Willow Spiling To Protect Farmland

Bank erosion of streams pollutes watercourses with fine sediments and phosphates that can disturb the local ecology. It also removes sections of the farm to be deposited downstream, either within the river or on someone else’s land when the river overtops it’s banks. There are numerous methods for controlling this process and so reduce the rate at which soils are lost to rivers. The obvious method is fencing the watercourse to remove stock from the bank side. This allows vegetation to flourish creating a strong root stock that binds soils together. But sometimes just fencing the river bank is inadequate as erosion has taken hold and every flood pulse takes another section of bank and then the watercourse begins to eat into fields.

More drastic hard engineering methods can solve this. Gabion baskets for example create a strong buffer between fields and water. Such methods can be successful but they operate against natural processes and so always look at odds with the landscape. Moreover they too can wash out adding an extra ugliness as they hang into the river spilling their gravel back to the water.

There are intermediate methods that are successful and work with the natural features of a river. Matt Neale, the local ranger for upper Wensleydale, has championed willow spiling to protect banks, and fields, from erosion. This method seems to do the trick on most occasions and adds habitat to the river providing refuge and shade for fish, birds and mammals such as otters. Having visited sites where the Eden Rivers Trust had successfully employed the method Matt identified sections of the Ure that would benefit from this method. The first place he trialed willow spiling was on the river Ure just below Hawes. Here the river was eating into the bank and soil was being lost rapidly from the farm with every high flow event.

The method is simple but effective. Stakes are driven into the river bed close to the bank, spaced at two or three metre intervals. Live willow rods are then woven between the stakes to provide a permeable barrier between the river and the bank. This helps to reduce stream power and thus erosion and it also slows the water enough to allow sediments to drop out behind the willow spiling. The bank then builds up as new sediment is deposited whenever the river overtops the willows. The added benefit with using willow rods is that they take root and flourish in these locations. The roots further bind the soil whilst the new tree growth slows the water down even further and more matter is deposited until the bank becomes completely revegetated. The habitat created by this is valuable and helps to protect farmland creating one of those rare win-win situations.

Since the first trial Matt has carried out the method on several other sections of the river Ure and Duerley Beck. He has refined the process and now builds up coarse gravel behind the spiling to encourage fine sediments to deposit out offering further protection to the willow rods whilst they take root. When possible soil is packed into the coarse sediments providing a substrate for the growth of bank side vegetation, again this further binds the bank providing a more stable environment which offers improved chances that the work will be successful. When it is appropriate the bank is reprofiled to create a less vulnerable slope.

All of the sites that Matt has worked on have been succesful with one exception. This is at a location where a large glacial deposit, possibly a recessional moraine, is eroding badly. The land slip is substantial and the processes causing the slip are not simply undercutting of the bank by the river since the slope has become unstable. These glacial desposits are unconsolidated, porous and permeable meaning that water seeps through them providing a good medium for failure points to emerge. Where the processes causing erosion have been caused by river water the method has been extremely succesful and the habitat created appears natural and undisturbed.

Change and restoration

Upland rivers are fascinating places that possess a raw quality. Full of rapid change, violent spates to low flows in quick time, they pose challenges to people and nature. It is this rapidity of change that keeps them alive, keeps people drawn to them and puts their ecosystems on a knife edge of existence. One severe flood, drought, landslip or pollution event can alter their ecology for years. And when a natural system flips between threshold states dragging it back to its original condition is complex and challenging, beset with frustrating feedbacks and full of unexpected consequences. The new science of restoration ecology has discovered this time and again. What has appeared to be an obvious intervention can force a natural system to skew off towards some other state not accounted for during intervention planning. This has resulted in more measured approaches to reverse interference and has revealed that ecology does not exist in easily flipped parallel states but occurs in complex dimensions of interacting multivariate factors that we can never fully understand.

But not fully understanding something does not mean that we cannot understand it enough. Beneath the aesthetics of these places lie scale upon scale of interacting processes within and between both the living and non-living components of a system that create forever fluctuating conditions. However, even though they fluctuate, under prevailing conditions they are always pulled back towards some unknown average. In order to understand enough we need to simplify these interactions down to basic assumptions that explain them in as much detail that is required in order to predict the outcome of an intervention. This is not a magical process and can only come from experimentation both in laboratories and natural systems. And we are beginning to understand enough though it would be arrogant to suggest that we will never make mistakes during restoration efforts, but with restoration ecology it is the mistakes that guide learning.

The biggest experiment (and probably mistake) we are undertaking is one we have not planned but has come as a consequence of modern economies and lifestyles. An article published in 2007 in Global Change Biology by Durance and Ormerord showed that upland stream macro-invertebrate assemblages may decline by 21% for every degree centigrade rise in water temperature. This is of concern to freshwater ecologists as after primary production these are the most important components of upland river systems. What we have learned in recent years is that such a change would be difficult to reverse and as future temperatures are forecast to increase for sometime it may be that these upland river systems are destined to become shadows of their present selves.

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.