. 2009. Scale-dependent habitat relations of birds in riparian corridors in an urbanizing landscape. Landscape and Urban Planning 92:3-4 264-275.

Hough-Snee, Nate, Brett B Roper, Joseph M Wheaton, Phaedra Budy, and Ryan L Lokteff. 2013. Riparian vegetation communities change rapidly following passive restoration at a northern Utah stream. Ecological Engineering 58:371-377.

Burger, B, P Reich, and TR Cavagnaro. 2010. Tajectories of change: riparian vegetation and soil conditions following livestock removal and replanting. Australian Ecology 35:980-987.

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Seger, Krystal, Peter C Smiley, Kevin W King, and Norman R Fausey. 2012. Influence of riparian habitat on aquatic macroinvertebrate community colonization within riparian zones of agricultural headwater streams. Journal of Freshwater Ecology 27:3 393-407.

Vosse, S, KJ Esler, DM Richarson, and PM Holmes. 2008. Can riparian seed banks initiate restoration after alien plant invasion? Evidence from the Western Cape, South Africa. South African Journal of Botany 74:432-444.

Jensen, Anne E, Keith F Walker, and David C Paton. 2008. The role of seedbanks in restoration of floodplain woodlands. River Research and Applications 24:5 632-649.

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