The morphodynamics of ancient rivers can be reconstructed from fluvial stratigraphy using quantitative techniques to provide detailed insights into the driving forces behind the sedimentary systems. This work explores how these drivers can be evaluated from Paleozoic stratigraphy. Field measurements are taken in fluvial sediments from the Westphalian (Bolsovian and Asturian; 315.2–308 Ma) Pennant Formation of South Wales, UK, to reconstruct the hydrodynamics and morphologies of these Carboniferous rivers, which were sourced from the Variscan (Hercynian) Mountain belt located south of the study area. Field data consist of cross-set heights, grain size, palaeocurrent directions, and the dimensions of fluvial architectural elements. Hydrodynamic properties, including flow velocities and discharge rates, are reconstructed using a suite of numerical approaches. Results suggest median formative flow depths of 2–3 m and palaeoslopes of 4-5 × 10−4 (0.02–0.03°). Quantitative planform prediction suggests these rivers were likely anastomosing but with distinct single-threaded reaches. Mean single-thread width is 55 m, while mean channel-belt widths of 100–200 m are reconstructed, suggesting bankfull discharges of 390–560 m3 s−1. This study resolves contrasting palaeohydrological interpretations for Pennant rivers, and demonstrates how sophisticated reconstructions of morphology, slope and planform can be obtained from fluvial stratigraphy.

Floods determine river behaviour in time and space. Yet quantitative measures of discharge variability from geological stratigraphy are sparse, even though they are critical to understand landscape sensitivity to past and future environmental change. Here we show how climate-driven floods in rivers in the geologic past can be quantified, using Carboniferous stratigraphy as an exemplar. Mass-preservation of woody debris coupled with the geometries of dune cross-sets demonstrate discharge-driven disequilibrium dynamics dominated fluvial deposition. Based on preserved bedforms, we quantified the magnitude and duration of flow variability, showing that rivers were perennial but prone to flashy floods lasting 4-16 hours. This is the largest stratigraphic interval over which disequilibrium bedform preservation has been documented and it demonstrates how climate-driven sedimentation events can be quantified in the geological past. We argue that signals of flooding may be ubiquitous but under-recognised in the rock record, indicating a significant preservation bias.

Floods determine river behaviour in time and space. Yet quantitative measures of discharge variability from geological stratigraphy are sparse, even though they are critical to understand landscape sensitivity to past and future environmental change. Here we show how storm-driven river floods in the geologic past can be quantified, using Carboniferous stratigraphy as an exemplar. The geometries of dune cross-sets demonstrate that discharge-driven disequilibrium dynamics dominated fluvial deposition in the Pennant Formation of South Wales. Based on bedform preservation theory, we quantify dune turnover timescales and hence the magnitude and duration of flow variability, showing that rivers were perennial but prone to flashy floods lasting 4–16 h. This disequilibrium bedform preservation is consistent across 4 Ma of stratigraphy, and coincides with facies-based markers of flooding, such as mass-preservation of woody debris. We suggest that it is now possible to quantify climate-driven sedimentation events in the geologic past, and reconstruct discharge variability from the rock record on a uniquely short (daily) timescale, revealing a formation dominated by flashy floods in perennial rivers.