Loch Dee (NX470790) is situated in the Galloway Hills of Southwest Scotland (Figure 4.1) approximately 40km east and 35km north of the Mull of Galloway. This westerly location with the consequent influence of Atlantic frontal systems results in a mean annual rainfall of 2200mm (1941-70) (Burns et al. 1984). The majority of the catchment lies to the south of the loch and consists of three main sub-
catchments, the Dargall Lane to the west, the White Laggan with its tributary the Black Laggan to the south and the Green Burn to the south-east. The natural catchment area of the Loch Dee basin is 13.5km2 of which the loch surface occupies 1.0km2. This has been increased, however, to 15.6km2 by the 1939 diversion of the headwaters of the Glenhead Burn, which formerly flowed into Loch Trool, and which now flows into the Dargall Lane as part of the Galloway Hydro-electric power scheme.
Catchment altitudes vary from 225m on the loch shore to 716m on Lamachlan at the head of the Dargall Lane, with almost two thirds of the catchment above 300m (Figure 4.1). The most rugged topography, with steep slopes and rocky outcrops, form the southern boundary of the Dargall Lane catchment and the watershed between the Dargall Lane and the White Laggan Burn. The remainder of the catchment is far gentler in relief with more rounded hills, especially in the Green Burn catchment.
4.2.1 Solid Geology
The solid geology of the catchment (Figure 4.2) is dominated by intrusive igneous rocks of the Loch Doon complex to the north and sedimentary rocks to the south. The igneous rocks of the Loch Doon complex are described by Brown et al. (1979) as decreasing in acidity from granites at the centre to hypersthene diorites and quartz monzonites at the margins. The sedimentary rocks consist of Ordovician and Silurian greywackes, shales and mudstones, the majority of which lie within the 2km wide metamorphic aureole of the igneous complex. These metamorphic rocks are more resistant
Boundary between igneous and non igneous bedrock
Metamorphic aureole boundary 0i--- 11 km
than the igneous rocks, thereby accounting for the ridge of high ground at the southern end of the catchment.
The northernmost half of the Loch Dee catchment lies within the hypersthene diorite/quartz monzonite margin of the complex. Most of the the southern half of the catchment consists of Ordovician greywackes, shales and mudstones, the majority of which lie within the metamorphic aureole, except for the headwaters of the White and Black Laggan Burns. There is also a band of Silurian greywackes and shales that runs through the headwaters of the Green Burn and into the White Laggan. In the Green Burn these are within the metamorphic aureole but pass beyond the aureole in the Black and White Laggan catchments. Further sediments of Silurian age occur at the head of the Dargall Lane in the form of an area of metamorphosed pyritic black shales.
The sedimentary rocks are described by Bown (1973) as having a chemical composition which differs little from those recorded for the acid igneous rocks in the northern part of the catchment. Despite the fact that the sedimentary rocks are more resistant than the granite to physical weathering their chemical breakdown is more rapid.
4.2.2 Drift Geology and Soils
The drift cover of the catchment is predominantly of Lateglacial origin consisting of lodgement tills and morainic ridges of poorly consolidated sands and gravels. Soils developed over the drift cover are dominated by blanket and basin peats which reach over a metre in thickness on the gentle slopes. On the lower slopes of the catchment
Figure 4.3: The major soil groups within the Loch Dee catchment (after Bown 1973)
there is a limited degree of podsolisation and these soils are also dominated by thick organic horizons. Bown (1973) divides the soils into two main associations, the Dalbeattie and the Ettrick, as shown in Figure 4.3. The parent material for the Dalbeattie association is the Loch Doon granite, whereas the greywackes, shales and mudstones form the parent material for the Ettrick association. From figure 4.3 the soil associations appear to be well developed across the catchment, more detailed site investigation suggests, however, that peat deposits are far more extensive than figure 4.3 implies.
4.2.3 Land-use and Vegetation
Land-use in the catchment is limited to sheep grazing and forestry. Prior to afforestation, vegetation in the Loch Dee area was dominated by a dry grass and heather moorland community on the steeper well drained slopes and bracken and Molinia communities on the lower slopes (Tivy 1954). The land capability classification, prior to afforestation, is described as land classes 6 and 7 so that land use is restricted to rough grazing and forestry (Bown 1973).
Between 1973 and 1982 the Forestry Commission have pursued a planting programme in the catchment, so that 28 % of the catchment is now afforested (Figure 4.4). Sitka spruce (Picea sitchensis
(Bong.)Carr.) is the principal species planted, with small pockets of
Lodgepole pine (Pinus contorta Dougl. var. latifolia), Norway spruce
Figure 4.4: Location of the afforested area within the Loch Dee catchment.
I
4.3 THE THREE SUB-CATCHMENTS
I The sections that follow will describe the catchment hydrology and
catchment water quality in each of the three main sub-catchments together with the land-use and land management practices that have been pursued on each since afforestation.
4.3.1 The Dargall Lane Burn
The gauged area of the Dargall Lane catchment, with the inclusion of that part of the catchment that formly drained into the Glenhead Burn just upstream of the gauging station, is 2.1km2. Topographically the catchment has a typical corrie form, with steep cliff-like slopes surrounding a semi-enclosed valley floor (Plate 4.1). The lower slopes and floor of the corrie are mantled by an accumulation of peat which can reach a thickness of 1.5m. The average gradient of the catchment is 11.6° (Little 1985), this gradient is, however, broken by a steep section where water leaving the corrie falls over a rock step into the lower catchment. The stream channel, incised into the peat, varies in width from l-2m and has a bed of well-rounded boulders and cobbles (200-300mm), which are liable to move in spate conditions (Burns et al. 1984).
The Dargall Lane catchment is open moorland and will be left uncultivated by the Forestry Commission until the 1990's, in order to act as the control catchment in the Loch Dee Project study. The catchment vegetation over much of the catchment is typical of upland moorland areas and consists of Sphagnum, Molinia and Gramineae
Plate 4.1: A general view of the Dargall Lane catchment showing the steep corrie walls surrounding the shallow valley bottom
Plate 4.2: The Green Burn catchment looking upstream away from the Loch
communities. Calluna and Erica spp. and Aquilinium pteridium become more important, however, in the lower part of the catchment.
4.3.2 The Green Burn
The area of the Green Burn catchment to the gauging station is 2.5km2. Topographically the catchment is far gentler than the Dargall Lane (Plate 2) with an average gradient of 7.2° (Little 1985). The stream channel is similar to the Dargall Lane in that it is incised into peat over much of its length and in that it has a cobble and boulder bed. The channel is, however, generally wider than the Dargall Lane at 2-3m and at several sites, usually at bends, the bed is overlain by gravel.
Upstream of the sampling station 70% of the catchment was planted with Sitka spruce (Picea sitchensis) between 1973 and 1975. This afforestation occurred up to an altitude of 460m at a stocking density of 7000 trees per acre (Kite 1984). Prior to planting catchment drainage was improved by ploughing. The resultant furrows now form a dominant part of the catchment drainage system with some of the major ditches connecting directly with the Green Burn channel. At the present time, 15 years after planting, the trees are about 3m in height and the canopy is just beginning to close (Plate 3).
4.3.3 The White Laggan
The total area of the White Laggan catchment to the gauging station, including its tributary the Black Laggan, is 5.7 km2. The catchment forms the head of a glacial valley and with an average slope of 10.4°
Typical coniferous vegetation in the Green Burn
i
(Little 1985) is topographically more akin to the Dargall Lane catchment than the Green Burn (Plate 4).
The main stream channels are incised into peat in the upper catchment and alluvial deposits in the lower catchment. In the lower reaches the width of the channels are 4m and 3m for the White Laggan and Black Laggan respectively. The beds of both channels are composed of gravel and cobbles, however, the cobbles in the Black Laggan are far more angular than those of the White Laggan (Burns et
al. 1984). •
As a result of afforestation in 1975 30% of the catchment, primarily in the Black Laggan sub-catchment, has been planted with Sitka spruce. Outside the afforested area vegetation is dominated by
Molinia and Sphagnum in the upper catchment and Calluna and Erica spp. and Aquilinium pteridium in the lower catchment.
In 1979 the conifers were cut back approximately 100m from the main channel for 1km upstream from the loch. The cut back area was subsequently replanted with hardwoods such as, alder (Alnus
glutinose), birch (Betula spp.) , willow (Salix sp.) and rowan (Sorbus aucuparia L.) in order to conform with the 'buffer zone' concept of
Mills (1980) (Plate 5). Mills also suggests that forest drains should finish short of water courses in order to prevent high sediment loads entering streams and to enhance the infiltration of precipitation. Consequently, the ditches in the replanted area that connected directly with the stream channel were infilled.
Plate 4.5: A view across The "Buffer Zone" of the White Laggan catchment
Plate 4.6 Limestone addition to the White Laggan Burn
In a further attempt to reduce the acidity of the White Laggan Burn (and thereby Loch Dee) calcium, in the form of queen scallop shells and powdered limestone, has been added to the White Laggan catchment on several occasions since 1980 (Plate 6). Precise details of the type and location of such applications are given in table 4.1.