Abstract The Paleocene-Eocene thermal maximum (PETM) was the most extreme example of an abrupt global warming event in the Cenozoic, and it is widely discussed as a past analog for contemporary climate change. Anomalous accumulation of terrigenous mud in marginal shelf environments and concentration of sand in terrestrial deposits during the PETM have both been inferred to represent an increase in fluvial sediment flux. A corresponding increase in water discharge or river slope would have been required to transport this additional sediment. However, in many locations, evidence for changes in fluvial slope is weak, and geochemical proxies and climate models indicate that while runoff variability may have increased, mean annual precipitation was unaffected or potentially decreased. Here, we explored whether changes in river morphodynamics under variable-discharge conditions could have contributed to increased fluvial sand concentration during the PETM. Using field observations, we reconstructed channel paleohydraulics, mobility, and avulsion behavior for the Wasatch Formation (Piceance Basin, Colorado, USA). Our data provide no evidence for changes in fluvial slope during the PETM, and thus no evidence for enhanced sediment discharge. However, our data do show evidence of increased fluvial bar reworking and advection of sediment to floodplains during channel avulsion, consistent with experimental studies of alluvial systems subjected to variable discharge. High discharge variability increases channel mobility and floodplain reworking, which retains coarse sediment while remobilizing and exporting fine sediment through the alluvial system. This mechanism can explain anomalous fine sediment accumulation on continental shelves without invoking sustained increases in fluvial sediment and water discharge.
Information about past environments is stored in sedimentary rocks via biogeochemical markers stored in the sediments. Using these markers, the signal of paleoclimate and other environmental factors can be reconstructed from the strata. However, because sediment accumulation occurs stochastically, the stratigraphic record is often difficult to reconstruct with confidence. It is generally thought that with a sufficient sample size though, noise averages out, and the true signal can be reconstructed. This assumption is valid when the statistics of erosion and deposition remain steady throughout the interval of interest. In fact, it is known that changes in climate can alter the statistics of erosion and deposition, but the impact of this effect on paleoclimate reconstructions remains poorly understood. This dataset describes a set of physical delta experiments conducted at the Tulane University Sediment Dynamics and Stratigraphy Laboratory. Throughout the experiment, the level of flooding intensity that the delta was exposed to alternated between two end-member values, with transitions of varying durations. We monitored channel dynamics, and reconstructed synthetic climate records from the strata to see how the changing statistics of sediment accumulation impacted the preservation of environmental signals in the strata. This dataset is an HDF5 dataset, which is a general format. The data largely consist of a set of 3D arrays that contain 2D topography and imagery data, where the third dimension is time. Each data object is paired with a 1D vector that links datasets across the time dimension, since data were collected at different intervals. The appropriate linking datasets are also included as CSVs.
These files contain the results from Allen et al. "Similarity of stream width distributions across headwater systems". locationStreamSurveys.zip : contains stream hydromorphology data collected in seven headwater catchments in North America and New Zealand. repeatStreamSurveys.zip : contains stream hydromorphology data collected in six repeat surveys in the Stony subcatchment in Duke Forest, NC. streamWidthModelOutput.zip : contains stream width model parameters and output data presented in Allen et al. Note: the code used to analyze these data can be found at https://github.com/geoallen/streamWidthAnalysis2017/
A more-detailed explanation of the field methods used to collect the data for this study, and the statistical tools used to analyze the data, in addition to a description of how the data file is organized. This information should be applied in conjunction with the data and code if readers are interested in using these data for future work.<br>
Abstract Avulsing rivers carve new pathways on the floodplain, and the associated flooding can profoundly impact society 1-4. River avulsions are thought to occur when the water column becomes perched above the floodplain5, or when the slope down the channel's flanks provides a steeper descent than the current path 6,7. However, neither idea has been suitably field-tested. Here we quantify the topography around avulsing rivers and reveal that these mechanisms work together. Near coasts, rivers avulse when the slope away from the channel is steeper, not because they are perched. The opposite is true near mountain fronts; on alluvial fans, alternative paths are all similarly steep, so rivers avulse when they are perched over the surrounding landscape. We reconcile these findings and present a novel theoretical framework that can predict which rivers are vulnerable to avulsion, and where avulsing rivers will go after they start. By clarifying the rules of river avulsion, it is evident that avulsion risks are underestimated in many coastal environments8 and that probabilistic predictions of avulsion pathfinding can efficiently project hazard with minimal information. Applying these principles for risk assessment may have outsized benefit in the world’s poorest nations, which we find are disproportionately impacted by avulsions.
The morphology and abundance of streams control the rates of hydraulic and biogeochemical exchange between streams, groundwater, and the atmosphere. In large river systems, the relationship between river width and abundance is fractal, such that narrow rivers are proportionally more common than wider rivers. However, in headwater systems, where many biogeochemical reactions are most rapid, the relationship between stream width and abundance is unknown. To constrain this uncertainty, we surveyed stream hydromorphology (wetted width and length) in several headwater stream networks across North America and New Zealand. Here, we find a strikingly consistent lognormal statistical distribution of stream width, including a characteristic most abundant stream width of 32 ± 7 cm independent of discharge or physiographic conditions. We propose a hydromorphic model that can be used to more accurately estimate the hydromorphology of streams, with significant impact on the understanding of the hydraulic, ecological, and biogeochemical functions of stream networks.
