Bar Sedimentology of the Squamish River, British Columbia: Definition and Application of Morphostratigraphic Units

Abstract

ABSTRACT In a 20-km river reach of the Squamish River, British Columbia, in which channel planform changes progressively downstream from braided to meandering, there is a corresponding down-valley change in bar type from mid-channel compound bars to bank-attached compound bars to point bars. On each of 10 bars studied, within-bar facies and particle size trends relate directly to the spatial distribution of local-scale depositional environments on bar surfaces. Four fluvial morphostratigraphic units, defined in terms of their surface morphology and sedimentary characteristics, are differentiated: bar platform, chute channel, ridge, and remnant floodplain. The morphology, scale, and sedimentologic character of each morphostratigraphic unit are described. Compound bars are composed of extensive bar platform units dissected by chute channels, with remnants of other morphostratigraphic units. Within-bar facies and particle size trends are highly irregular, whether viewed down-bar, across-bar or vertically. Sediment sequences on point bars exhibit both around-the-bend and lateral trends. Bar platform units are transitional laterally to alternating ridge and chute features away from the main channel, with floodplain sequences beyond. Analysis of bar sedimentology using fluvial morphostratigraphic unit analysis provides a more reliable basis for palaeoenvironmental reconstruction than do existing procedures based on facies associations at the channel bedform scale.