6.4.1 Climatic, geological and morphological setting of the Feshie River basin
The River Feshie is situated in the Cairngorm Mts. of Scotland, which belong to the paleozoic system of the Grampians. The river is the right−bank tributary of the River Spey, which flows north−east to the North Sea (Figure 21). The climate of the area is strongly oceanic. The annual totals of precipitation are high (from 830 mm to 2050 mm) and air temperatures mild. The bedrock geology of the drainage basin consists in 80 per cent of paleozoic metamorphic schists (Moinian schists). The rest is underlain by granite outcrops (Cairngorm granite) from the local batholits. These granites occur in marginal fragments of the basin and do not underlain the channel of the main river. The River Feshie valley has been remodelled by Pleistocene glaciers. It cuts the high elevated Gaick Plateau (700−1000 m a.s.l.) and its north−western slope. Except the uppermost fragment which cuts the Gaick Plateau, the upper part of the valley is narrow and features a typical glacial trough. In the middle and the lower parts (below 400 m a.s.l.) the valley is getting wider. Its bottom is covered with alluvial deposits and flanked with morainic and fluvioglacial material which forms five, well expressed late−glacial and Holocene terraces (Young 1976). Before the late Pleistocene the upper part of today’s Feshie valley belonged to the river system of Geldie. Then the low watershed between the two valleys was cut by transfluencing glacier and next by proglacial waters. As a result of this the upper part of the River Geldie was captured by the River Feshie (Linton 1949). This has caused the rapid growth of the drainage area of the River Feshie by about 30 per cent. Glen Feshie is mostly deforested. Scots pine clusters (Pinus silvestris) being the remnants of the ancient Caledonian forest occupy the fragments of the valley bottom. Most of the area is covered by heath (Calluna−Empetrum). At the highest elevations the typical mountain meadow communities dominate. Due to the steep valley slopes, poor bedrock permeability and well saturation of peat the infiltration of water is limited. This favours easy formation of overland flow, which results in rapid response of the river to rainfall events. Floods usually follow the intensive summer rainfalls of a convectional origin and autumn or winter ones of a frontal origin. The role of snowmelt in flood formation is minor (Werritty 1984). The lag between rainfall and flow peaks is short; thus the River Feshie is considered one of the flashiest rivers in Britain (Ferguson, Werritty 1983).
6.4.2. Human impacts within the Feshie River basin
During the late 18th and early 19th centuries, land drainage, construction of flood embankments, dredging and channelization significantly affected the middle and lower
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courses of many Scottish rivers. Towards the late 19th century, water supply schemes and early hydroelectric power generation schemes also resulted in significant changes to river flow. During much of the 20th century, there was a continuation of some of the early trends. River flow regulation and water abstraction increased markedly as a result of the development of large hydroelectric power and water supply schemes. More recently, smaller ‘run-of the river’ hydropower schemes have been developed under the Scottish Renewables Order. Surface water is also abstracted by freshwater fish farms, distilleries, paper mills and other industries, and for potato irrigation in eastern Scotland. Catchments with hydropower schemes cover 20% of the area of mainland Scotland. Hydropower schemes involve not only direct impoundment of the main rivers, but also substantial cross-catchment transfer, using an estimated 1000 off-takes to divert discharge from smaller rivers. In addition, there are over 400 groundwater abstraction boreholes of more than 15 m depth and many more small private abstractions, which may have indirect effects on river discharge through the depletion of baseflows (Robins, 1990). These modifications have been to the benefit of water supply for domestic, industrial and agricultural use, and hydropower production. There is little doubt that historical changes in Scottish rivers were accompanied by changes in hydrology, geomorphology, water quality and ecological quality, yet there are few data with which to quantify change.
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6.4.3 Study reach
The study reach lies within the uppermost braided section where the valley widens downstream of a deep glacial trough and braiding is extensive for over 2 km downstream. This reach has an upstream drainage area of 80 km2 and is underlain predominantly by schists which dominate the coarse bedload, and a small proportion of granite of the Cairngorm batholith. Average valley width varies between 200 and 500 m, the gradient in the study area is 1.5% and the median surface grain diameter (D50) is 65 mm. No contemporary flow record
is available for the reach, but a gauge 14 km downstream at Feshie bridge, which drains an area of 235 km2, reveals a highly variable flow regime. Instantaneous peak flows at Feshie bridge exceeded 100 m3 s-1 six times between 1998 and 1999, while the 50% exceedance percentile 1992–1999 is just 5.2 m3 s-1. The major flood events occur mainly during October- January with a smaller number in late spring resulting from a combination of snowmelt and rain on snow. Downscaling by area is difficult, however, this regime is consistent with records from a gauge 2 km downstream of the study reach maintained in the late 1970 which recorded a mean flow of 3 m3 s-1 and a mean annual flood of 70 m3 s-1 (Brasington et al., 2012).
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