Nature Notes: Jono Forgham on the environmental reasons behind planned removal of the weir on the River Stort at Grange Paddocks
With plans to remove the weir on the River Stort at Grange Paddocks raising concerns among the town's residents, Jono Forgham explains the environmental reasons behind the move...
News of the planned removal of the weir on the River Stort next to the new Grange Paddocks Leisure Centre under construction has led to many local residents questioning the thinking of this proposal from Hertfordshire County Council and the Environment Agency (EA). I saw this as an opportunity to carry out an in-depth study of the river around the weir and at another site half a kilometre upstream.
The River Stort is a precious chalk stream, one of just 200 or so in the world, with many being found in the south and south-east of England.
The river is monitored by several agencies on a regular basis and presently the EA has a status of moderate quality on the river. The River Ash in Much Hadham is stated to be poor.
These chalk streams are a vital and unique habitat and, when in prime condition, support a multitude of water plants and insects along with fish species and bird life. Once the river becomes moderate or poor quality, this ecological food chain breaks down rapidly.
I qualified with Herts and Middlesex Wildlife Trust as a river champion, a rather grandiose title for one who checks the quality of a stretch of river for signs of pollution.
Basically, this entails carrying out timed kick-sampling sessions in the river. This involves finding a gravel riverbed section, placing a net downstream and kicking the gravel, thereby disturbing it. Invertebrates which are caught in the net are tipped out into a study tray for identification.
Certain invertebrates such as freshwater shrimps (gammarus,) mayfly larvae, caddis fly larvae and tiny creatures called olives are what I would be looking for. The more of these, the better the quality of the river.
These creatures are highly susceptible to pollution, so by monitoring the river on a monthly basis its quality can be quickly established as a drop in numbers of these tiny creatures would indicate a pollution event and the authorities will then be noted post-haste.
Also, a lack of these creatures at a certain point along a river indicates that the speed of flow, the quality of the riverbed or the lack of gravel is not right for them to survive. In a good river habitat, thousands of gammarus would be expected from each kick sample.
Weirs do nothing to enhance the quality of a river and do plenty to damage it.
Above the weir, the flow of water slows down, meaning the sediment in the water sinks to the riverbed and settles there, suffocating the life out of what should be a healthy gravel bed.
Below the weir, the flow increases, causing erosion of banks, as can be seen on the Stort, and removes the gravel, which in turn damages the aquatic population of invertebrates.
As I entered the river directly below the weir with my net I wasn't anticipating to catch much from the small gravel area and this was borne out after five kick samples. Two bullheads (small fish), two mayfly larvae and five gammarus were all I found. An indicator of a poor river.
The pH of the water measured 7.1, just below the optimum expected of 7.4 to 7.9, but well within the expected range. It was clear that the weir was indeed impacting upon the ecology of the river and that its removal would benefit the habitat immensely. Add to this the fact the weir is damaged and has become a health and safety issue.
Whilst I appreciate this site has long been a favourite for paddling and netting small fish, there is no reason for it to remain. Within the project are plans to put access points to the river for paddling and for dogs. These will be much safer than the present badly-eroded access point by the weir.
Aquatic conservation is always a tricky one. We see trees being cut from woods and housing estates and, at times, question the environmental impact such removal has. However, what goes on underwater is invisible to us.
We see kingfishers and water voles and presume all is good with the river. However, just a summer check on damselfly and dragonfly species along this stretch show them to be in very low numbers, mainly the banded demoiselle, which has a larvae that remains underwater for a few years and can withstand a certain amount of pollution and disturbance. Also, the variety of native chalk stream flora is limited.
By removing the weir, the speed of flow will be enhanced, encouraging scouring of the silt on the riverbed and encouraging natural bankside plant species to get a hold.
By lopping some branches from overhanging willows (which will regrow), more sunlight will penetrate the water. These branches can be dropped into the river to act as collectors of the silt and alter the flow and ebb of the water.
Plants will get a head start on the silted areas behind the logs, known as deflectors. These logs are pinned in place. In the natural world trees would regularly fall into the river, so this is doing no more than replicating what would normally occur.
The overwintering kingfishers and local water voles will not be affected by any work done on the weir – in fact its removal will make the whole section a far more favourable habitat for such creatures.
All in all, removal of the weir will be excellent for the quality of habitat.
I phoned a vociferous champion of chalk stream improvement and enhancement, Feargal Sharkey. The former Undertones singer's passion was evident in our half-hour phone conversation and his knowledge on the subject extensive. He fully endorsed removal of the weir as a starting point to improve the whole section of the Stort. He also pointed out that weirs stop fish migration upstream to spawn. Although small fish are found upstream of the weir, these will undoubtedly be a resident population from before the weir's construction some 50 years ago when the river was diverted. Freshwater fish require gravel riverbeds to spawn successfully in well-oxygenated water. Very little gravel exists further upstream, where the speed of flow is too slow to scour away the silt.
Older readers may recall the river flowing along its natural course, through the Waitrose car park and joining today's canalised river near Wetherspoon's Port Jackson pub. Street names such as Old River Lane and Bridge Street hint at what used to be there.
The weir was constructed to aid this transition of flow and, on that basis alone, is now redundant. However, I realise that its removal may result in issues with silt further downstream that I trust will be dealt with using an environmentally-mannered approach.
Much more about the waterside project is to be announced and public engagement sessions will be held in the future when residents can hear a lot more, discover the benefits and hear what else the project has planned. The weir's removal is but a small part of the overall vision for the River Stort. Presently, with Covid restrictions, the engagement will be via newsletters. I look forward to attending these events and finding out more. All details will be posted in this column when announced.
I packed up my equipment having had a wonderful time, with many locals stopping for a pleasant chat and to hear about the weir etc. Children were pleased to see the bullhead fish, a food source of the kingfishers, which I noted on several occasions.
A flock of siskins fed on alder cones whilst a little egret fed and posed in trees all along the Grange Paddocks section. In the mud and pools on the football pitches, black-headed gulls fed and squabbled, a few already moulting into breeding plumage and showing their chocolate-brown heads.
I headed towards Cannons Mill Lane and Grange Paddocks Meadow, where a pair of common buzzards have taken up residence. Also here were tit species, redwings, magpies, goldcrests and crow species. Recently, a small duck called a teal had been seen on the river. An unusual bird for such a habitat and one that had probably been displaced from wintering quarters during the recent cold snap. No evidence of it on this occasion.
Signs of oncoming spring were evident, with common comfrey in leaf and the buds on male sallow trees just beginning to break open. In another couple of weeks these will be inundated with March flying moth species such as Hebrew character and common quaker, moths that signal the real beginning of the moth-trapping season.
To complete my two-day survey of this fascinating open space, I returned in the evening with a moth trap. Very early in the year for moths and I wasn't expecting to catch many. The temperature dropped lower than forecast and persistent drizzle meant moths were unlikely to move from their roosts. A chestnut was netted in flight whilst the trap attracted a seven-spot ladybird, several sylvicola species of winter gnats, an opiliones arachnid (harvestman) and a few fly species.
The harvestman is not a spider, having only one body part compared to a spider's two, and also shows two eyes on a turret on top of the body whereas spiders have a variety of eye combinations.
I checked the meadows for a barn owl that was reported from the Rye Street side, but no sign before I packed up around 7.45pm. I finished by checking the fence posts by the level crossing where a splendid Nuctenea umbratica (walnut orb web spider) lay in wait for unsuspecting insects.
The kingfishers along here are still attracting interested locals and now the little egret adds to the charm of the area.
Thanks to Indie photographer Vikki Lince for taking photos of me in the river and to Rick Stead, Nicola Gipps and Ian Plume for answering my call for good-quality photos of siskins.