OR/15/040 Introduction

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Whitbread, K. 2015. Channel geometry data set for the northwest Scottish Highlands. British Geological Survey Internal Report, OR/15/040.

Channel geometry data were collected for 139 reaches forming 77 km of main stem channel of three montane rivers in the northwest Scottish Highlands between October 2009 and April 2011. Reaches were designated as stretches of channel between 50–2000 m in length characterised by the channel morphology and dominant substrate making up the bed and banks of the stream, i.e. the percentage of alluvial bed cover. Bedrock reaches were defined as having less than 30% bed cover and where both banks are in intact rock. Reaches with over 99% of the channel were defined as alluvial, and reaches with between 99 and 30% bedcover, and/or at least one alluvial bank, were defined as mixed bedrock-alluvial. The method of estimating bed cover is described by Whitbread et al. [2015][1].

The reach average channel width and depth were derived from 450 individual bankfull measurements as the arithmetic mean of between 2 and 10 measurements per reach. Uncertainty on the channel width and depths (U(W) and U(D) respectively) were derived as standard error of the mean reach average value. Full details of the reach designation and measurement methods are provided by Whitbread et al., [2015][1] and Whitbread [2012][2].

The three study catchments are:

  • The River Carron, which drains north and east from the northeast slopes of Beinn Dearg into the Kyle of Sutherland; the catchment reaches a maximum area of 300 km2, and 78 reaches were surveyed along 44 km of channel length;
  • The River Elchaig, which drains west from the mountain slopes west of Glen Affric to the head of Loch Long at Killilan; the catchment reaches a maximum area of 97 km2, and 42 reaches were surveyed along 18 km of channel length;
  • The River Canaird, which drains west from the ice-scoured uplands northeast of Ullapool in to Loch Kanaird (Loch Broom). The catchment reaches a maximum area of 95 km2, and 19 reaches were surveyed along 15 km of channel length.

Hydrological analysis of the 5 m grid, 1 m vertical resolution NEXTMap digital terrain model (Intermap Technologies, 2007[3]) was conducted to derive channel slope, catchment area and along-stream distance for each reach. Channel slope is recorded as the elevation fall per unit reach length. Catchment area is taken as the mean of all catchment areas derived for points at 5 to 7.5 m spacing along the reach length (no reach includes a major tributary junction). The along-stream distance is measured from the mouth to the top of each reach along the channel line extracted from the digital terrain model; this method yields channel lengths that are typically longer than measurements derived from channel lines on topographic maps.

The northwest Scottish Highlands are dominated by metasedimentary rocks including gneissose to schistose psammites and pelites. The main lithology underlying each reach was determined from published geological maps and field observations. In some areas the channels are developed along the lines of faults. The faults typically occur as zones of fractured rock with localised fault gouge and mineralisation. Affected reaches are recorded as ‘Fault’ in the geology column of the following table.

These data form the basis for the analyses of substrate, slope and sediment controls on channel geometry developed and discussed by Whitbread et al. [2015][1].

References

  1. 1.0 1.1 1.2 WHITBREAD, K, JANSEN, J, BISHOP, P and ATTAL, M. 2015. Substrate, sediment and slope controls on bedrock channel geometry in postglacial streams. J. Geophys. Res. Earth Surf, 120. doi:10.1002/2014JF003295.
  2. WHITBREAD, K. 2012. Postglacial evolution of bedrock rivers in post-orogenic terrains: the NW Scottish Highlands. Unpublished PhD Thesis, University of Glasgow.
  3. INTERMAPTECHNOLOGIES, 2007. NEXTMap Britain: Digital terrain mapping of the UK, NERC Earth Observation Data Centre.