One year on now the weir has gone

A year has already flown by since we (me at WTT, Pete Turner at EA & Kevin Sunderland at Aire RT) worked in partnership to remove the weir at Coniston Cold on the Aire, North Yorks. I’m currently champing at the bit awaiting a bespoke permit from the EA to tackle another. The funding is in place. The owner has granted permission for full removal having seen the transformation at Coniston Cold. The next weir will present its own challenges… but more on that in another blog.

Here, I’ll focus on developments at the site formerly known as Coniston Cold Weir. Claire Bithell, the EA Geomorphology Specialist, predicted that a deposition bar would form on the inside of the bend (left bank) and extend downstream, thereby effectively returning the channel to more natural proportions. This has been one of the most obvious developments, despite the relative lack of spate flow in the past 12 months. The Aire has experienced one decent spate (in March 2019) which approached winter ¹⁵⁄₁₆ record level but was by comparison of extremely short duration (24h compared to what seemed like 3 weeks at the time in ¹⁵⁄₁₆).

Time-lapse sequence of the site 1 month (Jul/​Aug) after weir removal

Time-lapse sequence of the site 4 months (October) after weir removal

So, instead of a 19m wide impounded reach, the wetted channel is now ~5m wide with obvious flow paths. I attempted to capture some of the geomorphological change via time-lapse photography but weather and battery life construed to make that less successful than I had hoped. Nevertheless, one can observe gradual change in the first few days of the August video as gravel and small cobble fractions get pushed through. Then, when one compares to the deposition bar evolution in the October video, taken from the same vantage point, the ​‘level playing field’ we left behind using the 8‑tonne machine has clearly been sculpted by the river into a more natural channel form. 

Fixed point photography of the site development

The photo sequence above shows the evolution of the bar from downstream, with the wonderful but slightly confounding addition in the latter months of some seriously large woody debris!

We are always keen to stress the tight ecological interlinkage between the riparian (bankside) habitat and the organisms within the river (for shade, for shelter, for food etc), but we rarely expound on the benefits of ​‘instream’ work for terrestrial critters. Exposed gravel bars are dynamic habitats that may ultimately become vegetated and fully stabilised via succession. However, during the early stages of development, those seemingly bare deposition bars provide relatively small but fairly distinct and important habitats for terrestrial and semi-aquatic invertebrates (predominantly beetle and spider species). 

Mammals and birds key into these invertebrate resources too. Trail camera footage has revealed eight species of mammal using the bar including hedgehog and otter, and in May 2019 an oystercatcher was incubating eggs almost exactly where the weir once crossed. How about that for multiple benefits?

But what has happened instream to the fish community? Pre works assessment of the community by Durham University Aquatic Ecology Lab staff, as part of the AMBER (Adaptive Management of Barriers in European Rivers) indicated that while all common species (brown trout, bullhead, stone loach and minnow) were present above and below the weir, numbers were much greater on the downstream side (3x and 6x as many individuals for trout and bullhead, respectively). We could surmise that the lower numbers of fish in the reach upstream of the weir reflected the poorer habitat quality for flow loving species in that impounded section. 

Durham University staff from the AMBER project sampling pre weir removal, and Lancaster University students and staff from Wyre RT following up that work post removal

Fast forward 12 months post removal and would the fish community have changed at all because the river was free flowing? I suggested to Jessie Moravek, a MSc student from Lancaster University, that data from Coniston Cold would make a great contribution to her wider thesis on fish response to various passage easement projects, so we put together an electrofishing team to replicate the work of Durham University. Thanks also to Tom Myerscough from Wyre RT for using this venture as a training exercise! 

The results.… well, the small species were still more abundant in the equivalent downstream reach perhaps unsurprisingly as the habitat was still more varied downstream and probably always will be below the bend. However, the disparity between upstream and downstream for all species was markedly reduced (eg bullhead was now only 3x more abundant), as the habitat upstream has become more suitable again. But what of the trout?

Distribution of trout from standardised upstream (U/S) and downstream (D/S) reaches (relative to the original weir position) pre and post weir removal 

(Month: ‑1 = May 2018; 2 = Aug 2018; 11 = May 2019)

The clearest redistribution was exhibited by trout. Instead of being more abundant downstream (compare the first two columns in the figure above), on the two occasions of assessment post weir removal, trout numbers have been higher in the upstream reach than in the downstream reach. Note in the figure that trout numbers swelled considerably in August reflecting a pulse of young-of-year fry arriving in the reaches. Total number of parr and adult fish inhabiting the reaches was remarkably consistent at the equivalent time period (12 individuals sampled in May 2018 and 2019, pre & post works). And big fish, indeed bigger fish, are still present….

What was a sluggish, overly-wide, impounded reach (upstream), now has much more varied flow, still retains depth, and there are plenty of refugia. The increase in habitat complexity offers up a greater number of ​‘microniches’ for individual trout to occupy. Plus, of course, there is now unimpeded passage up and downstream through a formerly fragmented section of river so fish can move wherever and whenever they need to. 

Typical Aire trout from the sample reaches

I have to reiterate, as with many of the projects for which we have rudimentary monitoring in place, that the dataset is limited and the inferences we draw from the data aren’t particularly robust on an individual site basis. However, that is the beauty of a project like AMBER which has initiated sampling of fish communities prior to and following on from many weir removals across Europe and can highlight the trends and commonalities from a much larger dataset. I’m hoping Jessie will also share some of the findings of her thesis on the WTT blog when she has completed the analyses of all the data, not just the snapshot I have portrayed here.

In summary then, both the geomorphology and the fish community have responded as predicted. Obviously, the reaches will continue to adjust, especially the raw bank edges which will slowly become colonised and further stabilise the formerly sunken rock revetment, so it’s still a work in progress, just that nature is doing the (majority of the) work now instead of me!

For more info on why weirs are an issue, please see the excellent post by my colleague Paul Gaskell.