River Conwy Acidification Investigations

Environment Agency Wales surveys of spawning streams in the Afon Conwy catchment have declined from an annual survey down to one every five/six years and evidence pointed to a decline of fish stocks in the upper Conwy both of wild brown trout and migratory salmonids. This is despite the commissioning of the fish pass at the Conwy falls and in 2010 the Trust decided to investigate the reasons for this apparent decline in the fish population.
To assist with the investigation, Environment Agency Wales installed an in-river monitoring programme using a Sond detector specifically looking at water pH and flow levels. Over a one year period, these showed a 100% correlation between river spates and alarming falls in pH. Ambient levels on the upland stream are to be expected at a pH of around 6.8, ie. Slightly acidic. Consistent with river spates, pH levels were seen to fall as low as 4.9 with levels in the region of 5-6 common. Such acid flushes can severely impact invertebrate populations and where these coincide with aluminium rock substrate, can even kill juvenile fish.

The water quality analysis was followed up with a series on invertebrate surveys and electrofishing surveys at a number of sites both upstream and downstream of Ysbyty Ifan. Trust volunteers were assisted by Pierino Algieri of Environment Agency Wales and Dr Nigel Milner, an eminent fishery scientist and a Trust Trustee.

Apart from just upstream of the bridge in the village, poor results were recorded for both invertebrates and fish although two generations of salmon parr were discovered in the short stretch of river above the bridge. Very few stone loach were observed; another good indicator of a watercourse in trouble. At the upstream site, actually on the moor itself, only one juvenile trout was discovered in a 100metre stretch of stream and the only invertebrates discovered were large cased caddis, a species relatively intolerant to pollution. Significant brown algae was also observed at this site, initially appearing to indicate nutrient enrichment but later believed to indicate minimal natural grazing by invertebrates.

The conclusions were that the upper Afon Conwy was suffering from acidic flushes in periods of high rainfall where acid stored within the upland moor is leached out into the stream during spates causing major spikes in acidity to levels where invertebrates could be wiped out. The most severe spikes and resultant low invertebrate populations are inhibiting fish populations.

An obvious solution to this problem would be to lime the stream in a number of different ways, from liming Llyn Conwy to lime dosers to spreading lime on grazing land around the ditches. These approaches were rejected as much of the key sections of stream reside within a SSSI. CCW also use Llyn Conwy as a monitoring site and would not entertain any modification of water acidity.

Funding of this project from the European Fisheries Fund was also restricted to capital funding and would not allow annual purchases of lime. The Trust was aware of the potential advantages of slowing down run off from upland bogs and was aware of the RSPB/National Trust ditch blocking programme on the Migneint. The Trust also reviewed scientific data showing correlation between river flows and invertebrate populations showing that physical slowing of flows through holding back water can positively impact water quality and consequent invertebrate populations.

Following much consideration, it was decided that an extension to the previous ditch blocking programme would contribute significantly to the acidic flush problem. Following discusions with National Trust Officers, it was agreed that the Trust would block ditches around the headwaters of the Afon Serw, an important upper Conwy tributary.

The Trust’s ditch blocking programme on the Serw catchment was completed in late 2011 and approx’ 20km of main stem upper Conwy is benefitting from improved water quality – this figure is in line with the water quality target that would have been achieved as a result of the original liming project concept.

Peat drainage as a result of the extensive drainage ditching (moorland gripping) in the Migneint area of the Upper Conwy has led to peat drying, acidic “flushes” and highly variable flows, resulting in channel and gravel instability, washout of eggs and fry and downstream flooding issues. Approximately 100km of ditches were blocked by peat dams and were re-profiled during February/March 2011 through a joint RSPB / National Trust LIFE project with the aim of restoring the active blanket bog. The Rivers Trust helped block a further 25km of grips to complete this section through its EFF project during the winter 2011.

Initial results from completed work have shown that the increase in overland flow and pooling within blocked drains appears to have led to a less flashy river system. Peak flow responses in both drains and upland streams are less severe, with more rainfall being retained within the bog. The moderation of flash floods achieves multiple benefits by preventing the discharge of acid ‘spikes’, lessening the erosion damage to the river channel and reducing the scouring of spawning gravels. It is also reasonable to suggest that grip blocking all the NT-owned Migneint will almost certainly have some long term beneficial effects against flooding downstream at Llanrwst with the 8,000 acre moor acting as an effective sponge.

Reporting an environmental incident

To report an environmental incident such as suspected poaching, pollution or wildlife crime, the dumping of hazardous or industrial waste and illegal abstraction from watercourses etc, call Natural Resources Wales’ incident hotline on 0300 065 3000 (24 hour service).

Afonydd Cymru
The Rivers Trust
Natural Resources Wales
The Wild Trout Trust