Grain size along two gravel-bed rivers: statistical variation, spatial pattern and sedimentary links

Abstract

A new set of field data facilitates a detailed analysis of variations in bed material grain size within two confluent gravel-bed rivers in northeastern British Columbia, Canada. A preliminary assessment of grain-size variability establishes a basis for examination of the spatial pattern of grain-size change. Standard ANOVA techniques are inappropriate because individual samples have unequal variances and are not normally distributed. Alternative tests for homoscedasticity and comparison of means are therefore utilized. Within-site, between-sample variability is not significant. The grain-size distributions that were obtained at individual sites are therefore representative of the depositional environments that were sampled. In both rivers mean grain size does vary significantly between sites and there is therefore a basis for examining the data for spatial patterns such as downstream fining.Textural variations along the two rivers studied here are complex and show negligible overall fining (in over 100 km). This is the consequence of a large number of tributary inputs and non-alluvial sediment sources which are the legacy of Late Pleistocene glaciation. The identification of lateral sources like these is fundamental for understanding textural changes within rivers. The sedimentary link (a channel reach between significant lateral sediment inputs) provides a means of isolating fluvial maturation processes (abrasion and sorting) from contingent lateral inputs. Strong fining trends are apparent in most links and classification of grain-size measurements according to their location within particular links greatly improves the statistical explanation of textural variation. Identification of sedimentary links provides a means of applying models of fluvial fining processes, so isolation of link networks will aid the development of basin-scale models of textural variation.

Downstream fining in a rapidly aggrading gravel bed river

“…One approach to elucidating the controls on spatial patterning is therefore to delineate the fining sequence that occurs between "significant" lateral sediment sources [Rice and Church, 1998]. Another is to focus on fluvial systems in which a consistent fining gradient is preserved, because the perturbations imposed by lateral sources are, for one reason or another, damped [Hoey and Bluck, 1999].…”

Catchment‐scale mapping of surface grain size in gravel bed rivers using airborne digital imagery

“…Such techniques are therefore limited to small river reaches and are impractical when grain size information over very large scales (>10 km) is of interest. For example, Rice and Church [1998] used grain size data sampled at the river scale to show that downstream fining is not continuous along a given river. This allowed for the development of the concept of the sedimentary link.…”

The Impact of Local Source Sediments on Bed Texture in the Fall River, Rocky Mountain National Park, USA

“…Local sediment sources can greatly alter the distribution of sediment in a reach, especially if they significantly differ in grain size distribution from upstream source areas [15,16]. This paper explores spatial differences in surface texture within an individual stream reach that result from the incorporation of comparatively coarse alluvial fan sediment along the left margin of the channel, and are unrelated to channel morphology.…”

“…This paper explores spatial differences in surface texture within an individual stream reach that result from the incorporation of comparatively coarse alluvial fan sediment along the left margin of the channel, and are unrelated to channel morphology. Our study builds upon many reports [15,[17][18][19][20] that examine longitudinal grain size variations by focusing on local supply-induced variations in bed texture at the subreach scale.…”