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Summary of Impacts of both livestock and wildlife grazing on water quality and riparian habitat (Objective 1).
Influence of Large Ungulates on Water Quality
Any pollutant that enters a stream from other than a discernible, confined and discrete source is considered nonpoint source pollution (Wyoming Department of Environmental Quality 1990). The impacts of range livestock and free roaming wildlife on water quality are considered nonpoint sources of pollution. The impacts caused by feedlot situations are point source pollution and will not be addressed in this report.
Because nonpoint source pollution is not discrete it is difficult to determine cause, especially in areas with multiple-land-use practices. Natural processes of erosion and sedimentation also contribute to changes in water quality and unless their levels are quantified, it is difficult to assess the contribution of other sources.
The major nonpoint source pollutants that result from grazing are sediment, bacteria and nutrients. Sedimentation is most detrimental in terms of associated biological and economic problems (Chesters and Schierow 1985, Crosson 1987, Robinson 1988). Increased bacteria levels and nutrient loads from grazing are of short duration and do not seriously impact water quality, except in situations of obvious overgrazing by livestock (Van Haveren et al. 1985).
Sediment
Sediment is detrimental to water quality and streams for many reasons (Lynch et al. 1977). Increased sediment loads cause changes in physical properties of the water, impacting habitat for stream flora and fauna. Increased sediment loads also alter stream morphology. Sediment functions as a reservoir for bacteria, nutrients, and chemicals. Sediment also interferes with human uses of water.
Sediment affects stream organisms while it is in the water column as suspended solids (turbidity), and after it has settled to the bottom (Lynch et al. 1977). Excessive levels of sediment can change species densities, species diversity, and community structure of a stream.
Groups of organisms in the food chain are affected by suspended solids (Cordone and Kelley 1961). Suspended solids decrease light penetration, effectively reducing photosynthetic rates of phytoplankton and periphyton, thus reducing the numbers of these primary producers. Decreased numbers of primary producers influences the populations of secondary consumers, ultimately changing the community structure of the stream (Windell 1983). Suspended solids also influence the thermocycles of the water column. Ellis (1936) found that as suspended sediments settle they interfere with the heat transmission of the water. The metabolic rates of aquatic organisms may be affected by change in these cycles.
Suspended solids have direct and indirect impacts on fish. Physiologically, the function of the gills is inhibited by the amounts of suspended solids in the water column. Vision is impaired by turbidity, thus affecting a fish's ability to capture prey items. Suspended solids also affects the drift behavior of macroinvertebrates (Gammon 1970) which are prey for fish.
Once sediments have settled, they cause different impacts on stream organisms than suspended solids. Sediment changes the complexion of the substrate as interstitial spaces become imbedded with fine sediment material (Cordone and Kelley 1961). Sediment covers gravel substrates, rendering these areas unsuitable as fish spawning habitat. When sediment covers redds the amount of gas exchange for the eggs is impaired, decreasing egg survival (Armour 1977). Duff (1977) found survival of chinook salmon and steelhead eggs was reduced when gravel interstices were filled with fines. Cover used by young of the year fish is reduced causing a decrease in the total population (Alexander and Hansen 1986). Imbeddedness also alters macroinvertebrate community structure because the habitat required for some species is lost (Nuttall and Bielby 1973, Alexander and Hansen 1983).
Sediment, in association with discharge and gradient, is a major component influencing stream morphology (Miller 1987). Although streams have the ability to adjust for changes in flow regimes and sediment loads, excessive sediment loads can damage a stream's ability to maintain its state of dynamic equilibrium (Apmann and Otis 1965). Changes in stream morphological characteristics, including channel width and bed form can result from excessive sediment loads.
Scour and deposition processes alter streams banks, thus altering stream width. In alluvial rivers the bank material, which is primarily wash load (fines transported with the same velocity as that of the water), is constantly being scoured and deposited (Einstein 1972). A stream channel is in equilibrium relative to sediment level when scour and deposition rates are equal. A stream not in equilibrium with its wash load will scour sediment from the banks if the wash load is low, and deposit sediments when the flow is insufficient to carry the load (Apmann and Otis 1965). Channel width change through scour and deposition is dependent on flow rates. When flow rates are low relative to stream width, aggradation at the banks results and the stream becomes narrower (Andrews 1982). Bank encroachment results in greater habitat uniformity. Restriction of flow to a narrower channel causes many habitat features such as undercut banks to be lost.
The stream bed form is also altered by excessive sediment. When sediment loads are in excess of the water velocity's ability to flush the stream system, deposition results (Foster and Meyer 1977). Sediment deposited along the bottom alters the contour of the bed form by filling in pools and backwaters. This process creates a more physically uniform stream and reduces habitat diversity.
Another detrimental characteristic of sediment is it acts as a reservoir of bacteria, nutrients and organic matter (Windell 1983). Sediment is derived from run-off and land erosion. Because of its source, sediment is high in organic material, and in areas that are grazed, bacteria and organic wastes contained in animal feces will enter the stream (Cole et al. 1986).
Kittrel and Furfari (1963) found that coliform organisms tend to adhere to suspended sediments and settle to the bottom. Acting as a sink for bacteria sediment may cause prolonged and heightened levels of bacteria in streams. Bacteria levels within the water column may diminish rapidly. But a storm event can causes re-suspension of bottom sediment triggering increased bacteria levels (Stephenson and Rychert 1982).
Sediment may act as a sink for organic matter and nutrients (phosphorous and nitrogen). Fredriksen (1972) found that a significant portion of nutrients entering streams are attached to eroded soil particles. Animal wastes are washed into streams as in run-off and may settle out of the water into sediments. Presence of these components accelerate the eutrophication process.
Sediment loads in streams also act as sinks for chemical substances (Ellis 1936, Auer and Auer 1990). The interface between the water column and the substrate is important for biochemical processes that take place in the stream ecosystem. At this interface, chemicals are often harmful because they can impact oxygen levels. Unsaturated chemicals result in oxygen depletion (Ellis 1936). Decomposition of organic matter at the water column substrate interface depletes oxygen causing toxic reduced chemical species to be produced including ammonia and hydrogen sulfide. Fish embryos in contact with contaminated sediments may have reduced viability (Auer and Auer 1990). Sediment in streams is costly to humans both economically and socially (Crosson 1987). Both problems motivate the development of best management practices.
The economic problems created by excessive sediment loads include decreased water storage and reduction of sport fishing and recreation opportunities. Sediment loads inhibit water storage by filling in reservoirs, lakes, and irrigation ditches (Holochek 1980). Building sediment storage areas into reservoirs and sediment dredging processes are costs incurred from excessive sediment delivery. Revenues generated by sport fishing and associated economic benefits of tourism decrease when fishing opportunities are reduced by the destruction of fish habitat (Tiner 1984). Other recreational opportunities are lost when sedimentation from high water turbidity renders waters undesirable for body contact watersports (Fanning 1986).
Bacteria
Grazing animals increase levels of bacteria in streams from feces delivery (Tiedemann et al.1988, Gary et al. 1983, Milne 1976). Bacteria do not broadly impact stream ecology as sedimentation does. But they are detrimental to water quality. The major problem with high levels of bacteria are the dangers posed to human health. The presence of excessive levels of pathogenic bacteria renders water unsuitable to human consumption or contact (Hammer and MacKichan 1981).
Nutrients
As with bacteria levels, nutrient loading in streams results from inputs of ungulate excreta (Doran et al.1981, Schepers and Francis 1982). Nutrient loading does not have the far reaching impact on the stream ecosystem as sedimentation does (Johnson et al. 1978, Smeins 1975). The more significant impact of nutrient enrichment from grazing is the downstream cumulative impacts of eutrophication (Likens and Bormann 1974, Cole et al. 1986). Nutrients (primarily phosphorous and nitrogen) from animal excreta entering the stream may pass through the stream system with little impact, but provide increased nutrients to downstream bodies of water. Excess nutrient loading provided to aquatic vegetation results in increased photosynthetic rates followed by will diminish light transmission through the water. Decreasing light decreases photosynthetic rates, causing plants to consume oxygen for respiration, leading to oxygen depletion of the water (U. S. Environmental Protection Agency 1979). Decomposition of increased plant biomass also requires greater amounts of oxygen. oxygen depletion from eutrophication impacts the ecological balance of the body of water (U. S. Environmental Protection Agency 1979).
Riparian zone
The preceding discussion of the major nonpoint source pollutants addresses grazing impacts in both the upland and the riparian zones. Large ungulate grazing in the riparian zone as well as the upland areas influences the quantities of sediment, and bacteria and nutrients from animal excreta that enter the stream. But some impacts to water quality and stream ecosystems are unique to grazing impacts on the riparian zone (Skovlin 1984). Riparian zones are defined as having a high water table and distinct vegetation and soils (Kauffman and Krueger 1984, Hansen 1988). They form transitional zones between the terrestrial and aquatic ecosystems (Ewel 1978). Because of their unique position as an ecotone between the two systems they are particularly important to the maintenance of water quality (odum 1978). Animals grazing in riparian zones remove and trample vegetation (Severson and Boldt 1978). These activities promote the following three types of changes to water quality and the stream ecosystem: changes in water temperature, alteration of allochthonous inputs, and changes to the physical features of the channel.
Shading provided by riparian vegetation is important for regulating solar energy inputs to streams (Miller 1987). Removal or damage to vegetation overhanging the stream reduces shading, thus increasing water temperature (Meehan et al. 1977). Warmer water temperatures can reduce habitat for sensitive organisms such as salmonid species. overhanging vegetation also provides cover to fish (Boussu 1954). Loss of this type of cover is equivalent to loss of fish habitat (Wesche et al. 1987).
Streamside vegetation is an important source of allochthonous detrital inputs to the stream, particularly in heterotrophic headwater streams (Knight and Bottorff 1984). Allochthonous inputs provide nutrition for fungi, bacteria and macroinvertebrates and are important for the energy flows within stream. Loss of these materials results in changes to the community structure of macroinvertebrates which are an important food source for fish (Armour 1977). Also, streamside vegetation supports many terrestrial insects that are important food sources to fish once they fall into streams (Armour 1977). These populations are impacted when streamside vegetation is altered by grazing.
Streamside vegetation removal and trampling alters bank stability, instream vegetation, and quantity of large woody debris that influence the physical features of the stream. Loss of streamside vegetation weakens bank structure because the binding of root systems no longer provides bank stability (Groeneveld and Griepentrog 1985). When vegetation is lost, banks are more susceptible to breakdown from animal movements and from erosional forces of the stream flow. Destablization of streambanks can result in breaking and sloughing of bank material into the stream (Marcuson 1969, Groeneveld and Griepentrog 1985). Stream morphology is changed by the resulting stream width increase and stream sediment contribution (Platts et al. 1985).
Another feature of riparian zones are overhanging banks. Overhanging banks provide cover for fish and influences stream morphological processes. In a study of eight small streams in southeast Wyoming, Wesche et al. (1987) found that overhanging banks were the most important cover type for trout. Also, vegetation on overhanging banks slows water flow and promotes deposition (Clifton 1989). Thus loss of overhanging banks from breaking and sloughing not only reduces an important component of fish habitat but also influences factors of stream morphological processes.
Large woody debris is an important component influencing stream morphology (Swanson et al. 1982). Sediment trapping, stream bank protection, routing of water and sediment, and creation of instream habitat structures such as plunge pools are functions of large woody debris in streams (Keller and Swanson, 1979). Loss of large woody debris in the stream may result in morphological changes.
Riparian vegetation is important as a buffer strip between the upland areas and the stream ecosystem (Meehan et al. 1977, Odum 1978, Miller 1987). It decreases the levels of sediment that reach the stream and is also a sink for nutrients (Omernik et al. 1981). Loss of riparian vegetation allows for greater delivery of sediment to the stream. The riparian zone functions in denitrification processes and in phosphorous demobilization (Green and Kauffman 1989). Water quality is influenced by these biogeochemical functions of the riparian ecosystem. As the preceding discussion demonstrated impacts to stream water quality from large ungulate grazing, particularly sediment, bacteria and nutrients, have many ramifications for the stream ecosystem and for the socioeconomic uses for human beings. To maintain water quality, standards have been established.
Water Quality Standards
Nonpoint source pollution standards for various streams in Wyoming differ based on the stream's classification. All streams are organized into a four class system based on water quality potential with 1 being the highest quality and 4 being the lowest (Wyoming Department of Environmental Quality 1990). The type of fishery that is supported by the particular water, warmwater or coldwater, and the level of body contact with the water are other rating categories used in conjunction with the four class system to define Wyoming waters.
Water quality standards that are associated with ungulate grazing impacts include those for: turbidity, settleable solids, fecal coliform bacteria, and dissolved oxygen. The standards set for each of these nonpoint source pollutants qualifies that the standards are set for degradation levels that are "attributable to or influenced by the activities of man" (Wyoming Department of Environmental Quality 1990)
The water quality standards for turbidity and settleable solids address the problems of sedimentation. Standards set for Class 1 and 2 coldwater fisheries type waters require that human activities not increase the turbidity more than 10 nephelometric turbidity units. In Class 3 waters and Class 1 and 2 warmwater fisheries waters, increases of 15 nephelometric turbidity units are allowable. The standards for settleable solids are less quantitative. In all Wyoming waters, sludge formation and bank or bottom deposition from human activities are not allowed if they affect the aesthetic value or other human uses of the water. Also, these impacts may not adversely affect the aquatic organisms, plant life, or wildlife.
The standards used to address the problem of bacterial loads that can be caused by grazing animals vary by water class and by rating of body contact. In all Class 4 waters levels of the indicator bacteria (fecal coliform bacteria) may not exceed a geometric mean of 200 bacterial groups per 100 ml sample year round. In waters classified as full body contact recreation waters the same standard applies for during the recreation season. For waters classified as secondary contact recreation waters, fecal coliform groups may not exceed a geometric mean of 1000 per 100 ml sample.
Dissolved oxygen levels can be impacted from ungulate grazing because of nutrients contributing to the process of eutrophication. The Department of Environmental Quality has designated specific standards for dissolved oxygen levels for Class 1 and 2 coldwater fisheries, and Class 3 and Class 2 warmwater fisheries. These standards are based on dissolved oxygen levels necessary for viability of different life stages. The standard deems that human activities may not introduce wastes harmful to aquatic organisms into waters.
Grazing may influence water temperature by the removal of vegetation that shades the stream. The Wyoming standards prohibit changing natural water temperatures by human activities. In Class 1 and 2 coldwater fisheries changes of more than 1.1 ° C are not allowed. A change of more than 2.2 ° C is not allowed in Class 3 waters or in Class 1 and 2 warmwater fisheries.
Mechanisms of Impact by Grazing
The degree of degradation by sediment, bacteria and nutrients to water quality is dependent upon delivery processes, primarily erosion and run-off (White et al. 1983, Cole et al. 1986). Coupled with the mobilization of nonpoint source pollutants are the actions of grazing animals that alter landscape features, thus, enhancing the delivery of pollutants to the stream (Marcuson 1977a). Before mobilization and the influence of grazing animals on nonpoint source pollution can be discussed, the importance of the riparian zone to maintenance of water quality must be reviewed.
Riparian Zone
By virtue of the riparian zone's proximity to the aquatic and the upland area it has the ability to directly influence stream water quality and to buffer streams from upland impacts (Meehan et al. 1977, Odum 1978).
Riparian zones directly influence instream water quality by their function in water storage, stabilizing banks, and sediment removal (Hansen 1988). Riparian areas store water during periods of increased flow and help to maintain water flow during low water periods. The bank storage function decreases the magnitude of flooding reducing the likelihood of excessive bank erosion. Riparian vegetation binds bank soils, enhancing bank stability and reducing bank erosion. When riparian vegetation is inundated by high stream flows it will trap sediment, increasing riparian soil and improving water quality (Clifton 1989). Also greater channel roughness is achieved helping to control water velocity. The slowing of water velocity by riparian vegetation hanging within the stream channel allows for deposition and helps to reduce erosion of the stream channel (Lowrance et al. 1985, Clifton 1989).
Sediment movement is regulated by riparian vegetation. Eroded materials contained in run-off from upland areas are effectively filtered by the riparian vegetation before entering the stream (Meehan et al. 1977). Riparian areas also act as sinks for 24 nutrients from the uplands (Lowrance et al. 1985).
Besides water quality maintenance, riparian areas are integrated with nonpoint source pollution problems due to their relationship with large ungulates. Riparian areas are of disproportionate importance to both wildlife and livestock (Thomas et al. 1979). The greater abundance and palatability of forage, proximity to water, and availability of cover and shade increase the that 37 to 43 percent of Wyoming terrestrial wildlife species are to some degree dependent on riparian habitat for survival. Livestock preference for riparian areas over upland areas has been noted by many authors (Ames 1977, Kennedy 1977, Hansen 1988). Because the riparian zone is important in maintaining water quality and important for wild and domestic ungulates, a potential for negative impacts to water quality exists when animals overuse riparian areas.
Erosion
The primary mechanism by which sediment is delivered to streams is overland flow of precipitation (Foster and Meyer 1977, Meehan et al. 1977). This run-off mobilizes eroded soil particles and carries them into streams. Subsurface flow can deliver sediment but because velocities are low, this process is of little consequence (Statham 1977).
Many factors influence the quantities of soil that will erode and be mobilized by run-off, including climate, topography, ground cover, and soil (Foster and Meyer 1977). The activities of large ungulates alters the functions of ground cover in protecting water quality and also changes soil properties.
Vegetation influences hydrological processes including infiltration, interception of surface run-off erosion and deposition of soil components (Smeins 1975). Ground cover acts to protect the soil from the erosional forces of rainfall (Branson and Owen 1970, Buckhouse 1984a). Vegetation protects the soil by either preventing precipitation from reaching the soil or by buffering the impact of the rain drops. Upland vegetation can trap sediment mobilized by run-off. Bare soil not bound by vegetation nor protected from rainsplash by vegetation or litter, is more susceptible to erosion. The impact of rainsplash is higher on soils that are not protected by vegetation (Barrett 1984). The velocity of run-off is slowed by vegetation allowing more time for absorption. Vegetation also increases soil permeability allowing more water to penetrate (Statham 1977).
Soil properties also determine its erodibility. Moisture content, bulk density, and soil infiltration rates affect how efficiently precipitation will be absorbed rather than becoming surface run-off (Buckhouse 1984a). Trampling by large grazing ungulates compacts the soil, increasing bulk density of soil. The resulting lowered infiltration rates cause increased run-off and erosion.
Bacteria
The mobilization of bacteria and nutrients from animal excreta is also due to run-off (Cole et al. 1986). Fecal material is eroded by precipitation and mobilized by run-off. The amount of mobilization depends on whether feces were deposited in a location where it would be susceptible to erosion (Cole et al. 1986). Also bacteria and nutrients associated with the soil will be eroded along with the soil. Thus the same factors that influence sediment delivery will also influence bacteria and nutrient loads. Omernik et al. (1981) states that many nutrients may eventually reach the stream by subsurface flow.
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