Letchworth Garden City Angling Association

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River Ivel Riffle Project

For those of you visiting Poppy Hill over the Christmas break, you would have noticed 3 very large piles of stone dominating the car park. In a joint project with Letchworth Garden City Angling Association and the Environment Agency this stone was used to create a large Riffle in the River Ivel, just behind Lake One.

So whats a Riffle I hear your ask? Well if you do a Google search it suggests a gun thinking you misspelled, and no its not a type of card shuffle. It's actually an area of shallow water over and uneven river bed where the flow is increased and ripples or rolls over the stones. Depending on the riverbed makeup these do occur naturally, if it is a stony, gravely bottom the water scours and washes stone downstream. If the riverbed makeup is not suitable then these can be created artificially. Any artificial structure should always aim to work with the natural flow of the river rather than against it.

It was decided by the Environment Agency to put the Riffle in for a number of reasons. The section behind Lake One was fairly slow paced and a lot of silt and weed growth had built up together with low oxygen levels it's a pretty poor habitat for river life. The creation of the Riffle will allow an increase in water flow, adding more oxygen as the water is turned over the stones and create a better habitat, giving fish a perfect environment for breading and encouraging other aquatic life.

Using a 360-degree tracked excavator the bank was first dug out and lowered at the site to give the machine access and the reach needed to get to the far bank. A large dumper was then used to ferry the materials from the car park. Over looked by the Environment Agency, some 125 tonnes of stone was carefully positioned in the river to create a bar across the river bed about 12"-18" below the surface. Best described as half a Pear in shape if you was to look at it in profile and slightly dished in cross section to protect against bank erosion by directing the main force of the flow towards the middle. Three different grades of stone were used, Firstly very large cobbles were used to form the foundation and basic structure to give weight and prevent the whole structure from being washed downstream. Next large gravel aprox 30-40mm was used over the top of the cobbles. Finally pea shingle was placed over the top to provide a fine bed of gravel, perfect for fish spawning and other aquatic life. A bit of tweaking and sculpting was required after letting the stones settle and observing the change in river flow, the rain we recently had did not make the task easy as the river levels were up and down. Once work was completed the bank and pathways were reinstated as the excavator tracked back out to the car park.

I guess it will be a some time before we see what effect it will have on the fish population in our stretch of the River Ivel, the next couple of Temporal Surveys carried out will be interesting reading. But certainly looking at the changes in the river now it appears to be a vast improvement with more movement and flow and a slight increase in depth above the Riffle.