Threats to the River
What is the current ecological health of the River Sid, based on invertebrate surveys?
Based on surveys of river fly invertebrate populations (larvae and nymphs of species like mayflies, stoneflies, and caddisflies), the water in the River Sid is considered reasonably healthy, although not entirely pristine. The presence of targeted river fly species indicates a functioning ecosystem, as these invertebrates act as "canaries in a coal mine" for water quality. The presence of kingfishers, which require clean, well-oxygenated water with fish, also suggests a relatively healthy river environment.
Significant physical modifications impacting the river include weirs, particularly the large weir in The Byes, which acts as a barrier restricting the upstream movement of aquatic life, including migratory sea trout and European eels. Coastal structures like the training wall and East Pier also impede sediment movement onto East Beach. Modified riverbanks within the town boundary lead to habitat loss, and the construction of the new bridge at Fortescue destroyed sediment beds, impacting the local Brook Lamprey population. Changes in water flow due to abstraction or damming can also affect plant growth and aquatic communities.
Pollution sources are varied and include both agricultural and urban runoff. Agricultural runoff includes fertilisers, pesticides, herbicides, high slurry runoff, and potentially toenicitinoids. Urban runoff comes from vehicles, road abrasion, garden chemicals, and corroded metals. Other potential sources include a historical landfill site and the chemical content of sewage sludge. Poor maize cropping practices are also highlighted as contributing to runoff and pollution risk.
High water temperatures have been recorded in the River Sid and its tributaries, particularly in July and August. Temperatures reached 18°C on Lincombe Stream and Burscombe Brook and 20°C on Bickwell Brook in 2022. While the specific impacts are not fully detailed, high water temperatures can stress aquatic organisms and affect dissolved oxygen levels, impacting the overall health of the river ecosystem.
Total Dissolved Solids (TDS) levels generally increase as the river flows downstream. Upper catchment areas like Plyford have low readings (under 70 ppm), while downstream sites like Sidford and School Weir are usually above 165 ppm and can reach 200 ppm. Burscombe Brook and Woolbrook show particularly high levels (often 200-250 ppm, with peaks of 288 ppm and 270 ppm), only dropping after heavy rainfall. Higher TDS levels generally indicate an increased presence of dissolved substances, which can impact water quality.
Fish populations, particularly migratory species like salmon and sea trout, are threatened by barriers to migration, such as weirs like School Weir, which prevents upstream movement. Loss of suitable spawning substrates like clean, loose gravel due to sediments and pollutants is also a concern. Other pressures include the overall ecological condition impacted by physical modifications, pollution, and invasive species.
Himalayan balsam and Japanese knotweed are identified as significant invasive non-native species in the River Sid catchment. Himalayan balsam is particularly widespread, infesting over 50% of the river's length, and clearing these species is considered a priority action.
Challenges include the increasing frequency of intense rainfall events, leading to significant runoff from land. Insufficient clearing of road drains, overgrown roadside hedges blocking drainage, and slow engagement with the local council on flooding issues are problems. The potential negative impact of new development increasing impermeable surfaces and root ingress into sewers is also a concern. There is a need for better management of highway drains and more urban water retention solutions like water butts, rain gardens, and SuDS. Contract farming and the catchment being a low priority for relevant agricultural schemes are also highlighted as contributing factors.