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Flocculation controls mud sedimentation and organic carbon burial rates by increasing mud settling velocity. However, calibration and validation of floc settling velocity models in freshwater are lacking. We used a camera, in situ laser diffraction particle sizing, and suspended sediment concentration–depth profiles to measure flocs in Wax Lake Delta, Louisiana. We developed a new workflow that combines our multiple floc data sources to distinguish between flocs and unflocculated sediment and measure floc attributes that were previously difficult to constrain. Sediment finer than ∼10 to 55 µm was flocculated with median floc diameter of 30 to 90 µm, bulk solid fraction of 0.05 to 0.3, fractal dimension of ∼2.1, and floc settling velocity of ∼0.1 to 1 mm s−1, with little variation along water depth. Results are consistent with a semi-empirical model indicating that sediment concentration and mineralogy, organics, water chemistry, and, above all, turbulence control floc settling velocity. Effective primary particle diameter is ∼2 µm, about 2 to 6 times smaller than the median primary particle diameter, and is better described using a fractal theory. Flow through the floc increases settling velocity by an average factor of 2 and up to a factor of 7 and can be described by a modified permeability model that accounts for the effect of many primary particle sizes on flow paths. These findings help explain discrepancies between observations and an explicit settling model based on Stokes' law that depends on floc diameter, permeability, and fractal properties.

Lateral migration of meandering rivers poses erosional risks to human settlements, roads, and infrastructure in alluvial floodplains. While there is a large body of scientific literature on the dominant mechanisms driving river migration, it is still not possible to accurately predict river meander evolution over multiple years. This is in part because we do not fully understand the relative contribution of each mechanism and because deterministic mathematical models are not equipped to account for stochasticity in the system. Besides, uncertainty due to model structure deficits and unknown parameter values remains. For a more reliable assessment of risks, we therefore need probabilistic forecasts. Here, we present a workflow to generate geomorphic risk maps for river migration using probabilistic modeling. We start with a simple geometric model for river migration, where nominal migration rates increase with local and upstream curvature. We then account for model structure deficits using smooth random functions. Probabilistic forecasts for river channel position over time are generated by Monte Carlo runs using a distribution of model parameter values inferred from satellite data. We provide a recipe for parameter inference within the Bayesian framework. We demonstrate that such risk maps are relatively more informative in avoiding false negatives, which can be both detrimental and costly, in the context of assessing erosional hazards due to river migration. Our results show that with longer prediction time horizons, the spatial uncertainty of erosional hazard within the entire channel belt increases – with more geographical area falling within 25 % < probability < 75 %. However, forecasts also become more confident about erosion for regions immediately in the vicinity of the river, especially on its cut-bank side. Probabilistic modeling thus allows us to quantify our degree of confidence – which is spatially and temporally variable – in river migration forecasts. We also note that to increase the reliability of these risk maps, we need to describe the first-order dynamics in our model to a reasonable degree of accuracy, and simple geometric models do not always possess such accuracy.

The grain size 2 mm is the conventional border between sand and gravel. This size is used extensively, and generally without much physical justification, to discriminate between such features as sedimentary deposit type (clast-supported versus matrix-supported), river type (gravel bed versus sand bed), and sediment transport relation (gravel versus sand). Here we inquire as to whether this 2 mm boundary is simply a social construct upon which the research community has decided to agree or whether there is some underlying physics. We use dimensionless arguments to show the following for typical conditions on Earth, i.e., natural clasts (e.g., granitic or limestone) in 20 ∘C water. As grain size ranges from 1 to 5 mm (a narrow band including 2 mm), sediment suspension becomes vanishingly small at normal flood conditions in alluvial rivers. We refer to this range as pea gravel. We further show that bedload movement of a clast in the pea gravel range with, for example, a size of 4 mm moving over a bed of 0.4 mm particles has an enhanced relative mobility compared to a clast with a size of 40 mm moving over a bed of the same 4 mm particles. With this in mind, we use 2 mm here as shorthand for the narrow pea gravel range of 1–5 mm over which transport behavior is distinct from both coarser and finer material. The use of viscosity allows the delineation of a generalized dimensionless bed grain size discriminator between “sand-like” and “gravel-like” rivers. The discriminator is applicable to sediment transport on Titan (ice clasts in flowing methane/ethane liquid at reduced gravity) and Mars (mafic clasts in flowing water at reduced gravity), as well as Earth.

Arctic river systems erode permafrost in their banks and mobilize particulate organic carbon (OC). Meandering rivers can entrain particulate OC from permafrost many meters below the depth of annual thaw, potentially enabling the production of greenhouse gases. However, the amount and fate of permafrost OC that is mobilized by river erosion is uncertain. To constrain OC fluxes due to riverbank erosion and deposition, we collected riverbank and floodplain sediment samples along the Koyukuk River, which meanders through discontinuous permafrost in the Yukon River watershed, Alaska, USA, with an average migration rate of 0.52 m yr−1. We measured sediment total OC (TOC) content, radiocarbon activity, water content, bulk density, grain size, and floodplain stratigraphy. Radiocarbon activity and TOC content were higher in samples dominated by silt as compared to sand, which we used to map OC content onto floodplain stratigraphy and estimate carbon fluxes due to river meandering. Results showed that the Koyukuk River erodes and re-deposits a substantial flux of OC each year due to its depth and high migration rate, generating a combined OC flux of a similar magnitude to the floodplain net ecological productivity. However, sediment being eroded from cutbanks and deposited as point bars had similar OC stocks (mean ± 1 SD of 125.3±13.1 kg OC m−2 in cutbanks versus 114.0±15.7 kg OC m−2 in point bars) whether or not the banks contained permafrost. We also observed radiocarbon-depleted biospheric OC in both cutbanks and permafrost-free point bars. These results indicate that a substantial fraction of aged biospheric OC that is liberated from floodplains by bank erosion is subsequently re-deposited in point bars rather than being oxidized. The process of aging, erosion, and re-deposition of floodplain organic material may be intrinsic to river–floodplain dynamics, regardless of permafrost content.

Abstract The timespan recorded in deposits from fluvial activity is a key gap in our understanding of ancient environments on Earth and Mars. Because riverine sediment transport occurs under time‐variable water discharge, common models that represent sediment transport with a single bankfull discharge require an intermittency factor. For this reason, the ability to predict intermittency factor values based on environmental factors would improve estimates of time from fluvial deposits. To address this knowledge gap, we calculated intermittency factors from 201 modern rivers and six fans and deltas with depositional timespans of months to millions of years. Intermittency factors range from 0.0064–0.73; they are uncorrelated with averaging timescale, bed‐material grainsize, or climate aridity, but are larger in river catchments with greater rates of denudation relative to precipitation. Application to ancient fluvial systems on Mars indicates long‐lived depositional river systems for up to 104–106 years.

The mass of carbon stored as organic matter in terrestrial systems is sufficiently large to play an important role in the global biogeochemical cycling of CO2 and O2. Field measurements of radiocarbon-depleted particulate organic carbon (POC) in rivers suggest that terrestrial organic matter persists in surface environments over millennial (or greater) timescales, but the exact mechanisms behind these long storage times remain poorly understood. To address this knowledge gap, we developed a numerical model for the radiocarbon content of riverine POC that accounts for both the duration of sediment storage in river deposits and the effects of POC cycling. We specifically target rivers because sediment transport influences the maximum amount of time organic matter can persist in the terrestrial realm and river catchment areas are large relative to the spatial scale of variability in biogeochemical processes.Our results show that rivers preferentially erode young deposits, which, at steady state, requires that the oldest river deposits are stored for longer than expected for a well-mixed sedimentary reservoir. This geometric relationship can be described by an exponentially tempered power-law distribution of sediment storage durations, which allows for significant aging of biospheric POC. While OC cycling partially limits the effects of sediment storage, the consistency between our model predictions and a compilation of field data highlights the important role of storage in setting the radiocarbon content of riverine POC. The results of this study imply that the controls on the terrestrial OC cycle are not limited to the factors that affect rates of primary productivity and respiration but also include the dynamics of terrestrial sedimentary systems.

Objective We aim to assess which variables are associated with recruitment failure of obstetrical and gynaecological randomised controlled trials (RCTs), leading to an extension of the study period.Design Nationwide study.Setting A cohort of RCTs supported by the trial centre of the Dutch Consortium of Obstetrics and Gynaecology.Population We included 83 RCTs that recruited patients between 1 March 2003 and 1 December 2023.Main outcome measures Main outcome was recruitment target not achieved within 6 months after the preplanned recruitment period. Secondary outcomes were recruitment target not achieved within an extension period of at least 12 months and premature termination of the trial. In all RCTs, we collected information on variables with a potential effect on recruitment failure, recorded at five levels; patient, doctor, participating centre, study organisation and study design.Results In total, 46 of 83 RCTs (55%) did not achieve their targeted recruitment within the preplanned study period with a maximal extension period of 6 months. The most relevant variables for recruitment failure in multivariable risk prediction modelling were presence of a no-treatment arm (where treatment is standard clinical practice), a compensation fee of less than €200 per included patient, funding of less than €350 000, while a preceding pilot study lowered this risk.Conclusions We identified that the presence of a no-treatment arm, low funding and a low compensation fee per included patient were the most relevant risk factors for recruitment failure within the preplanned period, while a preceding pilot study lowered this risk. Awareness of these variables is important when designing future studies.