Rivers and Their Catchments: Physical Impacts of Agriculture - Essay Example

THE IMPACTS OF AGRICULTURE ON RIVER SYSTEMS AND MEASURES TO MITIGATE THEM Riverine ecosystems change continuously, alternating between episodes of relative stasis and dramatic change (Odum, 1993), influenced by the landscapes through which they flow (Vannote, et al., 1980). In recent times, such a landscape has increasingly become man-made and agricultural lands have encroached in river floodplains to take advantage of the readily available supply of water (Gore & Petts, 1989). Agriculture is, of course, important; the immensity of its benefits is underscored by the fact that nowadays it feeds 6,000 million people (Tilman et al., 2002). But it has also been increasingly recognized that agricultural activities are now threatening the ecological integrity of river ecosystems, impacting habitat, water quality, and the biota (Strayer et al., 2003). Agriculture as a Potentially Destabilising Phenomenon on River Systems Although many features of the dynamic river channel are mutually adjusting (Church 2002), the present rate at which human activities alter water or sediment supply has destabilize the existing channel shape, setting off a complex cascade of changes that has disrupted geomorphic processes and ultimately degrading the river habitat (Junk et al., 1989). Such actions, called potentially destabilizing phenomena (PDP) – or factors that are the most likely causes of management problems – must be tackled in developing sustainable river channel management solutions (Downs & Priestnall, 2003). All river systems are vulnerable to the potentially deleterious impacts of agricultural activities but this report is focused on regions of humid mid-latitude climates mostly because of the exacerbating factors such as winter, frequent thunderstorms, and mid-latitude cyclones (Pidwirny, 2006) that contribute to detrimental effects of some aspects of agricultural practices and to emphasize the necessity of proper mitigation and management actions (Clark, n.d.). These destabilising actions of agricultural activities are summarized in Table 1 and their causative relationship is discussed in detail in the following sections. Impacts of Agriculture on Sediment System and Flow Regime Agriculture requires the clearing of riparian vegetation and ploughing to cultivate the soil (Clark, n.d.). Riparian vegetation allows the gradual infiltration of water that comes down as precipitation into the soil and without it, surface run-off increases (Odum, 1993). The increased surface run-off coupled with the loose soil due to cultivation results to increased sediment transport and soil erosion (Clark, n.d.; Allan, 2004); this effect is more pronounced during winter when soils are more exposed and transport is aggravated by stronger winds (Clark, n.d.). The increased run-off is usually mitigated by land drainage but this has resulted to the faster rise and fall of the water levels where the drainage is discharged (Clark, n.d.). The trampling of livestock, on the other hand, causes soil compaction which increases surface run-off further (Quinn 2000). Increased surface run-off, in turn, may result to bank instability especially in the absence of bank vegetation (Allan, 2004). Soil erosion and bank instability mean the increased deposition of sediments in rivers resulting to changes in flow regime and river morphology (Clark, n.d.; Allan, 2004). Alteration in a river’s flow regime is also brought about by water abstraction and dam operations for use in irrigation (Clark, n.d.; Dougherty & Hall, 1995; Basson,2004). Low flow regime means deposited sediments are no longer flushed out (Old and Acreman 2006), thereby exacerbating the sedimentation caused by soil erosion. Apart from inundating floodplains as dam reservoirs fill up with water, the reservoirs also retain sediments which causes the reservoir to become shallower in the long run (Basson, 2004); this has been the reason for several reported dam failures. Worldwide, reservoir sedimentation occurs at an annual rate of 0.3% reducing the life expectancy of most reservoirs to only 300 years (Palmieri, 2003). Downstream of dams, flow regime is much decreased while there is also a reduction in sediment supply. The exponential relationship between river flow and sediment transport means that most sediment transport occurs during periods when flow is maximum (Simenstad et al, 1992); the flood attenuation function of dams disrupts this pattern of sediment transport. Impacts of Agriculture on River Morphology Studies have indicated that increased sedimentation has resulted to the extensive widening of rivers and has made them shallower due to bed aggradation by coarse sandy bedload (Brooks & Brierley, 1997). Alterations in the flow regime have also changed the width and depth of rivers (Allan, 204). Morphological changes also result from the rapid rise and fall of water levels which is brought about by land drainage (Clark, n.d.). Land drainage also reduces the natural buffering capacity of the catchment against floods resulting to increased magnitude and frequency of floods downstream (Clark, n.d.) and altering channel dynamics in the long term (Allan, 2004). Farmers have traditionally employed piecemeal bank protection to mitigate soil erosion; they do this by dumping hard materials such as concrete or boulders on destabilised river banks or by using gabion baskets and constructing ripraps for longer-term bank protection (Clark, n.d.). But these intrusive methodologies alter the river’s flow dynamics by deflecting the main current thus causing erosion immediately downstream or in the opposite bank (Clark, n.d.). Impacts of Agriculture on Geomorphically-Influenced Physical Habitat Rivers are a mosaic of microhabitats supporting a rich diversity of species (Odum, 1993). Such species have restrictive ecological requirement making them vulnerable to changes in flow and sedimentation (Dougherty & Hall, 1995). Increased sediment deposition results to the destruction of interstitial habitat by in-filling thereby endangering invertebrates and fishes who occupy these crevices and for which the gravel bottom substrate is crucial for spawning. Sedimentation also results to the expansion of areas of shallow water habitats, which usually lack structure and are more easily warmed (Richards et al., 1996). Hydrological alterations, meanwhile, specifically impair substrate suitability for periphyton and biofilm production (Allan, 2004). Because velocity is a significant factor affecting the distribution and assemblage of stream invertebrates (Statzner et al., 1988), flow regime alterations may also disrupt spawning and can cause the downstream displacement of species in their early life stages (Schlosser, 1985); respiration, feeding biology and behavioural characteristics will also be adversely affected (Petts, 2008). Due to the high intolerance of niche-specific species to flow regime alterations and sedimentations, a reduction in species diversity often results (Allan, 2004). Dams and low flows due to water abstraction have been known to impede the migration of salmonids and other migratory fishes. Because of this alteration, juvenile salmonids could not access the upper reaches of river catchments which are important for their production (Old and Acreman 2006). Table 1: Agriculture as Potentially Destabilizing Phenomena and Its Impacts on the River’s Sediment System, Flow Regime, Morphology and Geomorphically-Influenced Physical Habitat AGRICULTURE ACTIVITIES IMPACTS ON RIVER SYSTEM ECOLOGICAL CONSEQUENCES Crop cultivation and ploughing; Livestock trampling Impacts on sediment system Increased sediment transport and soil erosion; decreased bank stability. Increased sediment deposition in stream bed. Impacts on morphology Sedimentation widens river and makes it shallower. Impacts on geomorphically-influenced physical habitat Infilling of interstitial habitats. Land drainage Impacts on Flow regime Water levels rise and fall more quickly. Water abstraction for irrigation Impacts on flow regime Reduces flow in rivers. Impacts on morphology Reduced flow causes alteration in river depth, width, velocity patterns, shear stress. Impacts on geomorphically-influenced physical habitat Modify distribution and availability of instream habitat. Displacement of early life stages and disruption of spawning. Shallow water habitats are more easily warmed. Impacts on sediment system Sediments will not be flushed out in low flows. Dam operation for irrigation Impacts on sediment system Retention of sediments in the reservoir. Bank instability and bank collapse. Impacts on flow regime Impacts on downstream flow variability. Impacts on morphology Reservoir inundates the floodplain. Downstream tributaries will be narrowed. Impacts on geomorphically-influenced physical habitat Disruption the migration and distribution pattern of some fish species. Mitigation Options for the Adverse Impacts of Agriculture Activities on River Systems Traditional mitigation actions address the symptoms – such as bank collapse and erosion – by hard engineering measures which in turn alter channel dynamics all the more (Clark, n.d.). Because of these dire consequences, ecologists have proposed mitigating the potentially destabilizing impacts of agricultural activities by addressing the root cause (Gray, 2008). Several best management practices are now being employed which includes: river restoration through natural channel design; soil conservation methodologies; and increasing irrigation efficiency to limit water abstraction. Natural channel design River restoration implies that rivers are restored back to their pristine state (Poudevigne, 2002; Doll et al., 2003). This is, of course, unrealistic given that agricultural activities are dependent on readily available water supply. But if the extractive traditional practices are not alleviated or that channelization remains to be of the hard engineering kind (berms, levees, and floodwalls) exclusively, such agricultural processes will not be sustained in the long term (Tilman et al., 2002; Doll et al., 2003). River restoration then is a technique that still controls riverine process but it does so by employing a more natural channel configuration in order to effectively mitigate fluvial erosion hazards by impeding bank erosion or minimising lateral channel migration (Doll et al., 2003). One of the most popular techniques for river restoration is by natural channel design. This approach involves using reference channel morphology as templates for channel design (Rosgen, 2007). Reference channels are those reaches in the same river that are least altered. In mimicking reference channel morphology in restoration sites, slope differences, sediment differences, sediment transport capacity and competence, and flow capacity must be accounted for. Natural channel design aims to “reconnect the stream with the floodplain; to restore the proper pattern, profile and dimension; and to restore [aquatic] habitat” (Jones & Chandler, 2005: 1). In a project conducted by Jones and Chandler (2005: 1) which employed the natural channel design, they reported that the “restoration reach has successfully endured multiple bankfull events with no failures of structures” and was able to provide aquatic habitat and has restore the river’s sediment transport competence while reducing shear stress on the channel banks. Mimicking natural design may also be applied to dam operations to mitigate their deleterious impacts on hydrological regime without necessarily reducing the dam’s efficacy (Dougherty & Hall, 1995). Downstream demands must be identified in order to determine minimum compensatory flows thus minimizing the impact of an altered flow regime. Modifications may be necessary to dam take-off facilities in order to mimic natural flooding so that flow variability downstream will not be disrupted significantly. Dougherty and Hall (1995) particularly recommended to “[pass flood flows] early in the season to enable timely recession agriculture [which] may have the added advantage of passing flows carrying high sediment loads”. Soil conservation methods Sediment is one of the most vulnerable to disturbances in the riverine ecosystem because it is the most dominant river-borne component (Simenstad et al, 1992). Excessive sediment transport results not only to sedimentation of rivers but also to alterations of flow regime. Mitigating actions to address the problems of sedimentation specifies the need to conserve the soil in the agricultural area so that it is not easily eroded by surface run-offs (US EPA, 1999). These methods are summarised in Table 2. Table 2: Best management practices (BMP) for soil conservation. SOIL CONSERVATION METHOD DEFINITION IMPACTS ON SOIL AND/OR SEDIMENTATION Conservation tillage Planting crops with minimum disturbance of surface soil Minimise loose soil and prevents sediment transport and erosion even during periods of increased run-off. Agricultural riparian vegetation buffers Linear wooded areas or strips of grass along rivers with buffer width of at least 10m Filter sediments during run-off and prevents their massive deposition in water bodies. Wooded buffers also help to stabilize river banks. Afforestation Planting native species in agricultural lands, targeting highly erodible areas Promotes soil retention and prevents massive erosion. Fencing Impeding sediment transport and retaining the soil on site. This also refers to placing livestock inside fenced areas far away from rivers. Prevents massive deposition of sediments into river. Contour farming or terrace farming Consist of ridges and channels constructed across the slope. Contour farming is for moderate slope while terrace farming is for steep slopes. Minimises soil erosion and promotes water conservation by decreasing surface run-off. Crop rotation Gyration of types of crops planted instead of monoculture Helps to aerate soil without need for increased tillage. Increased irrigation efficiency and limiting water abstraction Water abstraction, as discussed, is detrimental to the river ecosystems and their morphology because it hugely alters the flow regime (Dougherty & Hall, 1995). To mitigate this, water must be conserved (Dougherty & Hall, 1995). In the UK, one way of addressing this concern is through the issuance of abstraction licenses to water users (Gray, 2008). Starting in 2004, new licenses have time limits and the Environmental Agency has the power to revoke licenses of owners who will be proven to have caused environmental damage (Environmental Agency in Gray 2008). Before 2004, however, licences were permanent and in their case, it is required that compensation must be paid to the licence holder (Gray, 2008). As most licences are permanent, the need for compensation has prevented the removal of damaging abstractions. What exacerbates the problem of abstraction is that, in most areas, the water withdrawn for irrigation is largely used inefficiently (Dougherty & Hall, 1995). Dougherty and Hall (1995) reports that as much as half of water diverted for agriculture does not even yield any food; therefore, even modest water conservation measures could free up large quantities of water and reduce the pressure on rivers that supply them. Some of the measures that could be employed include the following (Dougherty & Hall, 1995): Implement more efficient irrigation methods such as drip system instead of surface irrigation; Reuse waste and drain water; Find alternative ways to dispose drainage effluent; Prevent or reduce canal seepage through lining; and Build reservoirs which fill naturally during winter; this can provide an alternative water source for irrigation during summer (Clark, n.d.). In mitigating the potentially destabilizing impacts off agricultural activity, two things must be underscored: one, the removal of key stressors is crucial in the long term restoration of the river’s flow regime, morphology and the habitats it supports; and two, mitigating measures such as soil and water conservation actions or river restoration must not be considered in isolation but linked with water quality, river morphology, and as part of a broader landscape (Gray, 2008). Taking a landscape perspective in designing river management practices makes it all the more complex and this has presented fundamental challenges (Poudevigne, 2002). It is nevertheless necessary because the object of mitigating agricultural impacts is first and foremost the re-establishment of the natural interactions between physical environment and biota which the destructive impacts of agriculture has simplified (Jones & Chandler, 2005; Gray, 2008). It is also with this re-establishment to a more natural ecosystem design that will make agriculture more sustainable and which will improve both the river and agricultural land’s environmental resilience (Gray, 2008). This is essential not merely for ecosystem health but more for the benefits people derive from catchments (Gray, 2008). As well as that, it must be remembered that a river is a continuum and impacts in a particular reach will invariably affect downstream reaches (Vannote et al., 1980). Conclusion: The key points of this report are as follows: Agricultural activities are considered potentially destabilising phenomena because of their detrimental impacts to the flow regime, morphology, and habitat of river systems; Increased surface run-off and loose soil results to soil erosion which causes massive sedimentation in catchments; Water abstraction and dam operations may also result to increased sedimentation and altered flow regime; Sedimentation may cause alterations in river morphology and flow regime and may destroy aquatic habitats; Mitigation actions include natural channel design to restore the river, employment of soil conservation measures, and to make water use more efficient to lessen the need for excessive withdrawal; and Key aspects in employing mitigation measures include removal of stressors and considering such actions in landscape perspectives. REFERENCE LIST Allan, J.D., 2004. Landscapes and riverscapes: the influence of land use on stream ecosystems. 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The River Continuum Concept. Canadian Journal of Fisheries and Aquatic Sciences, 37, 130-137. Read More