So, first of all let us explain what a “shaggy margin” is! Basically, the term describes the naturally irregular edge of a river, created by features such as fallen trees, branches, woody debris, and deposited sediment. These features increase habitat diversity by creating sheltered areas of slow-flowing water for fish and aquatic invertebrates, trapping sediment that supports riverside vegetation, and varying river flows to expose clean gravels suitable for fish spawning. 

Shaggy margins also help slow water during high flows, encouraging rivers to reconnect with their floodplains and contributing to Natural Flood Management (NFM) by reducing flood risk downstream.

Here’s Sarah and Peter to tell you more:

Sarah Perry, Rivers Restoration Manager, Herts and Middlesex Wildlife Trust: 

Back in March, I worked with the Park Manager at Stanborough Park to train up the Grounds Team in delivering more and managing large woody habitat in the River Lea.

Adding to our Species Survival in Hertfordshire Programme, which restored 11.5km of chalk stream in 2024/25, including the River Lea, we set to work last week adding even more interest and variety to the river at Stanborough Park.

Working with Better Leisure, who manage the Park, next to our Nature Reserve at Stanborough Reedmarsh, I trained up the Grounds Team over three days, helping ensure the river restoration project's long-lasting success.

Large woody habitat (felled whole trees, large hinged limbs and brushwood berms) were introduced to enhance natural processes along the re-aligned river. These natural features, essential for chalk stream health, act to energise flows, aid sediment transport, create living margins along the river and scour its bed. 

These natural processes, often absent from our chalk streams due to mis-management and historic impacts like re-alignment, are essential for creating healthy conditions to support native wildlife.

Wild Brown Trout, Barbel and other salmonid fish like Grayling, will use the cleaned and energised riverbed to spawn. Dragonfly larvae will take refuge in the slowly decaying woody material. Mayfly will reap the rewards of silt free areas of riverbed. And iconic plants like Water-crowfoot will start to regrow in the dappled shade created by felling trees into the river.

Our River Champion Volunteer, Peter White, lent his support at Stanborough and has taken his enhanced ideas about "shaggy margins" back to another local river, the Beane.

Peter White, River Champion:

In their natural state, rivers tend to have trees falling into them in stormy weather or as they age, and branches and debris that wash down stream too.  These create river edges that are not smooth and neat, but instead have plenty of ‘interesting' features. This ’shaggy material’ interacts with the flow in ways that help create areas of still water with plenty of hidey holes for young fish and invertebrate larvae, and also trap silt which leads to areas where riverside vegetation can establish. In other places, the fallen trees vary the way the water flows, creating faster sections where the scouring action clears away silt to leave clean gravels - ideal for fish spawning - and create meanders in the channel. In times of high water, these features also help to slow the flow and encourage the river to naturally spread out into its floodplain, reducing the flood risks further downstream (a process now referred to as Natural Flood Management, or NFM).

For decades we've been ‘obsessed’ with cleaning rivers up: pulling out the fallen trees, tidying up the edges and generally making it all look ‘pretty’. But nature doesn't like pretty - nature loves messy!

Now, with a renewed understanding of just how important trees are to rivers, more ‘large woody debris’ (or LWD) is being added back into river systems as part of restoration work. Even now though, in many cases this is done with ’neatly trimmed logs’ and all carefully planned and ‘installed’.  Although better than nothing, these ’tidy’ interventions are not really as beneficial as they could be and sometimes are actually detrimental.  

By felling or hinging whole trees and leaving all the branches on them, the benefits are so much greater.  It's what a good chalk stream should look like. (Hinging is only cutting part way though the trunk and bending the still-living tree over, a bit like hedge laying.) Building ‘brash build outs’ is another feature of many recent restorations. But again, these are often too neat and simply too small to be effective. 

Recent schemes along the Beane have introduced some woody material in this way. But it’s not really ‘big enough’ and certainly not ’shaggy enough’ to have a real impact. What I hope to be able to do is show landowners, contractors and volunteers the work that's been done at Stanborough to demonstrate that they need to ’think big’, be bold, and stay away from ’neat’: opening their minds to the need to ‘go large’. And shaggy :)

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