Down-delta hydraulic geometry and its application to the rock record

Abstract

Copyright . Palaeodischarge estimation is largely undertaken within fluvial settings. However, there are limited palaeodischarge estimates specifically from delta deposits, despite their significance globally. Estimating water palaeodischarges for deltas using catchment-based approaches developed using data from fluvial settings requires estimating parameters from the rock record (for example, palaeotemperature, palaeoslope and palaeorelief). These may be difficult to determine, leading to under-estimation or over-estimation of palaeodischarge values due to differences in process-form relationships between alluvial rivers and deltas. When a sediment-conveying fluvial channel enters a standing body of water, delta lobes develop through repeating mouth bar deposition due to flow deceleration, forming a deltaic morphology with distributary channel networks that differ morphologically from those developed in unidirectional flowing alluvial rivers. This study provides empirical relationships determined across five climate regions, using 3823 measurements of distributary channel width from 66 river deltas alongside the trunk river bankfull discharge that feeds into the entire delta, using a hydraulic geometry scaling approach. Empirical relationships are developed from the global delta dataset between bankfull discharge and catchment area (Qb–A), and bankfull discharge and median distributary channel width (Qb–Wmed). These empirical relationships produce very strong statistical correlations, especially between Qb and Wmed, across different climate regions (Qb = 0.34 Wmed1.48, R2 = 0.77). However, both Qb–A and Qb–Wmed relationships have outliers that may be explained by particular hydrological or geomorphic conditions. These new empirical relationships derived from modern systems are then applied to Cretaceous outcrops (Ferron Sandstone and Dunvegan Formation). The comparatively simple scaling relationships derived here produced palaeodischarge estimates within the same order of magnitude as palaeodischarge values previously obtained using existing, more complex approaches. This study contributes to source-to-sink investigations by enabling palaeodischarge estimates that intrinsically account for climate impacts on channel geometry at the time of deposition, using measurements of channel width or catchment area of a deltaic outcrop.