Morphological expression of bedforms formed by supercritical sediment density flows on four fjord‐lake deltas of the south‐eastern Canadian Shield (Eastern Canada)

Abstract

High‐resolution swath bathymetry data collected in fjord‐lakes Pentecôte, Walker and Pasteur (eastern Québec, Canada) allowed imaging in great detail the deltas of four rivers in order to understand the factors controlling the formation and downslope evolution of bedforms present on their slopes. The morphometry and morphology of 199 bedforms reflect the behaviour of sediment density flows. The shape of the bedforms, mostly crescentic, and the relationships between their morphological properties indicate that they were formed by supercritical density flows and that they are cyclic steps. The crescentic shape suggests an upslope migration while the aspect ratios and increasing wavelengths with distance from the shore (and decreasing slopes) are compatible with a cyclic step origin. At the rollover point, the acceleration of the density flows on steep slopes produces tightly spaced hydraulic jumps and favours short wavelength and symmetrical bedforms. Further downslope, decreasing slopes and increasing specific discharge increase the wavelength and asymmetry of the bedforms. The wavelength and asymmetry are increased because density flows require longer distances to become supercritical again on lower slopes after each successive hydraulic jump. Bedform morphometry and morphology are used to reconstruct density flow behaviour downslope. Froude numbers are high near the rollover point and gradually decrease downslope as the slope becomes gentler. Conversely, the specific discharge and flow depth are low near the rollover point and gradually increase downslope as the flow either erodes sediments or becomes more dilute due to sediment deposition and water entrainment. The supercritical density flows are believed to be triggered mainly by hyperpycnal flows but some evidence of delta‐front slope failures is also observed. The differences in delta morphology and bedform development between the four deltas are linked to basin morphology and watershed hydrology, but also mainly to the fjord heritage of the lakes that allowed the focusing of sediment at the delta front.

Morphological signatures of deglaciation and postglacial sedimentary processes in a deep fjord‐lake (Grand Lake, Labrador)

“…A higher concentration of sediment waves is observed on the upper slope in coarse sediments. Previous studies have demonstrated that seasonal fluvial pulse discharges can generate gravity flows (Desloges and Gilbert, ; Van Rensbergen et al ., ; Gilbert and Butler, ; Gilbert et al ., ) and form sediment waves on delta frontal slopes (Wynn et al ., ; Girardclos et al ., ; Clarke et al, ; Hilbe and Anselmetti, ; Normandeau et al ., ; Symons et al ., ). The activity of gravity flows as a trigger mechanism for such sediment waves fits with the morphostratigraphy of the deltas in Grand Lake as stacked mass‐transport deposits are observed in the deltaic unit (U3a).…”

Sediment dynamics in paired High Arctic lakes revealed from high‐resolution swath bathymetry and acoustic stratigraphy surveys

“…Known turbidite thickness in East Lake (millimeter scale) and the runoff of the river which attains peaks of 1–2 m 3 /s during snowmelt (peak discharge) also suggest that the hyperpycnal flows are rather dilute. These low‐energy hyperpycnal flows are not erosive enough to produce large canyon‐channel systems that can concentrate flows downslope [e.g., Kremer et al ., , ; Normandeau et al ., ]. Instead, the flows being triggered at the delta front remain largely unconfined.…”

Morphodynamics and depositional signature of low‐aggradation cyclic steps: New insights from a depth‐resolved numerical model

“…Developments in physical and numerical modelling of supercritical‐flow bedforms (Kennedy, ; Jorritsma, ; Foley, ; Winterwerp et al ., ; Parker & Izumi, ; Alexander et al ., ; Fagherazzi & Sun, ; Sun & Parker, ; Taki & Parker, ; Fildani et al ., ; Kostic & Parker, ; Alexander, ; Sequeiros et al ., ; Spinewine et al ., Kostic et al ., ; Paull et al ., ; Cartigny et al ., , ; Kostic, ; Balmforth & Vakil, ) have sparked a large number of observations of supercritical‐flow bedforms in modern systems (Fildani et al ., ; Lamb et al ., ; Duarte et al ., ; Jobe et al ., ; Babonneau et al ., ; Maier et al ., ; Covault et al ., ; Hughes Clarke et al ., ; Fricke et al ., ; Tubau et al ., ; Zhong et al ., ; Normandeau et al ., ; Symons et al ., ). Despite this common and well‐documented occurrence of supercritical‐flow bedforms, outcrop examples of deposits indicating these flow conditions in a fluvial setting (Fielding, ; Duller et al ., ; Fielding et al ., ; Ghienne et al ., ; Lang & Winsemann, ) or in a (deltaic‐) marine setting (Postma et al ., , ; Ventra et al ., ; Dietrich et al ., ) are sparse.…”