Notes on Large Woody Debris

_DSF5368Learn from nature: LWD is rarely better than when an old tree falls in.

Large Woody Debris (or in plain English ‘Trees in Rivers’) – if done well – is a very effective river restoration / rehabilitation technique.

Until very recently we tended to tidy rivers up. In fact we still do. I often see pictures posted on Instagram and Twitter of tidying up exercises on chalk-streams masquerading as river restoration: “clearing out” a stream, or uniform and tidy constructions – faggot hurdles, bank edges – that would be more at home in a garden or golf course. 

Traditionally river-keepers tended to pull out trees and branches that had fallen into their streams: the aesthetic was one of a ‘tidy’ river, the practical intention was to make the river easier to fish, with fewer obstructions to casting, and no tangly obstructions to lose fish in. It was about creating an environment that suited anglers more than it suited the fish they were seeking.

State bodies like the Environment Agency have colluded with this mindset. When a tree falls in they remove it, motivated by the need to avoid ‘flood-risk’.

But slowly rivers-keepers and the Agency are changing how they look at this issue. That’s because in both fishing and habitat terms and in terms of flood risk, removing trees and large branches from rivers is self-defeating, the very opposite of what we should be doing.

IMG_1698Trout love cover: remove the cover, remove the trout.

From the fish and habitat point of view it’s simple: fish love cover. A tree in a river provides fabulous refuge from predation. In New Zealand’s spring creeks which are not tidied up at all, the largest trout will always lie within bolting distance of a submerged tree, or some form of log-jam or snaggy overhead cover. This makes catching them tricky, but on the other hand they wouldn’t be there if the LWD wasn’t there too.

Research carried out by Dr Murray Thompson has also shown that LWD is also fabulous habitat for invertebrates, that invert numbers are far higher in rivers where there is plenty of submerged LWD.

LWD is also great from a morphological point of view, especially in chalk-streams.

Chalk-streams are very low energy rivers. The forces which shaped them (melting glaciers) have long since retreated from our landscape. We have since that time heavily modified their channels by straightening, dredging and impounding them. Chalk-streams very rarely achieve the flows needed to overcome these modifications and re-shape themselves to a more natural planform. Thus their shape and their habitat is locked in a man-made straight jacket and the rivers are effectively imprisoned.

But a fallen tree can make a huge difference, releasing gravel and stones into a system that lacks material (quick morphology lesson: when a river is modified, by dredging, say, the natural processes of the river work to erase that modification, depositing material into the enlarged space until the river reaches the correct dimensions again. To repair itself, therefore, a river needs material, but chalk-streams are too gentle to supply it, except in the form of fine particles like silt. When a tree falls in, however, this creates the material and energy that enables self repair). A single tree can kick-start a whole sequence of channel repair and re-meandering.

I own a fishery at Frampton in Dorset where the river once flowed through a stately park, impounded by ornamental weirs. All the bends had been taken out. When I bought the fishery the then owner removed fallen trees all the time. He even removed one “for me” after he had sold me the river. I had to explain that I really didn’t want him to.

DSCF5328One large filled tree is visible in the distance, the other to the left of the shot. Ten years ago this reach was dead straight. The entire green bank on the left is new and the bend to the right is new. The same has happened in the opposite direction downstream of the lower tree.

DSCF5610Looking upstream from the lower tree-fall. The vegetated island in the middle of the shot and the meander to the left were created by a single fallen tree upstream.

_DSF43882013 when the lower tree fell in.

DSCF53272014, one year later: we’re starting to see a braided channel, a deep pool, a meander.

DSCF6881The third big fallen tree lies upstream of the island and gravel riffles which it created.

Since then three really big willows have fallen in. I have left them where they fell and watched what happened. Those willows have catalysed a whole sequence of channel repair, far better than any river restoration guru could ever have designed. The Frome is quite a powerful chalk-stream, so these changes have been on fast forward compared to gentler rivers, but even so they are extraordinary: hundreds of yards of re-meandered stream, of pool and riffle sequence, all catalysed by three fallen trees.

Those willows and many other instances where I have observed the aftermath of a natural tree fall, suggest to me that even now with LWD “restoration” we choreograph our work far too carefully. I’ve done it myself. In fact on the 7km LWD project I did with Simon Cain on the river Nar the work got more cost-effective, more natural and far, far more efficient as we progressed. To me good LWD imitates the impact of a fallen tree: it begins and ends with that.

_DSF4010Early phase of LWD work on the Nar: this was pretty good. It worked well but was quite intensive.

DSCF2512This was better! Less intensive, more natural, more cost-effective.

Another important learning curve has been where and when to use LWD. Basically there is a depth and gradient beyond which LWD doesn’t really work very well, at least from a morphological point of view.

LWD makes it best impact in relatively shallow, wide reaches, where the bed is more or less intact and where the river is not impounded. It is perfect for repairing an overwide river, but is not nearly so good at repairing a dredged river or an impounded river. In the latter two cases you need to resolve the river-bed and gradient issues first.

Do’s and Dont’s of LWD in chalk streams

• Don’t overly choreograph your LWD. Resist the desire to “build” something tidy or uniform.

• Look at and learn from natural tree fall and try to copy it.

• Use big timber, whole trees or very big branches.

• Work with the natural meander pattern of the river. Work out what the natural meander wavelength should be (see my other notes on this) and use LWD in sync with that, on the insides of bends or what will become bends.

• Think about daylight: the LWD will allow sediment to accrete in the slackened flow downstream. This sediment will only consolidate with vegetation and vegetation needs daylight.

• But don’t overdo the above. Rivers need shade too. Try to create a mosaic of light and shade.

• Be judicious about which trees you cut down. Alder is best. Willow can be great but is difficult to control later. Ideally you will have multi-stand alders and you can use some and leave others.

• Don’t expect LWD to do much in an impounded or overly deep river. Really you should remove the impoundment (weir or mill hatches) first. When you do you will have a fast flowing but overly wide and homogenous channel. That’s when LWD can work it’s magic.

Murray Creek. West Coast.Natural LWD in a New Zealand spring creek: spot the trout within a fin-flick of tangled cover. Note in the pictures below how the really big LWD has shaped the channel. It’s like this the whole way up the river!



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