Bank and Land Erosion on Rivers – Natural or Aggravated?

Introduction

Agricultural practices have increased stream sediment load worldwide (Zimmerman et al., 2003; Naismith et al., 1996). Whilst fine sediment inputs to water courses can be a result of natural processes when the rates are enhanced they act as a pollutant (Waters 1995). In the UK much of the spatial and temporal variation in diffuse pollution is due to land management and there has long been concern that modern agricultural practice increases erosion rates and surface runoff (O’Connell et al., 2007). Soil cores have shown that sedimentation rapidly increases after woodland clearances. These increased rates of sediment delivery are especially noticeable when woodland has been cleared for conversion to agricultural usage suggesting that all UK river systems are prone to aggravated sedimentation. Following a rivers course through woodland into open farmland highlights the difference in erosion rates between land use types clearly showing the difference between natural and agriculturally enhanced sedimentation.

Process cascades and scale

Catchment scale studies of hydrology and processes reveal that hydrological response has been altered due to land use change; this has enhanced runoff response to rainfall events creating flashier systems (Bunn et al., 2010). As a consequence there is now a greater risk of fine sediment delivery. These changes in hydrological processes and sediment transfer rates are good examples of why shifts at the river scale should be viewed in the context of the wider catchment (Kondolf, 1995). Many hydrologists and ecologists now support large scale approaches and identify the catchment as the core unit for river management (Chorley, 1969; Newsom 1992; Burt and Pinay 2005). This shift in thinking has been driven by recognition that most degradation occurs as a cascade across large areas of the catchment which are often driven by catchment land use (Bond and Lake, 2003). Many temperate river catchments are now dominated by land use methods that enhance sediment transfers from land to streams due to vegetation conversion to pasture or the removal of riparian trees (Larsen and Ormerod, 2009). In such agriculturally-dominated catchments land management practices have been shown to alter soil surface roughness and subsequently the magnitude of erosion rates (Gilley et al., 2002). Associated impacts that are transferred to streams, due to enhanced surface flow and erosion rates, include the delivery of nutrients, pesticides, pathogens and heavy metals.

It is important to understand such effects in terms of a functioning (or malfunctioning) catchment that is subject to large-scale human influence. This is essential when aiming to identify the important impacts and contextualise these in terms of the landscape with all its processes and multiple impacts. Studies have helped uncover impacts from catchment scale management at the in-stream habitat scale and so highlight the process cascade from source to recipient stream. This shows how upstream management can place significant controls on river ecosystems through, for example, the delivery of sediments or changes in hydrological regimes. Such impacts transcend scale and move through catchments via pathways controlled by hydrological connectivity with negative impacts being noticeable at small-scale riffle habitats. Thus, understanding the cascades is essential for river managers.

Observing a single stream within a catchment may miss the pertinent information that a catchment approach captures by providing information on the relative condition of a river and its tributaries. Setting the incorrect spatial scale in which to explore systems can result in dubious findings. For example, Larsen et al. (2009) found that sedimentation of gravel beds was directly linked to eroding banks within 500m upstream. When they increased the scale of inquiry they discovered that bank erosion was negatively correlated with riparian and catchment woodland extent. Small-scale processes such as bank erosion place limiting factors on brown trout and it is now becoming increasingly accepted that such processes must be viewed in the context of upstream land use such as extent of riparian cover, stocking rates and woodland (Jutila et al., 2001; Lane, 2008; Larsen et al., 2009).

Land use impacts

Trimble and Mendel (1995) comment that cows can be important drivers of geomorphological change through trampling and poaching which expose soils and erode river banks. Theurer et al (1998) reinforce this when they argue that livestock farming results in bank erosion through poaching and subsequent deterioration of the grass sward, and thus root depth. Within upland rivers Theurer et al (1998, p.6) identified problems associated with enhanced delivery of fine sediments including, ‘accelerated stream bank degradation from livestock, major gullying of steep hillsides resulting from overgrazing by livestock and the introduction of grips.’

Soils are increasingly prone to erosion by livestock poaching and heavy machinery compaction which reduces infiltration (Marshall et al., 2009). This results in high rates of surface flow and increased Critical Source Areas and fine sediment delivery to streams. Sediment loss from agriculture is a cause for concern due to both on-farm practical and economic implications (Boardman et al., 2003) as well as the impacts sedimentation has on stream habitats and ecology (Owens et al., 2005; Theurer et al., 1998).

Ecological impacts

Fine sediment delivery is a key concern in drainage basins affected by anthropogenic disturbance (Wood and Armitage, 1997). In recognition of this it has been argued that the more pernicious controls on population are not driven by competition but habitat quality, especially habitat patches that have been degraded by anthropogenic impacts (Klemetsen et al., 2003; Ormerod, 2003; Gosset et al., 2006). For example fine sediment accumulation in gravel spawning beds (Ojanguren and Brana, 2003) negatively impact brown trout survival rates.

The early life stages of brown trout have quite specific requirements. Egg development requires gravel and pebble substrate (16 to 64mm) with a minimum dissolved oxygen concentration of approximately 5mg/l, though this can be as high as 7mg/l depending on the developmental stage of the egg (Louhi et al., 2008). Any sustained dip below these requirements reduces survivorship. Such requirements carry over to the fry life stage. However, habitat heterogeneity can enhance survival of fry by providing refugia and increasing habitat availability for prey species including macroinvertebrates. High macroinvertebrate abundance and richness is positively correlated with medium to large substrates which provide stability, interstitial space for refuge, oxygen exchange, attachment sites for filter feeders and diverse microbial, algal and detritus food supply (Allan, 1995; Wood and Armitage, 1997).

Through deposition within the interstitial space fine sediment reduces intergravel flow and oxygen replenishment. Particle size <1mm can result in a film on the redd surface inhibiting fry emergence (Kondolf, 2000) whilst very fine sediment <0.125mm can block the micropore canals in the egg membrane thus reducing waste transfer (Lapointe et al., 2004; Grieg et al., 2005; Julien and Begereron, 2006). Larsen and Ormerod (2009) showed that fine sediment addition to riffle habitats increased macroinvertebrate drift density by 45% and propensity by 200%. Whilst benthic macroinvertebrate composition remained the same population density declined in treated reaches by 30 to 60% and the effects remained consistent between seasons and streams. In short the infiltration of fines reduces the porosity of gravel matrix surfaces which can then reduce salmonid egg survivorship, habitat availability, refugia and also increase macroinvertebrate drift response (Grieg et al., 2007). If management of river systems is to become more sustainable then it is the root causes of degradation that must be addressed.

Restoration

How different types of land cover modify soil structure, surface flow and propensity for erosion must be understood in order for restorative measures to be taken. Marshall et al. (2009) found that shelter belts of trees as young as ten years old significantly reduce overland flow through 1) the presence of trees and 2) the absence of sheep. Mature forests are known to reduce peak flows due to a number of processes including evaporation of canopy interception, transpiration and an increase in soil water storage capacity beneath trees (Robinson and Dupeyrat, 2005). In comparison, pasture land reduces interception and, due to both livestock trampling and heavy farm machinery, soil compaction occurs lowering soil water capacity. This inevitably increases runoff rates in comparison to woodland given the same topographical conditions (Marshall et al., 2009). Zimmerman et al., (2003) found that lethal concentrations of fine sediment on fish could be reduced by up to 98% due to alterations in land use including the installation of riparian buffer strips, conservation tillage and the encouragement of a permanent vegetation cover. These findings support the work carried out at the catchment scale at Pont Bren (Jackson et al., 2008).

In order to prevent the delivery of pollutants such as fine sediment, substantial changes in agriculture are being discussed (Krause et al., 2008). These changes involve breaking the connections between CSAs and the river or changing the land use method that creates the initial problem. Such measures that can be carried out at catchment or field scale include gill planting, grip blocking, buffer strip creation along riparian zones which delimit terrestrial and aquatic systems (McGlynn and Seibert, 2003), moving gateways from the downslope section of fields to areas where water is less likely to accumulate or completely changing the farming method in some fields or farms. All of these methods would be appropriate in upland UK catchments but farmers require advice and grant input in order to manage such change.


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