Advisory Walkover Cock Beck, Wharfe Catchment November 2017
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Advisory Walkover Cock Beck, Wharfe Catchment November 2017 Index Key Findings Introduction Link to Google Map with images Catchment & Fishery Overview Habitat Assessment Recommendations Quick Wins Riparian Habitat Spawning Habitat Pollution Fish Passage Making it Happen 1 Key Findings Cock Beck is severely challenged. It has been stripped of its ability to perform many natural geomorphological processes through historic dredging, in places with associated straightening or realignment. In essence, it has been converted into a drain. To reinstate this would require significant raising of the bed level throughout, to allow for better connection with the flood plain. Resource and a change in land management are likely to be prohibitive. Water quality is an ongoing issue. No specific point sources of nutrient pollution were witnessed. Diffuse pollution of fine sediment is rife, with some clearly defined point sources identified around fords, poorly conceived field drains and/or field buffers, cattle drinks, and bank degradation due to poaching. These point sources of fines provide opportunities for quick wins through installation or reinstatement of fencing. Daylighting some of the lower reaches and livestock exclusion around the mid reaches, both to promote better, natural riparian growth should encourage the Beck to self-heal through diversifying the bank profile and pinching the channel through instream colonisation. Encouragement of geomorphological process by judicious installation of woody material where the channel is already trying to renaturalise will aid retention and sorting of substrate, in particular the gravels required for spawning and invertebrate habitat. Value added benefit will be realised through extending the interventions above and below those sections. Working to maintain, enhance and extend the ‘better’ reaches of habitat will probably provide more value in the first instance by providing relatively regularly spaced island ‘refugia’ between more barren, less hospitable sections. 2 1.0 Introduction This report is the output of site visits to Cock Beck, a tributary of the River Wharfe, W. Yorkshire. The visit was requested by David Morley (Fisheries Officer, Environment Agency) and Marie Taylor (Project Officer, Yorkshire Dales Rivers Trust) for the ‘Rivers in Elmet’ project. Jonny Grey of the Wild Trout Trust undertook various walkovers with David and Marie. Normal convention is applied with respect to bank identification, i.e. left bank (LB) or right bank (RB) whilst looking downstream. Upstream and downstream references are often abbreviated to u/s and d/s, respectively, for convenience. The Ordnance Survey National Grid Reference system is used for identifying locations. Map 1. Overview of Cock Beck catchment highlighting areas walked: black rectangle – Map 2; red rectangle – Map 3; yellow rectangle – Map 4 Map 2. Cock Beck upper section, from Throstle Nest Farm to Aberford and the A1M culvert 3 Map 3. Cock Beck from Lead Hall Farm to Stutton village bridge at Mill Lane Map 4. Cock Beck from A1M culvert to Lead Hall Farm (completed later) A map with imported photos from the walkover has been created in Google at: https://drive.google.com/open?id=1Maf9ert1G7RXiMWAOA4J2yOlYX Z3uB9I&usp=sharing 4 2.0 Catchment / Fishery Overview Cock Beck is a ~40km tributary of the Lower Wharfe (Cock Beck Catchment; Environment Agency waterbody ID# GB104027063940; Maps 1-4), arising on the NE outskirts of Leeds and wending its way slowly to the tidal Wharfe just downstream of Tadcaster. The shallow valley and low-lying topography (~50m a.s.l.) reflects the underlying magnesian limestone which produces freely draining, lime-rich loamy soils ideal for arable farming. Over two cycles of assessment for the Water Framework Directive, Cock Beck has deteriorated from Moderate to Bad Ecological Status, primarily for nutrients and water quality issues (drivers of the Rivers in Elmet project), but also for fish diversity. Quite surprisingly, Cock Beck is not designated as artificial or heavily modified despite having been straightened or realigned in many sections, and dredged in its entirety; there is barely a discernible difference in channel cross- sectional proportions, i.e. trapezoidal, for the ~22kms viewed. Anecdotal evidence from historical angling reveals that grayling of ~1kg were caught and that the Beck provided spawning habitat for Atlantic salmon; indeed, there is a suggestion that the etymology of Cock Beck stems from male salmon. Just after the turn of the century, the EA installed several paired upstream deflectors, and introduced gravels to improve spawning habitat at Leyfield Farm. The latest EA fish data show that only trout were detected in low numbers in the vicinity of those works (u/s of Aberford), and a few salmon and small coarse species were found at Stutton, relatively close to the Wharfe confluence. 3.0 Habitat Assessment The reporting that follows considers the Beck from the most u/s point (Throstle Nest Farm; SE 40790 35620; Fig 1) to the most d/s point (just below Stutton on the Mill Lane bridge; SE 48206 41599). The channel has been dredged to a trapezoidal shape for much of this length, over-deep and over-capacity for the majority of flows, especially as the catchment lacks gradient. As a consequence, the bed is deep with fine silt, especially where flow has been impounded by natural wood fall or artificial rock ramps serving as fords. In the upper reaches, it appears that the Beck is entirely disconnected from the flood plain. Hence, the impoundments simply build up until a spate of sufficient size ‘blows’ them out, or the EA Ops Team removes them all (as was intimated upon meeting a team during the visit). 5 The bed has been dug down to a clay base (probably throughout the entire length but certainly evident in the upper sections) and the Beck lacks the power even to erode down and sideways on the outside of meanders, resulting in the retention of the trapezoidal cross-section. While gravels of appropriate size for the redds of resident trout (~10- 25mm) were present in the upper reaches, they were effectively ‘welded’ to the clay bed or so clogged with silts as to be of little use for invertebrate habitat, let alone fish spawning habitat. In terms of riparian habitat, Cock Beck is a bit of a curate’s egg. The wooded sections and wooded riparian fringe provide ample litter and woody debris but are so dense in some sections that the banks were bare of understory; this is a particular problem in the lower reaches where Himalayan balsam is present. Some fields were fenced but in general, even those were dilapidated to the extent that livestock could access the water edge and cause severe poaching and sediment ingress. Several formalised cattle drinks were observed but none were appropriately constructed and had been churned to a slurry. 6 Fig 1. Cock Beck at Throstle Nest Farm. The u/s & d/s sides of Parlington Lane bridge (upper & lower panel, respectively), demonstrating the impounded nature of the watercourse, generally deep, and with dubious colouration of the water. A saving grace is that the riparian vegetation in the upper sections is mostly native and dense. 7 Fig 2. A twin-pipe field drain to the Beck. The incision through the bank was ~2m at its deepest. Since this was adjacent to a recently ploughed, bare field on a relatively steep gradient with no bund or buffer strip in between, then it is likely to be a considerable source of soil ingress. Fig 3. A straighter section of the Beck. From a distance, the quality of the riparian fringe might suggest that the Beck is in good shape but even from this image it is evident that the channel bed is flat and featureless with some shallow depositions of fine silt. 8 Fig 4. A natural log jam and accumulated leaf litter, unlikely to be an issue for fish passage as a) it will be ephemeral, and b) the water was flowing through and underneath it. Even such a structure provided little evidence of out of channel flow paths. Note that despite meandering here, the channel still retains the trapezoidal form resulting from dredging. Fig 5. A rare example of substantial erosion on the outside of a bend, likely exacerbated here by livestock grazing right to the bank top and hence little physical matrix from the roots of a naturally diverse vegetation community to help resist erosion. 9 Fig 6. Image taken from d/s of the Ass Bridge (SE 40986 37429) depicting typical nature of the Beck – some narrow, native and mature riparian fringe but heavily straightened (here) between pasture and arable fields. Fig 7. At Leyfield Farm (SE 42138 37983), there appears to be an old mill race / culverted weir (inset) which represents a considerable obstacle to free fish passage. Below, the sudden increase in gradient and energy has led to the widening of the channel (weir pool) but without much increase in depth (again probably reflecting that even here, the Beck has little erosive power downwards). A deposition bar had been colonised with aquatic plants creating a short braiding of the channel and some much needed flow variation. This was the most u/s site where fish (trout) were observed. Horses had complete access to the bank top so the riparian vegetation was not as diverse. 10 Fig 8. Paired, upstream deflectors installed by the EA, circa 2001, were still functioning as intended, creating a scour pool and sorting some of the substrate. Gravels contained less silt fractions compared to u/s sites, and were generally looser. Approximately 15 years after being placed instream, these structures are testament to potential for further augmentation works to increase the spatial extent of the benefits (see Recommendations). 11 Fig 9. Three images from a short section assumed to be within the Parlington Estate.