Characterizing the differences in bankfull channel geometry across the tidal-fluvial zone of micro- to macro- tidal fluvial systems: Lower Trinity River, TX, USA vs Chehalis River, WA, USA

Abstract:

Bankfull channel width measurements and bankfull stage-discharge relationships, coupled with published and/or collected channel depth sounding readings, were utilized to examine bankfull channel geometries (in the single-thread meandering channel reaches only) spanning from the fluvial ‘normal flow’ moving downstream through the tidal-fluvial ‘backwater’ hydraulic regime of two rivers that are micro- (lower Trinity River) and macro- tidally influenced (Chehalis River). This analysis reveals that moving downstream from the fully-fluvial ‘normal flow’ regime through to the tidal-fluvial ‘backwater’ regime, the micro-tidal lower Trinity River displays: (a) a decrease in bankfull channel width and an increase in bankfull channel depth, (b) a decrease in bankfull channel width/depth ratio, (c) a bankfull channel cross-sectional area that remains nearly constant, and (d) both measured and calculated bankfull discharge remains constant at approximately 900 m³sec^-1. Conversely, the macro-tidal lower Chehalis River displays: (a) an increase in both bankfull channel width and depth, (b) bankfull channel width/depth ratios that remain constant, (c) a bankfull channel cross-sectional area that increases significantly, and (d) both measured and/or calculated bankfull discharge values range from approximately 800 (normal flow) to 10,000 m³sec^-1 (downstream end of backwater regime). Importantly, along the Chehalis River a maximum of ~ 2,000 m³sec^-1 of the total bankfull water discharge (10,000 m³sec^-1), at the downstream end of the ‘backwater flow’ regime, can be accounted for by the Chehalis River proper and two tributary inputs. This suggests, at this channel cross-sectional location, that the additional 8,000 m³sec^-1 of total bankfull water discharge must be supplied by the downstream tidally-sourced component of total water discharge. These results, coupled with the rates of change of measured and/or computed metrics from above, will be utilized to provide insight into the fluvial vs tidal dominance of ancient geologic deposits, such as the Cretaceous McMurray Formation, Alberta, Canada.