This report is an ecological assessment of proposals to increase water abstraction from Loch Lomond, and to dredge part of the River Leven. The implications of increasing abstraction without dredging are also considered.
The authorised abstraction rate is 100 million gallons/day (mgd) and the current rate of abstraction is about 50 mgd. The reliable yield of the Loch with the present river bed profile has been determined at 69 mgd. In order to meet the authorised abstraction rate of 100 mgd dredging of about 300 mm of ricer bed over a 600m reach would be required to allow minimum compensation flow to be sustained at slightly lower Loch levels than at present. The proposal is to obtain authority to abstract up to 125 mgd, based on recommendations in a report by Crough Hogg Waterman. Dredging work would entail the removal of 400 mm of river bed over the 600m reach.
The development proposals would have comparatively little effect on mean Loch levels. Since the commissioning of the Barrage in 1971, there have been fewer very high and very low Loch levels by comparison with calculated natural (unregulated) levels. The fluctuations in level have in effect been dampened. The increase in abstraction would result in little change in this regime, but there would be a fall in mean Loch level of about 1.8 ins. The effect of the Barrage since 1971 has been an increase of 0.7 ins, so abstraction to 125 mgd would result in a net fall in level (relative to natural levels) of 1.1 ins.
In general, if the proposed developments were implemented the seasonal changes in loch level movement would be more marked than existing conditions and actually tend towards the seasonal variations that would prevail under natural conditions.
Three comparisons can be drawn:
a. The proposed abstraction to 125 mgd would result in seasonal falls relative to the existing (50 mgd) regime ranging from 3 ins in May to a peak of 8.7 ins in August. However, to complete the comparison these seasonal falls in level for the proposed abstractions to 125 mgd would remain above the natural loch levels by 8.5 ins in May to 0.5 ins in August.
b. The maximum effect of an increase from the authorised (100 mgd) abstraction rate to the proposed rates would be substantially less (3.6 ins in August) than the figures quoted above. Thus much of the anticipated increase would result from already authorised abstraction.
c. The seasonal falls in level for the proposed abstraction to 125 mgd would remain above the natural Loch levels by 8.5 ins in May to 0.5 ins in August.
In the last 20 years rainfall in the Loch Lomond area has increased by about 25% and this has increased mean Loch levels and resulted in occasional flooding. The available evidence indicates that the incidence of both flooding and very low Loch levels would tend to be reduced by the water management proposals.
The number of days at or below the 26 ft control level (where the Barrage regulates the flow in the River Leven to the minimum compensation flow (163 mgd)), under existing conditions is 159 per year on average. With the authorised abstraction of 100 mgd, assuming no dredging takes place, the number of days per year would rise to 174. For comparison, on average the natural Loch level would have been at or below the 26 ft OD control level for 177 days/year of which on average about eight would have been below the minimum compensation flow rate. In the case of the proposed dredging with 125 mgd abstraction the number of days on 163 mgd flow would be 199 per year.
With the current rate of abstraction, barrage control and River bed, the River would not have failed to pass the compensation rate of flow during the period 1951-1991 except in the exceptional year 1984. If the present authorised abstraction rate of 100 mgd had been implemented failure would have occurred 1.9 days/year. With the River channel dredged, and abstraction at 125 mgd, failure would rarely, if ever happen, but without dredging failure would average about 5.8 days/year. These figures compare with an unregulated average of 8.2 days/year.
Issues and concerns about the development proposals were ascertained by consulting interested individuals, agencies and organisations. Some concerns proved to be well founded, whereas others were based on misconceptions about the development proposals, or could not be substantiated.
Concerns arising from the proposed dredging were mainly related to the immediate damage that would result to the River bed, and its structure, vegetation and invertebrate life, as well as the impacts on spawning and juvenile salmonids and sea lampreys. There were fears that replacing the River bed so as to retain its ecological characteristics might be only partially successful, or a failure. Possible impacts on angling could be substantial both in the short and longer term.
Perceived issues concerned with Loch levels included possible difficulties for boating due to lower levels, and impacts on powan, salmonids and birds.
An ecological resource inventory was prepared that detailed the main ecological land use and recreational features of Loch Lomond and the River Leven. This was compiled from existing published and unpublished information, supplemented by additional field surveys where there were insufficient data available. Additional surveys involved recording signs of otters, vegetation and invertebrates in the River Leven and the composition of the River bed.
The bed survey provided important new information which shows that although the proportional fine material increased with depth below the surface, the composition of the sub-surface gravels is within the size range acceptable for salmonid spawning.
The Loch Lomond area is of high ecological interest, with the north and south basins differing substantially in geology, soils, topography, water chemistry, vegetation and invertebrates. There are a large number of SSSIs around the Loch and a number of other landscape, Regional Park and Forest Park designations.
The River Leven is of lower interest, but is of high ecological value as a spawning and nursery area for salmonids and sea lampreys. An appreciable proportion of the salmonid redds and all the known spawning areas for the lampreys are within the proposed dredging reach.
The invertebrate fauna of the Leven indicates that the River quality is high. The River is of comparatively little interest as bird habitat, and otters are apparently absent, although they are present in several of the tributaries of the Loch.
There are important game fishings in both the River and Loch, and coarse fishings on the Loch.
The Loch Lomond area supports a wide range of formal and informal water and shore-based recreation activities.
The possible impacts of the development could mainly result from the dredging of the River and any changes in Loch levels.
Dredging and subsequent bed profiling would have a substantial short-term effect on water quality and on the fishing and fishery in the Leven, as well as on the vegetation and invertebrates in the dredged reach. There could be minor impacts on birds and on River amenity and the navigability of the River, although already of low value, would be temporarily affected.
In the post-construction phase most of these impacts should decline but the speed and effectiveness of recovery are uncertain. Monitoring over several years or longer would be needed to adequately assess the long-term effects of the dredging/
Impacts of the water management proposals on the Loch would be mainly minor in character. There could be benefits to birds from the increases in exposure of mudbanks for feeding, and some effects on boating could result from the changes in Loch levels. Larger areas of exposed beaches could benefit recreation.
Possible mitigation measures to reduce effects include improving liaison between angling interests and the CSWDB over freshet releases and compensation flows, and timing and construction constraints during the dredging and profiling operations. Some small-scale dredging at boat landing areas could reduce the risk of grounding. There is a possibility of improving some of the fishing on tributaries of the Loch, for example at the River Fruin.
The no-dredging option would effectively mitigate most of the substantial dredging impacts on the Leven but would have the penalty of increasing the frequency with which the River would fail to pass compensation flows. Reducing the effects of failure could involve purchase of additional water from the Scottish Hydro Electric Loch Sloy Reservoir. In the event of predicted failure the board could seek a Drought Order t permit a temporary reduction in the statutory compensation flow rate.