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Quantitative investigations of ancient rivers usually provide insights into either instantaneous or mean flow conditions. There is a critical gap between these time scales of investigation, which reflects the intermittency of flow and sediment transport, and closing this gap is crucial to fully explore the dynamics and evolution of ancient fluvial landscapes. Here, we combined fluvial stratigraphic data sets, flow and sediment transport models, and paleoclimate general circulation model (GCM) results to develop new methods to estimate intermittency in the geological past, specifically flow intermittency factors (Iw) and sediment transport intermittency factors (Is), and we show how they can be used to explore past hydrograph shapes. We illustrated these methods using the Upper Cretaceous Last Chance Ferron Sandstone in Utah, western United States. For sand-transporting flow conditions in Last Chance Ferron rivers, we estimated Iw values of 0.54-0.90, which imply that channel-forming conditions were sustained for the majority of the year, consistent with perennial systems in which relatively large discharges were sustained. In contrast, for gravel-transporting flow conditions, Iw values of 0.28-0.38 suggest that the largest formative flows may have occupied Last Chance Ferron rivers for nearly a third of the year, which could be explained by a monsoonal system in which high-magnitude discharge events were sustained, or a subtropical system in which high-magnitude discharge events had short durations but high frequencies. Meanwhile, Is values of 0.075-0.15 suggest that annual sediment budgets could have been transported in as little as 10 days and up to 2 months, if channel-forming conditions were sustained, and these values highlight that small changes in the duration of channelforming conditions could significantly impact sediment budgets. Our results are consistent with independent facies- and proxy-based insights into Last Chance Ferron rivers, which point to a perennially wet system characterized by a monsoonal or subtropical discharge regime. Further, our results highlight new opportunities to use paleoclimate GCMs to constrain intermittency factors in the geological past. Going forward, paleoclimate GCMs will be particularly useful where the rock record is incomplete or inaccessible, or where stratigraphic approaches are limited, and they will enable us to tackle pertinent research questions pertaining to past surface processes on both Earth and other planets. [ABSTRACT FROM AUTHOR]

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. Dunes and woody-debris preserved in the rock record have been used to quantify the magnitude and duration of flow events in ancient rivers, revealing a fluvial system dominated by flashy, storm-driven floods 300 million years ago. [ABSTRACT FROM AUTHOR]