Chapter 8 : Riparian Zone

8.1  Introduction
8.2  Shade
8.3  Wind Breaks
8.4  Interception of Sediment and Nutrients
8.5  Source of Nearshore Habitat
8.6  Effects on Fish
8.7  Aesthetics
8.8  The Reservoir Manager in the Riparian Zone
8.9  Riparian Zone Management
8.9.1  Width
8.9.2  Three-Zone Buffers
8.9.3  Livestock
8.9.4  Bank Stabilization
8.9.5  Facilitation of Interactions between Reservoirs and Riparia
8.9.6  Residential Development Management
8.9.7  Effects of Drawdown on Connectivity to Riparian Zone
8.9.8  Conservation Easements

8.1 Introduction

Riparian zones are areas of biological, physical, and chemical interaction between terrestrial and aquatic ecosystems and typically have high abiotic and biotic diversity. Riparian zones represent the strip of land immediately bordering rivers and streams, generally beginning at the bank and moving inland a loosely defined distance. Riparian zones have been defined as encompassing the terrestrial landscape from the high-water mark toward the uplands to the limit of where vegetation may be influenced by elevated water tables and flooding (Naiman and Decamps 1997). The riparian zone may be narrow in small streams; larger in creeks, where it is represented by a distinct band of vegetation whose width is determined by long-term channel dynamics and the annual discharge regime; and large in rivers, where it is characterized by well-developed and physically complex floodplains with long periods of seasonal flooding, lateral-channel migration, and oxbow lakes.

The characteristics of riparian zones adjacent to reservoir shorelines are somewhat unique and different from those associated with rivers, but they can be managed in a way that offers similar functional values (e.g., shade, bank stabilization, water quality). Reservoir riparian zones resemble those of rivers only at the mouth of tributaries. Near their main body, reservoirs often lack a true riparian zone because the original river channel has been submerged and the new shoreline contour is at a higher elevation and fringed by upland vegetation that is not adapted to regular flooding. The upland vegetation along this new water line is also exposed to a higher water table, and the upland trees and shrubs that cannot adapt to wetter conditions do not survive. Without the root systems to stabilize soils, shorelines become vulnerable to water fluctuations, wave action from wind and boat traffic, and overland runoff. Shoreline erosion increases sedimentation and reduces habitat quality for invertebrate production and fish that depend on shore environments during some stage of their life cycle. Consequently, depending on water level, extent of water-level fluctuations, and shoreline slope, the riparian zone along many reservoirs can vary from upland terrestrial vegetation to nonvegetated mudflats.

Riparian zones play a critical role in linking aquatic and terrestrial systems (Naiman and Decamps 1997). In river systems, beneficial functional roles of riparian zones include shading, thermal buffering, providing woody debris and bank stability, and intercepting nutrients  and sediment (Pusey and  Arthington 2003). In reservoirs these roles are similar, but the importance of riparian zones shifts toward protection of the shore from strong fetch, bank stabilization (by armoring banks against wave-induced erosion), and interception of sediment and nutrients. In addition to the important biotic and abiotic roles, riparian zones provide an aesthetic visual barrier (Pusey and Arthington 2003) that helps maintain quality of recreational experiences, particularly in urban and agricultural settings.
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8.2  Shade

Shade provided by trees within the riparian zone is a feature of habitat structure and diversity (Figure 8.1). Fish use shade as a refuge from predation and as cover to launch predatory attacks (Helfman 1981). Shade also regulates thermal aspects of water quality by helping to moderate against extreme temperature fluctuations in the summer and winter (Quinn et al. 1992; Amour et al. 1994), and by moderating daily temperature fluctuations (Quinn et al. 1992).

Figure 8.1. A well-developed canopy along the riparian zone regulates light intensity, thermal aspects, and other environmental characteristics that promote fish species diversity. Additionally, the riparian zone controls wind fetch, is a source of leaf litter and woody debris, and can intercept sediment and nutrients. Photo credit: C. Watts, Mississippi State University.

In most reservoirs, the extent of shade provided by riparian zones in the nearshore environment is small relative to unshaded open-water areas, and the large open-water volume tends to neutralize effects of shading on water temperature and associated water quality (section 6). As a result, shade provided by riparian zones may not influence water temperature and water chemistry significantly in reservoirs, and its primary effect may be a reduction in light intensity in the nearshore environment. For example, in Columbus Lake reservoir, Mississippi, average light intensity in summer was 66% lower in shaded sites, but average temperature and dissolved oxygen were <5% lower in shaded sites (Raines and Miranda 2016).

The decrease in light intensity in shaded nearshore environments has the potential to influence biotic assemblages through competitive mechanisms associated with finding food, avoiding predation, and other aspects associated with visibility rather than through physiologic effects via water quality. In Columbus Lake, clupeids and most centrarchids were represented better in terms of abundance in unshaded sites, and percids were represented better in shaded sites (Raines and Miranda 2016). Shaded sites also tended to include intolerant species whereas unshaded sites did not. The diversity in light intensity and spectral composition of light produced at shaded and unshaded sites can create diverse mosaics of light-based habitats in nearshore environments that attract different species or life stages (Raines and Miranda 2016). This patchwork of light characteristics can enhance fish species richness and diversity and the variety of species associations. Damage to vegetation in riparian zones, or water- level reductions that move the shore away from riparian vegetation, can cause the diversity of light in nearshore environments to decrease and, generally, to become dominated by lighted habitat that lose nearshore biodiversity.
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8.3  Wind Breaks

Limited information exists on the effect of riparian zones on wind fetch and its concomitant effects on reservoir environments. In Canadian natural lakes, removal of trees within reservoir riparian zones through wind blowdown and wildfires tripled the overwater wind speeds and caused deepening of the thermocline (France 1997). Moreover, in a set of lakes where riparian trees had been removed a decade earlier through either clear-cutting or by a wildfire, thermocline depths were over 6 ft deeper per unit fetch length compared with lakes surrounded by mature forests. Therefore, changes in fetch caused by tree removal in riparian zones potentially can have substantial effects on reservoir water quality. However, these relationships have not been studied adequately and are likely to vary greatly with reservoir morphometry, water retention, and operation plan.
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8.4  Interception of Sediment and Nutrients

Nutrients and sediment enter reservoirs through tributaries and runoff from the surrounding landscape. A vegetated riparian zone precludes development activities that generate sediment and nutrients. Moreover, a vegetated riparian zone can trap sediment and nutrients in surface runoff, reducing sedimentation and eutrophication of the reservoir. Vegetated riparian zones also may reduce the velocity of sediment-laden storm flows, allowing sediment to settle out of water and be deposited on land (Magette et al. 1989; Daniels and Gilliam 1997).
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8.5  Source of Nearshore Habitat

Riparian zones are a significant source of nearshore structural habitat in the form of leaf litter and woody debris. Leaf litter can constitute a major source of organic matter to benthic organisms. The importance of this input depends on the characteristics of the riparian zone (e.g., development and vegetation composition), the degree of shoreline complexity, and the overall productivity of the aquatic system (Gasith and Hasler 1976). Most of these organic inputs are deposited as litterfall from the vegetated riparian zone. However, in some instances inputs of terrestrial insects can produce substantial subsidies of prey for aquatic predators and for reservoir nutrient cycles (Carlton and Goldman 1984). Large woody debris from riparian zones enters reservoirs as trees that have fallen into the nearshore environment, often resulting from natural forest succession, bank sloughing, or other causes. Large woody debris can trap sediment, cushion the effects of wave action on shorelines, and reduce or prevent scouring of the banks, which help maintain diverse nearshore habitat for aquatic biota. Woody debris also adds structural complexity to aquatic habitats, and habitat complexity is an important determinant of fìsh species richness along reservoir shorelines (Barwick 2004). The structural complexity of woody debris itself may be important in determining the degree to which it supports fish (Wagner et al. 2015). Woody debris provides protection from fish piscivores and avian predators. Woody debris may be used as cover for ambush predators and may be an important determinant of the growth rates of piscivorous fish, potentially influencing production (Bolding et al. 2004).
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8.6  Effects on Fish

Without suitable riparian zones along the reservoir periphery and tributaries, fine sediments are transferred from the watershed to shallow reservoir environments where they can affect littoral fish species. Increased turbidity due to suspended sediment and sedimentation alter food availability (e.g., algae and benthic invertebrates; Berkman and Rabeni 1987), affecting fish foraging behavior and efficiency (Bruton 1985) and altering interspecific interactions. Other detrimental effects include a reduction in habitat suitability for substrate spawners (Walser and Bart 1999), including increased egg mortality and reduction in rates of larval development and survival (Jeric et al. 1995). As the banks and associated littoral habitats degrade, density of fish that rely on the littoral zone during all or part of their ontogeny is likely to decrease.

Submerged woody habitat originating from the riparian zone influences composition of lacustrine fish assemblages. In reservoirs of the southern USA, species richness and centrarchid abundance are generally higher in coarse woody habitat (Barwick 2004) contributed from vegetated riparian zones surrounding the reservoirs. In a lake in Wisconsin, experimental removal of coarse woody habitat contributed from the riparian zone resulted in largemouth bass consuming less fish and more terrestrial prey and growing more slowly (Sass et al. 2006). Moreover, in the same lake, yellow perch declined to extremely low densities as a consequence of predation and little or no recruitment.

Deforestation in riparian zones can expose lake surfaces to strong winds. Excessive wind may deepen thermoclines and reduce habitat for cold stenotherms such as some salmonids (France 1997). Moreover, wind simply may mix the hypolimnion and epilimnion, resulting in loss of thermal refuge for species that rely on them during warm months, such as striped bass (Coutant 1985). Mixing may also cause periodic declines in water quality that could affect a subset of the fish assemblage negatively while favoring others. Excessive wind associated with deforestation of riparian zones has been linked to increased turbidity through resuspension of sediment produced by the interaction of fetch and depth in shallow reservoirs.
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8.7  Aesthetics

Riparian zones increase the aesthetic value of waterscapes (Brown and Daniel 1991; Emerson 1996). In agricultural areas, riparian zones can provide a buffer between the reservoir and the cultivated terrain. In urban areas, riparian zones can create park-like areas or natural areas that buffer the water body from the urban environment. In residential and campground areas, vegetated riparian zones provide visual contrast and relief and buffer the noise from nearby highways. Also, diverse types of vegetation in riparian zones provide further enhancement of aesthetic qualities and possibly enhance the filtering value of riparian buffers.
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8.8  The Reservoir Manager in the Riparian Zone

Ownership or control of the riparian zone along reservoirs varies by reservoir and among regions of the USA. In states west of the Mississippi River, riparian zones surrounding large reservoirs are often federally owned, whereas in states east of the Mississippi River, there is a greater percentage of land in private or local-government ownership. Similarly, riparian zone laws vary among states and are generally different between eastern and western states.

Most commonly, reservoir managers lack jurisdiction to manage reservoir riparian zones actively and thus must become partners in broader land management partnerships (section 2). These partnerships often include a combination of local, state, and federal government agencies, local municipalities, nongovernmental organizations, private  landowners,  and  various reservoir  users and  stakeholders. A diverse group of partners can provide the capacity needed to plan, fund, and complete restoration or management of riparian zones. The makeup of these partnerships will vary and is often influenced by the cultural, political, and economic landscape and societal values of the region.

As partners, reservoir managers can demonstrate and promote the linkage between the aquatic environment in the reservoir and the riparian zone. Reservoir managers can contribute technical guidance and planning assistance in development of restoration and management plans for riparian zones. Furthermore, reservoir managers can offer science-based expertise as to the effects that specific actions or management scenarios may have on reservoir water quality and biotic communities as management options are considered. To this end, a reservoir-specific riparian zone inventory documenting features important to reservoir condition is essential, focusing on critical areas representing major sources of problems likely to have large effects on the reservoir, such as long segments of agricultural ventures stretching down to the banks, periodic forest clear-cutting operations, developments in residential or commercial construction, and eroded shorelines. A focus on critical areas would result in the greatest improvements. Help in gathering this information can be enlisted from within the partnership and from reservoir associations and stakeholder groups (section 13).
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8.9  Riparian Zone Management

8.9.1  Width

Studies comparing multiple width riparian zones have shown that effectiveness increases with increasing width. Grass filter strips in particular have been shown to be very effective at trapping sediment particles. Neibling and Alberts (1979) estimated that 91% of incoming sediment to a grass filter strip was deposited in the first 2 ft of grass filter. Much of the larger particles of sediment may be removed in 15 ft of grass buffer, but finer particles may require 30 ft (Gharabaghi et al. 2002). The width required to optimize nutrient removal is less clear. Generally, 30-ft forested bands have achieved >70% retention of nitrogen, and almost 100% of nitrogen can be removed by bands 65–100 ft wide (Fennessy and Cronk 1997).

Slope is a key factor in determining sediment entrapment within vegetated riparian zones (Young et al. 1980; Peterjohn and Correll 1984; Dillaha et al. 1989; Magette et al. 1989; Phillips 1989). In general, riparian zones need to be wider when the slope is steep to allow more time for the velocity of surface runoff to decrease and deposit sediment (Barling and Moore 1994; Collier et al. 1995). In steep terrain, overland flow tends to concentrate in channelized natural drainage ways, giving rise to high-flow velocities.

Figure 8.2. Itaipu Reservoir on the border of Paraguay (left) and Brazil (right). Note the well-developed managed forested riparian zone on the Brazilian side. The riparian zone is a minimum of 500 ft wide. Image credit: Google Earth.

The pattern and intensity of rainfall are important factors in determining the effectiveness of riparian zones. It is expected that in regions where rainfall is uniform and light, narrower riparian zones may manage most of the sediment and nutrients that enter them effectively. In areas that experience seasonal storms of high intensity, even if few such events occur, wider riparian zones may be necessary because water residence time in the buffer is decreased.

There is no one-size-fits-all width for riparian zones appropriate for all reservoirs. Width depends on needs and objectives, on the intensity of the land use surrounding the reservoir, and local climate conditions. As a rule of thumb, a 75-ft riparian zone may be sufficient for a low-intensity land-use area and a 150–ft riparian zone for a high-intensity land-use area, but all recommendations are site specific. As the width of riparian zones increases, their buffering effectiveness may reach a point of diminishing returns compared with the investment involved. Therefore, managers may develop guidelines that remain flexible to site-specific needs to achieve the most benefits as is practical (Figure 8.2).
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8.9.2  Three-Zone Buffers

Riparian zones with multispecies strips may best protect water bodies against effects associated with land disturbances because of the different modes of particulate and dissolved contaminant transport (e.g., Schultz et al. 1995). This concept is based on three interactive buffer strips within the riparian zone that are in a consecutive up-slope order from shore: a strip of permanent forest, a strip of shrubs and trees, and a strip of herbaceous vegetation (Figure 8.3; Table 8.1). Width and vegetation composition of this basic model is adapted to the geographical variability of terrestrial plant communities and riparian zone condition (Sparovek et al. 2002). The first strip of forest influences the aquatic environment directly (e.g., temperature, shading, bank stability, wind break, source of coarse woody debris). The second strip incorporates shrubs and trees to control pollutants in subsurface flow and surface runoff; this strip is particularly important because this is where biological and chemical transformations, storage in woody vegetation, infiltration, and sediment depositions are maximized. The first two strips contribute to nitrogen, phosphorus, and fine-sediment removal. The third strip consists of grasses to spread the overland flow, thus facilitating deposition of coarse sediment. Grassy riparian areas trapped more than 50% of sediment from uplands when overland water flows were <2 in deep (Magette et al. 1989). Grassy buffer strips are effective at filtering sediment and sediment-associated pollutants (particulate phosphorus and nitrogen) from surface runoff. However, they are less effective in removing soluble nutrients such as nitrate, ammonia, and dissolved phosphorus. Nitrate removal from subsurface flows is considered to be greater in forested buffers, partly through uptake by plants (Fennessy and Cronk 1997; Martin et al. 1999). Wetlands and soils in riparian zones have a high capacity for denitrification compared with terrestrial and aquatic soils.

Figure 8.3. Three-zone engineered riparian buffer (see Table 8.1). Image credit: University of Kentucky Cooperative Extension Service, Lexington.

Riparian zones accumulate nutrients and absorb them into plant biomass, thus serving as nutrient filters. In North Carolina, riparian zones removed up to 80%–90% of the sediment leaving agricultural fields (Daniels and Gilliam 1997). In Vermont, reductions of approximately 20% in mean total phosphorus concentration and 20%–50% in mean total phosphorus load were observed (Meals and Hopkins 2002). In Lake Rotorua, New Zealand, riparian zone management reduced sediment loads by 85%; particulate phosphorus and soluble phosphorus by approximately 25%; and particulate nitrogen and soluble nitrogen by 40% and 26%, respectively (Williamson et al. 1996). These reductions were predicted to reduce the chlorophyll-a concentrations in the lake by approximately 5 ppb and help shift the lake’s trophic state from eutrophic to mesotrophic. The effectiveness of riparian zone restoration in sediment and nutrient reduction is diminished during periods of high runoff and outside the growing season, which, depending on geography, is often when the highest discharges occur.

Table 8.1. Three-zones buffer model for riparian areas. Width and vegetation composition of this basic model is adapted to the geographical variability of terrestrial plant communities and riparian conditions.

   Purpose Vegetation Management considerations
Zone 1
Creates a stable ecosystem adjacent to water’s edge; reduces runoff nutrient levels; provides shade; contributes organic matter and large woody debris Native riparian trees, shrubs, forbs, and grasses suited to a wet environment and of value to wildlife; use fast-growing tree species where banks must be stabilized Exclude heavy equipment; remove trees only for hazard reduction; discourage livestock; avoid concentrated surface runoff through use of flow spreaders
Zone 2
Provides contact time for carbon and energy sources to stabilize and store nutrients Predominantly native riparian trees, shrubs, forbs, and grasses Avoid gullying by maintaining vegetation and grading; management for timber or wildlife is encouraged, but leaf litter and shade levels should be maintained
Zone 3
Provides area to convert concentrated overland flow to uniform sheet flow; promotes deposition of sediment, infiltration of  runoff,  and uptake  of nutrients by vegetation Dense perennial grasses and forbs; an ungrazed grassland may serve as Zone 3 Maintain vegetation in vigorous growth stage; weed control may be needed; periodic reshaping may be necessary to prevent gully formation


Phosphorus accumulates in the soils of riparian zones and can be taken up by plants, but there is no process similar to denitrification that removes phosphorus to the atmosphere. Therefore, riparian zones potentially could become saturated with phosphorus, and their ability to trap phosphorus may decline with age unless sediment or organic  matter is removed from the riparian zone (Barling and Moore 1994). Thus, harvesting trees or plants from the riparian zone can provide a mechanism whereby phosphorus is removed.
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8.9.3  Livestock

Figure 8.4. Fencing installed to manage grazing along a reservoir in the Missouri River, Montana. Photo credit: Missouri River Conservation Districts Council, Helena, Montana.

Usually, livestock grazing adversely affects water quality, hydrology, riparian zone soils, and bank vegetation and stability (Kauffman and Krueger 1984). Livestock damage to riparian vegetation and soils destabilizes the banks and leads to mobilization of fine sediment, which in turn causes sedimentation in shallow-water areas adjacent to shore and causes reduced water clarity. The resulting increased sediment load is accompanied by particulate nutrients that may contribute to eutrophication. Further, livestock contribute nutrients directly to riparian areas through feces and urine. Fecal material deposited in damaged riparian zones may readily wash overland into the water with little opportunity for filtration.

Nevertheless, most riparian zones evolved with animals feeding on the lush vegetation and stepping on banks while accessing water. Although the original grazers were bison, moose, and deer rather than cattle, sheep, and goats, this evolutionary pressure resulted in regrowth of native riparian plant species following a period of grazing (Ohmart 1996). When farmers and ranchers displaced these occasional grazers with continuously grazing livestock, quality of riparian zones decreased. Provided with limited grazing area and little stimulus to move from one area to another, continuous grazers trample banks, congregate in the shade and cool breezes next to water, and overgraze the lush vegetation in these fertile areas. Occasional grazing of riparian zones may be unavoidable, particularly in the western USA. Strategies for attracting livestock away from riparian zones include providing alternative watering systems; planting palatable forage species on adjacent upland areas; using prescribed burning on upland areas to enhance forage production and palatability; and placing feed supplements such as salt, grain, hay, or molasses in upland areas of paddocks away from the riparian zone. Fencing often is required to prevent overgrazing and can be cost-shared with government agencies (Figure 8.4). Sometimes fences may not be intended to create riparian exclosures but rather to create riparian pastures that can still be grazed according to the goals for balancing livestock and natural resources.

Brush removal by cattle can maintain grassy buffers that help protect water.  Light grazing in riparian zones may be managed to mimic the activities of native wildlife by grazing small herds for a limited time and by grazing at different times of the year. In Wisconsin fisheries managers often contract with farmers to graze riparian zones rotationally (Lyons et al. 2000). Goats are used to control noxious weeds and nonnative brush species in riparian zones, allowing for the growth of plants that provide healthy riparian conditions. Detailed guidelines for managing grazing within riparian zones are available (e.g., Clary and Webster 1990; Leonard et al. 1997; Swanson et al. 2015).
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8.9.4  Bank Stabilization

Figure 8.5. General approaches to bank stabilization include live planting, bioengi- neering, and hard armoring. Photo credits:
U.S. Army Corps of Engineers, Savannah District (upper and lower photos), and The Nature Conservancy, Rhode Island (middle).

Overall, there are three general approaches to bank stabilization: live planting, bioengineering, and hard armoring (Goldsmith et al. 2013; Figure 8.5). Live planting involves sowing vegetation appropriate for the site and region. Bioengineering relies on a combination of structural components and plant material to produce a dense buffer of vegetation that serves as a “living system” to protect shorelines. Hard armoring involves installing structures such as breakwaters, revetments, and bulkheads (section 5.8.2). Shorelines with steeper slopes tend to experience greater erosion, so reducing the slope of a shoreline dissipates wave energy and lessens erosion. In reservoirs, a fourth approach may be to establish no-wake zones in nearshore, shallow areas.
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8.9.5  Facilitation of Interactions between Reservoirs and Riparia

Figure 8.6. Hinge-cut trees in Smithville Lake, Missouri. Photo credit: Missouri Department of Conservation, Columbia.


Of the methods used to provide woody structure in reservoirs, felling large trees most closely duplicates natural processes. In areas where trees in the riparian zone are numerous, the “hinge cutting” method of felling selected trees can accelerate development of nearshore fish habitat (section 5.8.2.12). The technique involves cutting selected trees near their base just deep enough so that the tree can be pushed into the  water  but  remain  attached to the trunk (Figure 8.6). Hinge-cut trees cut about two-thirds of the way through the trunk may continue to live for months to several years. Trees may be cut as clumps of two to three to maximize structural complexity. Younger trees work best because older trees may tend to break when felled. In Smithville Lake reservoir, Missouri, managers hinge-cut over 6 mi of shore- line and documented that hinge-cut trees were sheltering large concentrations of juvenile fish.
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8.9.6  Residential Development Management

Figure 8.7. Unmanaged residential development can reduce the ability of the riparian zone to perform its natural functions. Photo credit: D. Fogia.

Residential development of lakeshores is associated with changes in key ecosystem characteristics, including nutrient levels (Moore et al. 2003), aquatic vegetation (Radomski and Goeman 2001), and the spatial distribution (Scheuerell and Schindler 2004) and growth rates (Schindler et al. 2000) of fishes. Residential development increases the area composed of impervious cover such as driveways, parking lots, rooftops, sidewalks, and lawns and decreases the area composed of native plants and undisturbed soils (Figure 8.7). These artificial surfaces collect pollutants such as oil and gas, heavy metals, fertilizers, and pesticides, which can be washed into the reservoir. These surfaces also prevent water from infiltrating the ground. When storm water cannot infiltrate the ground, it is collected and discharged through storm sewers, drainage ditches, or some other means of conveyance, often into downstream reservoirs. Pavement can also lead to increased temperatures of the storm water entering the reservoir. Mitigation of residential development may be as simple as not mowing within a riparian zone or changing land management and yard care practices, or as complex as changing zoning ordinances or widening riparian zones through buyouts.

Residential developments in rural areas surrounding reservoirs generally depend upon septic systems as public sewer systems may not be available. On-site septic systems can be safe and efficient if designed, installed, and maintained properly. A septic system relies on natural bacteria in a tank to break down solid matter. A drain field then transfers the liquid waste into the soil for treatment. Malfunctioning septic systems can leak effluent with high concentrations of nutrients and bacteria into the reservoir. Nutrients entering the reservoir can cause algal blooms and excessive growth of unwanted aquatic plants.
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8.9.7  Effects of Drawdown on Connectivity to Riparian Zone

Drawdowns in reservoirs move the edge of the water away from the riparian zone. The regulated zone in storage or flood control reservoirs may be very wide in some cases and may be devoid of vegetation unless the drawdowns last several years. Frequent flooding within the regulated zone discourages establishment of terrestrial vegetation both by surface erosion and the physiological effects of periodic inundation on terrestrial plants. During periods of low water, the exposed regulated zone is colonized by herbs and seedlings of shrubs and trees. The extent of their development in the regulated zone reflects timing and length of the drawdown.

Frequency and duration of inundation of the regulated zone diminishes laterally away from the reservoir. At normal pool elevation, riparian vegetation in areas close to the main reservoir is characterized by younger stands, commonly composed of deciduous shrubs and trees. Portions of the riparian zone farther from the reservoir may contain older plant communities composed of either typical riparian species where tributaries enter the reservoir or upland species away from tributaries and in upland areas flooded by the reservoir. Thus, reservoirs with fluctuating water levels may have a riparian zone only part of the time; the rest of the time the riparian zone may be represented by a barren band or ring that follows the contour of the regulated zone. Providing diverse fish habitat within this contour is challenging. Reservoir managers have resorted to artificially introducing some of the features (e.g., large woody debris, and aquatic and terrestrial plants; sections 10 and 11) normally provided naturally by a functional riparian zone.

Reservoirs impounded low in a river basin over floodplain rivers, such as flood-control and navigation reservoirs, are unique because they may include within their upper reaches extensive shallow water stored over the original floodplain. Moreover, reservoirs that experience large seasonal water-level fluctuations as part of their operational objective periodically inundate and dewater floodplains associated with their upper reaches, partly mimicking the natural inundation of river floodplains. Because of their relatively flat topography and riverine origin, floodplains in the upper reaches of reservoirs provide broad expanses of facultative wetland vegetation within a narrow range of reservoir water-level elevations. Vegetation in these floodplains is important because wetland plant species can provide suitable vegetated habitats for fish at elevations below normal pool. These may be the only flooded vegetated habitats in the reservoir early in the spawning period when water levels may not be high enough elsewhere in the reservoir to flood vegetated habitats. Moreover, access to vegetated habitats below normal pool in the upper reaches of these reservoirs precludes the need to flood upland vegetation above normal pool every year, vegetation that would inevitably be impaired by regular flooding. Thus, management of riparian zones may also include management of floodplains in the upper reaches of reservoirs with the goal of preserving or restoring, through judicious water-level management, key vegetated habitats.
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8.9.8  Conservation Easements

Conservation easements can be a valuable tool when the manager perceives the need for the permanent conservation of a reach of riparian zone that is directly or indirectly providing quality fish habitat (Figure 8.8). A conservation easement is a section of land where the right to develop has been donated or sold by a landowner to a government entity or a nonprofit land trust. The purpose of a conservation easement is to preserve property in its predominantly undeveloped, natural, scenic, or forested condition and to prevent any use of the property that will impair or interfere significantly with the identified conservation value. The Nature Conservancy is one of the largest nonprofit holders of conservation easements in the USA (Kiesecker et al. 2007). The landowner retains ownership and pays property taxes. Although taxes continue to be paid, a landowner who donates a conservation easement may be eligible for a reduction in federal income taxes and a reduction in the value of the property for the purposes of property tax valuations and estate taxes (Morrisette 2001). Indeed, these tax factors can be a significant component of a landowner’s motivation to donate the easement.

Figure 8.8. Conservation easements are a legal tool to preserve riparian habitats of value to reservoir fish. Photo credit: U.S. Natural Resources Conservation Service.

Conservation easements are privately initiated land-use restrictions designed to protect and preserve private lands from development. They commonly are used to protect open space and scenic sites or preserve wildlife habitat and historical structures or cultural sites. The owner retains title to the land and may continue to use the land subject to restrictions imposed by the easement. Thus, the owner retains all rights to the property that the owner possessed prior to the easement subject to the restrictions imposed by the easement. The owner may continue to exclude the public from lands protected under a conservation easement, unless the easement provides for public access. The owner also may sell the property or pass it onto heirs, but the property remains bound by the terms of the conservation easement-conservation easements convey with the land and are usually perpetual unless the easement stipulates otherwise. Typically, a conservation easement prohibits any further development of the land unless it is related to a use of the land that is permitted by the easement. For example, a conservation easement on a large section of land along a reservoir may prohibit the owner (current or future) from subdividing the property; the owner, however, is permitted to continue using the property for its current use and is allowed to make improvements to the property that are related to its current use. Ensuring that the property remains in the current use may be the primary reason behind the conservation easement.
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