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.


References

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Lane S., Reid S., Tayefi V., Yu D., Hardy R., 2008, Reconceptualising coarse sediment delivery problems in rivers as catchment-scale and diffuse. Geomorphology, 98.
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Larsen S., Vaughan I., Ormerod S, 2009, Scale dependent effects of fine sediment on temperate headwater invertebrates. Freshwater Biology, 54.
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Soil Compaction

Many meadows and pastures suffer soil compaction due to the regular passing of heavy machinery and high stocking rates. This can increase run-off during rainfall events and result in the delivery of fine sediments and nutrients to watercourses that can have significant impacts on river ecology. Soil compaction also has impacts on farm yields. If compaction impedes the root stock from penetrating into the soil layers then plant growth, and nutrient uptake, is reduced. This can result in poor crops of silage and hay which are required for feeding stock through the winter months; which is especially important in upland locations where growing seasons are short and winters long.

There are methods to improve conditions where soil compaction has occured. Sub-soil ploughs can penetrate below the compacted layer and break the soil allowing root penetration. This is not always possible in upland regions where soils can be thin with bedrock and boulders close to the surface. Steep slopes can also hinder the use of sub-soil ploughs. Aerators are often a better solution in such locations. These simple rotary blades penetrate into the soil and through the compacted layer allowing greater yields due to oxygen replensihment of the soil and root growth beyond the compacted layer.

If this is carried out in conjunction with newer methods of slurry spreading then yields can be massively improved. Two methods that appear to improve nutrient uptake are dribble bars and slurry injectors. These reduce the liklihood of run-off and allow improved use of a nutrient resource. If carried out alongside aerators and soil testing, for nutrient levels and pH, then savings in time and money can be passed onto the farm enterprise. Often pH levels can be lower then optimal and testing can identify where lime is required. This raises the pH and improves plant nutrient uptake. Soil testing can also idnetify which fields have high levels of phosphates and so allows the farmer to reduce inputs saving them money on fertiliser purchase.

These kind of options, if built into the farm plan, can be beneficial to the farmer and help improve the ecological condition of rivers. Cost of the machinery can be prohibitive but presently there are grants that can help with purchasing the kit. For example Yorkshire Forward's Farm Resource Efficiency Programme grants (FREP: www.yorkshire-forward.com/helping-businesses/rural-businesses/funding/frep) will pay up to 60% of costs. It will even pay up to 50% of costs for the pruchase of one piece of kit for contractors. These grants can make such options feasible for the farmer, either to purchase the kit directly or through their contractors.

Hay time



Soil profiles can help identify if compaction has occurred



Slurry injector





Dribble bar on umbilical

summer sun

The rivers are very low after the long spell of dry weather. Askrigg and Newbiggin Becks are completely dry in sections and upstream of Worton the main Ure is turning green with algae. These periods of low flow, coupled with algal blooms, create seasonal bottlenecks that limit populations of fish and flylife. Algae can soak up the dissolved oxygen during the night, as plants switch from photosynthesis to respiration. During the early hours the river can become depleted of oxygen to the extent that severe fish kills occur. When the algae dies back it smothers gravel habitat and depletes oxygen as it decomposes through microbial action.

Under natural conditions upland rivers would be oligotrophic, or low nutrient, relying on seasonal inputs of leaf litter and dying salmon after spawning. But in urbanised and agriculutral catchments these dynamics are changed through the inputs of human-derived nutrients. It doesn't take much phosphate to lead to eutrophication (the change from a lower to higher nutrient status). When this occurs the ecology of the river changes too. This can be through reduced populations of typical species or invasions of species pre-disposed to survive under the emerging conditions. In dales rivers populations of fish, including salmon and trout, are lower then expected suggesting issues of pollution. But barriers, such as weirs, also limit their populations showing that impacts on rivers are multiple.

At one time pockets of de-oxygenated water moved with the tide along the Humber estuary stopping upstream migration of spawning salmon and trout, and downstream migration of smolts (salmon and trout that undergo physiological changes enabling them to live in saltwater). With more stringent regulations on industry, and the decline of the UKs indutrial base, this has been reversed and migrating fish stand a chance of reaching their spawning grounds. The rivers trust is trying to improve the habitat of these upland rivers to ensure that spawning streams are able to support populations of salmon and trout fry. Providing shade to river banks through tree planting, preventing cattle and sheep accessing rivers and adding structure to stream habitats (for example through the inclusion of large woody debris) all offer habitat for salmon and trount young.

Winter in the Yorkshire Dales

Winter has hit the dales in a way it hasnt for decades. The riverscapes are surrounded in white that glistens in the low winter sun. Sheep huddle together behind walls waiting for deliveries of food that arrives by tractor which, despite their traction, also slip across the surface. The rivers are running low as all the precipitation is stored in snow and ice, the freezing nights keeping water locked up.

Cars have deep coverings of snow and roads look like gorges as the ploughs scrape them free of snow leaving steep edges on either side. The forecast suggests we have at least two more weeks of this and with drifts already reaching above head height the next days promise impresive sights.



The thaw is held up by the sub zero temperatures dipping to -8 in the dales and colder still the further north one travels. When this huge store of water eventually flows the rivers are set to rise substantially. The worst case scenario for those living in towns downstream of here is a rapid temperature rise coupled with rain. Such conditions will undoubtedly lead to flooding, and misery, for many.

Research into land management suggests that compacted soils and extensive drainage exacerbates flooding by shifting water rapidly from land to river resulting in sharp spikes in the hydrograph. The key to the next few years, as we move towards the prescriptions of the EU Water Framework Directive, is to understand how land management effects water and more importantly identify methods for improving conditions whilst making sure upland farmers do not lose income. This is an exciting time for freshwater ecology as local scale perspectives are stretched to the catchment scale which provides many of the controlling factors on river ecology and quality.

The incredible thing is that the whole country is white, smothered in deep drifts and layers of weeks of snow. Satelite photos from NASA display this strange image of the UK. It looks like not only Yorkshire rivers are at risk. In the meantime the landscape looks fantastic and we all hope the thaw occurs in a slow, steady manner.

The rivers are quiet.

I keep getting reports of fish movement upstream but I haven’t had the luck to spot anything yet. Since the records came in the rivers have dropped again. Even though there’s been a day of rain it has only just made the dizzy heights of light drizzle and the river is stubbornly refusing to respond. I went down to Aysgarth Falls late afternoon on Sunday but the water was so low nothing would have made the attempt. A heron and a few mallard ducks sat above the upper falls and that was about all. The electro-fishing kit is in store for the year and my sampling nets are only going to get another day or so. Things in the rivers seem quiet and there is just the spawning to come and then it will be a matter of waiting for spring before I can start serious river surveys again.

Plenty to do in the meantime so boredom won’t be a factor. Down to Cornwall next week to learn about a farm survey method called PINPOINT that the river trusts and Natural England have organised. This aims to help identify where the sources of diffuse pollution on a farm are. It should help farmers meet the requirements of the Water Framework Directive, may even save some money. Then there are lots of farm surveys to carry out before I take three weeks leave. The plan is to lock myself away on Islay and get on with writing up the project I’ve been working on for the past three years. I’m hoping it will be miserable forcing me to sit in and get on with it...it would be just my luck for unseasonal sun!

The intricacies of the dales

The little intricate waterfalls of the dales are the hidden gems of this land. Everyone knows the big famous ones like Aysgarth and Hardraw (both of which have had visits by Kevin Costner) but tucked away on some of the small high altitude streams are jewels of waterfalls that are rarely visited. Some of these are virtually dry but surge into life once the clouds burst. These ephemeral places contain ecology trained by evolution to cope with constant change.

Walks over the hills brings a natural closeness with the land. You can become lost in the scales here. One moment watching pipits swarm across coarse grass swards to pick at tiny beetles the next taking in wide vistas or your vision being tunnelled down a far winding dale. Hills like Ingleborough and Pen-y-ghent always seem to break into the skyline. Their obvious stepped tops revealing millions of years of strata, borne from scales we can barely grasp.

Heading over the top from Ballowfields the sound of water rushing beneath scree piles highlights that you are walking over limestone. The rock is taken into solution by acidic rain and streams drop underground forming caves and caverns from the tiniest of sink holes that swallow water with a giants thirst.

The water here doesnt behave as expected. Below the summit of Addleborough, a brooding hill that decieves its low altitude, is a waterfall dry for most of the year. Water is swallowed from the stream above and as you walk over the land you can hear it bubbling and gurgling beneath the surface like a hungry stomach. And at the lower reaches of the redundant waterfall it re-emerges from two places before combining back into one of the best trout spawning streams of Wensleydale.

And if you walk the other way, towards Addleborough top, calcareous flushes clear and potable emerge from the ground. Just metres from their source they mix with dark peaty streams and then for a short distance two different waters flow side by side till they become mixed, their pH and chemistry settling somewhere between the two.

In many ways water has shaped this land. Over 300 million years ago coral reefs grew in tropical waters and their remnants now form the majority of rock in the dales. Then huge columns of frozen water scraped the land forming u-shaped valleys that are the prominent feature of the landscape. After this liquid water ran over the surface of these scraped clean valleys cutting narrow V's in the land and dissolving the rock to form the caves and caverns that form underground mosaics like fungal hyphae. This isn't a perfect timeline of the processes but the idea of how these places were formed is there.


In the last few thousand years people cultivated the land and our signals can be seen all over the floodplains and down to the Humber estuary. A soil core of these plains shows that the rate of sediment movement increased massively after we cleared the original vegetation. It shows that we can have bigger impacts on the land then we suppose. But it is this patchwork of fields delineated by dry stone walls created by farming that is one of the biggest draws to the dales, and rightly so.

Modelling at the catchment scale

Understanding how water moves across the land and what chemicals and sediments it delivers to rivers is vital to the understanding of river ecology. The rivers trust movement has played a key role in recent years by helping to decipher how catchment processes control river morphology and how morphology and water quality affect ecology.

Excessive fine sediment in upland rivers degrades river habitats by clogging up the spaces between the gravel. This reduces egg survival of Salmon and Trout and changes the macroinvertebrate communities by favouring organisms such as Chironimidae worms over Stonefly and Mayfly.

A number of rivers trusts including the Yorkshire Dales Rivers Trust have been using a modelling tool developed by Durham and Lancaster universities called SCIMAP (Sensitive Catchment Integrated Modelling and Analysis Platform). This provides detail on where fine sediment is likely to be delivered to a watercourse based on slope, landcover and its associated erosion risk and rainfall.

SCIMAP outputs show the average risk for in-stream fine sediment in any catchment being modelled with a number of risk classes either side of the average ranging from high risk (red) to low risk (green). Underneath this output is a second output in grey that highlights the land parcels of a catchment most likely to be the source of the sediment. From this it is easy to locate the red 'streams' and then look up stream for the most likely locations that are delivering the sediment. Some ground truthing is then necessary but once the risk has been established it is possible to enter negotiations for changing land management to methods that can reduce these inputs. Simple measures such as buffer strips or contour planting of trees slows surface run off allowing sediment to settle out before the water reaches a watercourse.

This has the benefit of reducing the degradation of rivers and streams and allows species composition to restore itself back to the natural community of the river type. It is these simple habitat measures, developed through catchment scale thinking, that will restore habitats to something like their potential. This kind of large scale thinking is constantly developing which makes working as part of a rivers trust an exciting and rewarding vocation.

October rain

After a September with little rainfall October 6th finally brought a decent spell of wet. The rivers responded rapidly swelling up and turing peaty auburn for the first time in weeks. By the time it cleared the Wharfe and Ure where racing and gathering pace. After a dry night they began to drop back and it seemed a good time to look for fish jumping. I headed down to Redmire falls armed with camera and a little hope. A Dipper was sat on a rock beneath Apedale beck and a Heron stood in the shallow edges at the far bank. It reluctantly took to the air, slow wingbeats just managing to get it out the water and away. I headed over the limestone 'steps' and down to the first set of falls. Getting my feet wet I found a good place to sit and I waited there, legs dangling against a mossy boulder, for a couple of hours till dusk set in but nothing stirred, not even a noticeable rise in the pool beneath the falls.

On the way home I stopped at Ballowfields to see if any big trout had moved into the stream there. Too high and fast to see anything. It cant be long now before the fish head up this way so I plan to maintain the vigil in the spare time I get. In the meantime plenty of mapping and writing to be getting on with.