So how do river managers square this circle of competing and contradictory demands? How can we ensure a project using wood has desired benefits but isn’t ‘messy’ or unpredictable? And how can a project achieve long term goals without riparian woodland to supply new wood? Part of the answer is that in-stream wood management is not a simple ‘one-size fits all’ solution, there are a myriad of ways of using wood to achieve specific goals which will have different effects (see sketch opposite). If we are talking semantics, many (most?) of these projects are not really ‘restoration’, but provided limitations are known in advance they can be viable management options.A key concern with natural in-stream wood is its mobility, something I covered in a recent post and journal article. Managers want wood to stay in the same place both to harness its effects in a desired location, but also to stop it moving off and causing problems elsewhere. The example to the right is of a logjam in Pickering, part of the “slowing the flow at Pickering” project. This structure is likely to prove effective and relatively long-lasting, but it is not really representative of a natural structure. Fundamentally this is engineering with natural materials, harnessing what we know about in-stream wood to successfully deliver a very specific effect in a particular location. However, it is only going to deliver this effect for the length of time the wood takes to rot, so would need ongoing monitoring and management.
Anchoring a structure in place is one way of overcoming the unpredictable nature of wood. Such an approach requires a fairly detailed geomorphological and hydrological assessment, as the local hydraulics will change to accommodate the structure. By being fixed in place the effects on erosion may be intensified, for example if a fixed logjam persists in a location for a very long time it may generate a substantial scour pool. Likewise a staked or cabled piece of bank reinforcement may focus flow towards the centre of the channel and cause bed scour and channel deepening.Such unwanted effects can be particularly problematic where wood is being used as an erosion control measure (see post on erosion control). The issue here being that the wood is doing nothing to address the imbalance in stream power which is driving the erosion. In a crude sense the river has ‘too much’ energy and is expending it on eroding its bed/banks. Reinforcing in one place (whether with wood or concrete) even if successful, often pushes the problem up or downstream.
A popular method of creating areas of slower flow and/or arresting erosion is to use brush or willow spilling. This can be a very effective technique in the right place, particularly where the willow can subsequently become established as riparian trees, sprouting out of the cut wood.So do these distinctions between engineering and “restoration” really matter? I would argue they do insofar as it is important for people managing (and often volunteering) on projects to understand the goals, the mechanisms, the outcomes and the likely time-scales the project is working over. Where a project is a true “restoration” to a new quasi-stable state (in reality extremely rare), once the project is completed (and if successful) the river should not require any further substantial management. However, where a project is fundamentally engineering based it follows that the project will either need continuous monitoring and upkeep, or the project is intended as a one-off site specific intervention designed to last for a given time before the structure fails. Either way there needs to be an acceptance the project is a long-term commitment to continuing low-level management of the channel using natural resources (potentially indefinitely). This then raises the issue that goals and likely success of a given piece of work need to be understood and communicated, in order that the stakeholders can absorb any potential individual project failures without losing commitment to the overall management scheme. It is in light of this that I think “restoration” can be an unhelpful and misleading term when applied to all management using wood. Potentially raising false expectations in stakeholders and volunteers of the outcomes and workload/commitment in improving a river using in-stream wood management.
There is no doubt that using wood as a method of enhancing the river environment is a useful and effective management tool. Debates around what terms we use for various measures can seem a bit academic. However, when so many projects to improve stream health are dependent on the goodwill and efforts of stakeholders and volunteers it is vital project goals are understood and clearly communicated and part of this is using clear language.
I’m always excited to hear about new projects using wood for river management so please let me know in the comments or on twitter if you are involved in one.