Bulletin 041

Bulletin 041

BUL 041

The Composition of Recent Alluvial Gravels in Alberta River Beds

Author(s) Shaw, J. Kellerhals, R. Date 1982-01-01

A large data set of grain-size distributions and lithologic composition is presented for bed material in 12 Albertan rivers over a total sampled length of 7 500 km. The hydraulic, hydrologic, and geomorphic characteristics of the rivers are described briefly. Details of sample collection including geographic location and site descriptions are given. The data are presented in tables of grain-size distributions and percentage contributions of lithologic types to grain-size classes coarser than 2 mm. An estimate of significance f the constituent grain-size distributions is given. Eight grain-size parameters are estimated from the sample distributions (i.e. geometric means (DG) for the granite, limestone, quartzite, and total gravel fractions). Plots are given for downstream changes in lithological composition and grain-size parameters.

The downstream change in lithological composition reflects the relative resistance of quartzite and limestone in upstream reaches, and also illustrates the increasing importance of erratic, granitic Shield lithologies which have been introduced to downstream reaches by continental glaciation. The westward limit of Shield erratics in stream bed materials is used to estimate the maximum limit of Laurentide glaciers. These estimates are shown to correspond closely with mapped limits, with the exception of the Athabasca and McLeod estimates which lie much further west than the mapped limits. The variability of grain size in large gravel-bedded streams is treated y analysis of variance. Variance between samples (at a site) is highly significant, while downstream variance is even more ignificant. Downstream changes in grain size are considered in the course of this study in relation to the region''s geomorphic history nd the processes of differential fluvial transport and abrasion.

Three distinctive reaches are noted in the large, well-sampled rivers. The first, the mountain reach, in which aggradation in lakes and behind alluvial fans takes place, shows an increase in grain size as one moves downstream. Then, there is the central gravel reach which shows an exponential decrease in grain size with distance, corresponding to Sternberg's relationship. This central reach terminates abruptly, however, with a change from a gravel to a sand bed, which constitutes the third type of reach observed. The sand hows little change in size regardless of downstream location.

Diminution coefficients for rivers and alluvial fans are presented toshow the dominant influence of differential transport in the ggrading fan environment. Comparison of diminution coefficients for various river gravels with abrasion coefficients established in ontrolled experiments, reveals that abrasion coefficients onsistently underestimate diminution coefficients. Analysis, which uses diminution and abrasion coefficients for different lithologies, reveals that the abrasion coefficient for rivers can be subdivided nto two components-'abrasion during transport' and 'abrasion at rest.' The analysis indicates that the condition of 'abrasion at est' is dominant in Albertan rivers. Grain-size distributions for alluvial gravels are commonly bimodal. The gravels of this study show a deficiency in the range from coarse sand to granules. This deficiency relates to the style of sediment transport, whether suspended or bed load.

Shaw, J. and Kellerhals, R. (1982): The composition of recent alluvial gravels in Alberta River beds; Alberta Research Council, ARC/AGS Bulletin 41, 156 p.