Scale dependence of lithological control on topography: bedrock channel geometry and catchment morphometry in Western Scotland

Jansen, J.D., Codilean, A.T., Bishop, P. and Hoey, T.B. (2010) Scale dependence of lithological control on topography: bedrock channel geometry and catchment morphometry in Western Scotland. Journal of Geology, 118(3), pp. 223-246. (doi:10.1086/651273)

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We propose that a scale-dependent topographic signature of erodibility arises due to fluvial and glacial erosion acting on different parts of the landscape at different times. For 14 catchments in western Scotland, we define three levels of substrate erodibility in order of decreasing resistance: quartzite rocks, nonquartzite rocks, and zones of fault-related fracture. Then, using digital topographic and planimetric data coupled with field measurements, we identify regression based scaling relationships between substrate erodibility and morphometric parameters at two spatial scales. Catchment-scale morphometry shows a weak to variable relationship with substrate metrics overall. Erodibility can be inferred from catchment steepness indices (i.e., channel steepness index and relief ratio), but the existence of multiple exceptions could confound a more general application of this approach. Nonetheless, major valley troughs trace fault zones and nonquartzite rocks, leaving much of the higher and steeper ground formed in quartzite. At the reach scale, bedrock channel slope is far more sensitive to substrate erodibility than is channel width. Quartzite outcrops steepen bedrock channels by a factor of 1.5–6.0, and in terms of unit stream power, channels increase their erosional capacity by a factor of 2.7–3.5. Yet only 4%–13% of this increase is due to channel narrowing. Based on a large data set of bedrock channel width (n = 5825) from four rivers, we find that width scales with drainage area (in m<sup>2</sup>) as W = 0.01A<sup>0.28</sup>. Our results are consistent with the view that width-area scaling is similar in all single-thread rivers subject to transport-limited conditions but that for increasingly sediment supply limited settings, erosional thresholds at the channel boundary are the key determinants of bedrock channel width.

Item Type:Articles
Glasgow Author(s) Enlighten ID:Jansen, Dr John and Hoey, Professor Trevor and Codilean, Dr Alexandru and Bishop, Professor Paul
Authors: Jansen, J.D., Codilean, A.T., Bishop, P., and Hoey, T.B.
College/School:College of Science and Engineering > School of Geographical and Earth Sciences
Journal Name:Journal of Geology
Publisher:University of Chicago Press
ISSN (Online):1537-5269

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Project CodeAward NoProject NamePrincipal InvestigatorFunder's NameFunder RefLead Dept
390921Using the glacioisostatic uplift of N Britain to assess the controls on knickpoint recession in bedrock river channelsPaul BishopNatural Environment Research Council (NERC)NE/C510416/1SCHOOL OF GEOGRAPHICAL & EARTH SCIENCES