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The structural controls of sediment connectivity are poorly understood in low-gradient watersheds and stream networks. We developed a field- and modeling-based framework to understand the timescales and thresholds associated with the structural controls of sediment connectivity and continuity in the Inner Bluegrass region of Kentucky, USA. Field reconnaissance was carried out to assess stream morphology and sediment connectivity, which was leveraged to derive instream sediment connectivity indices for 181 pool-riffle sequences in the low-gradient bedrock stream network. Our model framework coupled a probability-based connectivity formulation with the sediment continuity formula to assess sediment transport and connectivity in the stream network. Results indicated that the bedrock system reflects an end-member in which lithology controls longitudinal stream morphology. Specifically, we found that regional morphology controls pool-riffle length scales via bedrock outcrops, which therein controls the timescales of sediment connectivity over multiple hydrologic regimes. The pool to riffle length ratio was found to be 4.6, an order of magnitude greater than the ratio for alluvial systems. The new formulation of the sediment continuity formula coupled with the probability of connectivity framework adequately captured both short-term and long-term continuous dynamics of sediment transport, as indicated by evaluation metrics with measured sediment flux and field reconnaissance. Based on the analysis of 362 riffles and pools, we found that sediment in pools upstream of bedrock outcrop barriers are disconnected 93 % of the time whereas riffles are connected and allow full continuity 99 % of the time. Longitudinal sediment connectivity within the stream network was found to be controlled by both functional connectivity and structural connectivity, which contrasts recent findings describing the controls of sediment connectivity over lateral pathways on hillslopes. • We investigate sediment connectivity with modeling- and field-based frameworks. • Probability of connectivity and sediment continuity equations form our approach. • Field reconnaissance assisted with soft validation of the model. • Pools are primarily disconnected whereas riffles demonstrate frequent connectivity. • Structural watershed properties control pool morphology and connectivity regimes. [ABSTRACT FROM AUTHOR]

• Overlapping crater populations constrain the total era of martian valley formation. • Martian valleys may have formed intermittently over hundreds of millions of years. • Long-term erosion rates would have been similar to the modern Atacama Desert. A number of studies have constrained the cessation of martian valley network activity to around the Noachian-Hesperian boundary, but the total time period over which these features formed remains poorly constrained. Most attempts to ascertain valley network formative timescales rely on an intermittency factor that varies widely for terrestrial rivers and is unknown for Mars. In this contribution, superposition relationships are used to identify crater populations that pre- and postdate valley network incision, resulting in maximum cumulative valley formation times of ∼108 years. These timescales correspond with long-term erosion rates of ∼0.25 m/Myr, similar to those calculated for the Middle to Late Noachian as well as the central Atacama Desert since the Late Miocene. Assuming sediment transport rates previously calculated for other martian valley networks, this implies a minimum fluvial intermittency of ∼10−5, several orders of magnitude lower than modern day Earth rivers but similar to values independently calculated for some martian fluvial deposits. This low intermittency suggests that martian valley incision may have been significantly hindered by erosion into existing bedrock or by boulder armoring of the valley floor. Alternatively, unlike river valleys on Earth, runoff events driving martian valley network activity may have been non-seasonal, Milankovitch-like cycles, with long eras of quiescence between periods of intensified fluvial activity. [ABSTRACT FROM AUTHOR]

Among many 234U–238U disequilibrium applications, the " comminution age " approach enabled quantitative constraints of sediment transport timescale since the genesis of detrital sediment. This method requires a critical assumption regarding the initial bedrock (234U/238U) activity ratios (A 0), typically assumed to be in secular equilibrium (=1). However, this supposition needs further validation as more authors proposed preferential leaching as a vital mechanism that produces U-series disequilibrium among bedrocks. To address this uncertainty and to examine the extent of impacts responsible for the bedrock (234U/238U) disequilibrium, we compiled the published A 0 covering a wide range of lithologies. In this study, the global A 0 values ranged from 0.700 to 1.280, with an average of only 0.981 ± 0.070 (2σ; n = 160). This widely observed 234U deficiency among global bedrocks cannot be resulted from the direct α-recoil of 234U alone but is attributed to the preferential release of 234U from the microfissures created in rock diagenetic history. To further evaluate the impact of A 0 on comminution age calculation, the Zhuoshui River (Taiwan Island) sediment comminution age was re-calculated using the newly measured A 0 from the Zhuoshui River basin, which is 0.978 on average. The newly derived comminution age was nearly three-fifths shorter than the values estimated using A 0 = 1 in the previous study. Our case study demonstrated that assuming bedrock secular equilibrium could lead to overestimated comminution age, inferring a significantly lengthier sediment transport timescale across the river basin; therefore should be carefully constrained to provide a holistic perspective of catchment-scale earth-surface processes. • Bedrocks are not always in a "closed system" due to the micro-fissures formation. • An "open system" through preferential leaching led to bedrock U-disequilibrium. • Zhuoshui River bedrock (234U/238U) is much more depleted than expected. • Comminution recalculation inferred nearly 3/5 shorter ages than assuming unity. [ABSTRACT FROM AUTHOR]

Using a suite of ultra-high resolution geophysical tools, remote operated vehicle dives, legacy cores, and isolated grab samples, we demonstrate at a regional shelf scale, the influence of antecedent geology (basement topography, shelf gradient and submerged shoreline features) on the evolving transgressive shelf stratigraphy of a subaqueous delta. The unconsolidated uppermost delta occurs as isolated remnants scattered across the low-gradient shelf. Seismic data reveal across-shelf heterogeneity in bedrock elevation, with prominent bedrock highs and depressions and several well-preserved aeolianite palaeo-shoreline complexes at water depths of −100, −60 and − 40 m. Analysis of bathymetric and seismic data demonstrates that these pre-existing shoreline complexes exert an overarching control on the distribution patterns of i) deltaic sediments, where they abut the landward flank of the shoreline form, acting as a barrier to seaward dispersal, and ii) shoreface sediments which remain sequestered on the mid-shelf on the seaward flank of the shoreline complex, hampering the landward translation of the shoreface in step with rising sea levels. Moreover, sediment distribution/accumulation is further constrained where elevated portions of the basement topography provide little to no accommodation, and act as zones of sediment bypass, and adjacent basement depressions accommodate sediment to seaward. The gentle antecedent slope, coupled with the gentle shoreline trajectory mediated transgressive erosion directly to landward. This is reinforced where local inflections in the wave ravinement profile form due to the presence of palaeo-shoreline complexes, aiding in the ultimate preservation of these submerged delta facets in the palaeo-shoreline lee. This study shows that low antecedent gradients, and palaeo-shoreline features, lead to development of transgressive coastal profiles that are predisposed to delta overstepping. The pre-existing basement topography and palaeo-shorelines constrain the positioning and morphology of the delta. We suggest that antecedent conditioning has partitioned accommodation for delta accumulation and moderated wave ravinement associated with transgression since the Last Glacial Maximum (LGM). The geological framework has acted as a recurring primary control on the geomorphic evolution of the submarine delta and shelf. • Seismic stratigraphy of the Thukela continental shelf • Reveal several submerged deltaic depocenters and shorelines • Show bedrock highs control depocenter positioning and preservation • Delta morphology and stratigraphy influenced by palaeo-shorelines • Palaeo-shorelines restrict ability for shoreface to migrate with rising sea level [ABSTRACT FROM AUTHOR]

The timing of surface uplift across the Appalachian Mountains, Eastern USA, remains a controversial topic. Thermal history modelling, geomorphological analysis, stratigraphic analysis, and river profile analysis have all suggested the region experienced surface uplift during the late Cenozoic. This is in contrast with many low-temperature thermochronology studies, which consistently present Mesozoic dates and typically infer the landscape to be the remnant of a much older mountain belt that has been slowly exhumed following the end of rifting in the Late Triassic-Early Jurassic. Resolving a regional exhumation history of the Appalachians through traditional low-temperature thermochronology methods is difficult due to the spatial constraints of bedrock sampling, while traditional detrital river sampling has limitations underpinned by analytical issues and grain selection bias. We aim to resolve the exhumation histories of large areas across the Appalachians using detrital laser ablation apatite (U-Th-Sm)/He dating (LA-AHe). LA-AHe is a new dating approach that produces a high number of apatite (U-Th-Sm)/He ages from modern river sands to infer the low-temperature (70–40 °C) exhumation history of the bedrock across a catchment. Supplementary apatite and zircon U-Pb dates were obtained to study sediment transport in each catchment and their respective bedrock geological histories. LA-AHe results show most dates to be Cretaceous (69%), with only a small Cenozoic fraction (6%), while thermo-kinematic modelling of data from four catchments can be explained by protracted Mesozoic-Cenozoic exhumation with rates ∼31–20 m/Myr. The lack of Cenozoic dates does imply the magnitude of exhumation has not been significant enough to greatly affect the apatite (U-Th-Sm)/He system, though models incorporating enhanced late Cenozoic exhumation between 2.1–1.4 km are consistent with data from three catchments. These results have implications for the study of elevated topography across passive margins and ancient orogens and highlight how laser ablation apatite (U-Th-Sm)/He dating can be effectively used to study the change of landscapes over geological timescales. • Appalachians are the result of protracted exhumation over last 200 Myrs (∼20 m/Myr). • Detrital LA-AHe and U-Pb dating reveals exhumation and sediment transport patterns. • LA-AHe analysis includes Cl wt% and zonation data, important for thermochronology. [ABSTRACT FROM AUTHOR]

This study examines the role of topography as a dynamic boundary condition that limits the efficiency of fluvial erosion in the post-glacial Teton Range landscape. The volume of sediment currently stored in two major catchments was estimated using high-resolution LiDAR and geometric reconstructions of depth to bedrock. Seismic reflection data in Moran Bay reveals post-glacial sediment preserved behind a submerged moraine, which isolates the bay from the larger Jackson Lake depocenter. The volume of post-glacial sediment stored in the canyons and bay totals 173.82 ± 19.5336.0 (×10−3 km3), which translates to a catchment-wide sediment production rate of 0.17 ± 0.02 mm/yr. The rock-equivalent sediment volume in Moran Bay is 4.4 ± 0.9 (×10−3 km3), only ∼2.6% of the total post-glacial volume. While the estimated sediment production rate in the canyons is similar to the uplift rate, the denudation rate derived from Moran Bay sediment is 0.004 ± 0.001 mm/yr, implying highly inefficient post-glacial sediment transport. The fluvial system has been disequilibrated by glacial erosion such that interglacial valley profiles lack the steepness needed to transport sediment, delaying sediment evacuation until the next glacial advance, or until uplift sufficiently steepens the fluvial system so that it regains efficiency. Furthermore, colluvial production rates in the deglaciated valleys are close to long-term denudation and uplift rates, suggesting that once topography has been equilibrated to glacial erosion processes, subsequent glaciers do not need to produce much bedrock erosion, but mainly sweep out accumulated sediment to maintain equilibrium. • Sediment budget quantified for two catchments in the Teton Range, Wyoming. • Flattened post-glacial topography limits efficient fluvial erosion. • Sediment transport delayed until next glacial advance or uplift steepens system. • Colluvial production rates are close to long-term denudation and uplift rates. • Subsequent glaciers may not need to produce much bedrock erosion. [ABSTRACT FROM AUTHOR]

Detailed quantification of material supply into stream channels is crucial for determining sediment (un)balance budget during flood events. Unfortunately, existing quantification methods require long-term field monitoring; otherwise, there is an insufficient amount of usable data. In this study, we introduce a new approach, based on dendrogeomorphic methods, to determine the volume of material supply generated from flysch high-gradient stream bank failures. This approach was supplemented by 1D sediment transport modelling, using a TOMSED model. We analysed 138 cross-sections from the roots of predominantly broad-leaved trees, which were exposed in bank failures from different floods. Using the spatial position of the roots and dating the exposure time, we determined the volume of material supply into the channel during the last flood, in 2010. In-channel sediment transport was analysed on 14 cross-sections, and the channel sediment thickness was estimated using the geophysically based ERT (Electrical Resistivity Tomography) method. The total volume of material loss during the 2010 flood within the study area, according to the TOMSED model, was found to be at least 300 m 3 . This means that the calculated total volume of sediment supply resulting from bank failures during the flood (63.9 m 3 based on dendrogeomorphic methods) was significantly lower than the calculated bedload transport for the entire study area. This finding implies that contemporary sediment sources cannot adequately cover the transport capacity during high-magnitude flood events, and in general, in-channel erosion processes prevail. [ABSTRACT FROM AUTHOR]

Abstract: This study examines the erosive resistance of the discontinuous, loose joint blocks of various sizes that comprise the bed material of the Etobicoke Creek, an incised bedrock channel found in an urbanized setting within the city of Toronto, Ontario, Canada. In order to accomplish this, both field and laboratory experiments were conducted on the bed material found in this channel. The field work involved repeated detailed surveying of the channel bed and estimations of distances of movement of various clasts during many sediment transport events. We found that rates of erosion and/or deposition can be significant during the snowmelt spring events and that there was a greater loss of bed material over the study reach than was gained in several spots, with bed lowering >0.10m in some locations. The field work also showed that the distances moved by the rock clasts scaled directly with shear stresses observed during rain events and that average distances of transport tend to be short. The frequency distributions of distance of transport scaled with particle size is log-normally distributed. Flume experiments demonstrated that the clasts offered different levels of resistance to movement, depending on their orientation. The platy nature of the rock fragments, clast orientation, and their imbrication significantly affect the threshold of erosion. The results have significant implications for the understanding of bedrock rivers from a management perspective, especially in the case of those rivers experiencing the effects of urban development in their drainage basins. [Copyright &y& Elsevier]

Abstract: We use the two-dimensional (2D) electrical resistivity imaging (ERI) method to detect the contact between sediment and bedrock in a modern fluvial system. We performed eight 2D electrical resistivity surveys along the Peikang River in Kuohsing area, central Taiwan, with a pole–pole array of 1-m electrode spacing to produce a spatial resolution of about 1 m. The resistivity data were inverted into subsurface electrical structures using the least-squares inversion techniques. Then, the contact between the bedrock and sediment was found by using an image processing technique called Laplacian edge detection (LED), which represents an objective approach in interpreting various geophysical images. Numerical modeling demonstrates that isoresistivity lines are not in good agreement with the hypothetical interfaces between the bedrock and sediment, but the zero-lines of the Laplacian operation are strikingly accurate. Our results indicate a well-defined boundary in the resistivity structure that can be used to estimate the quantity of sediments covering bedrock, thus highlighting the utility of this technique in studies of landscape evolution, sediment transport, and sediment budgets. [Copyright &y& Elsevier]

• Bedload transport rate was measured in a mixed bedrock–alluvial stream. • Patch classes were mapped to consider the effects of macro-roughness. • Equation predictions are improved by including the macro-roughness. • Equations predictions are also improved by including limited sediment mobility. • Flow resistance equations understate energy loss in mixed bedrock–alluvial streams. Mixed bedrock–alluvial channel beds are common in mountain rivers. It is well-known that in such system the balance between sediment supply and transport capacity defines either the level of the exposure of bedrock (in the riverbed) or its coverage by alluvial material. We investigate whether applying the representation of the energy losses (caused by macro-roughness) developed for alluvial channels improve the capacity of bedload transport equations to predict the bedload transport in a mixed bedrock–alluvial stream. Therefore, the purpose of this study was to evaluate the performances of five bedload equations against field measurements, while considering the effects of macro-roughness on the equations and the limited availability of mobile sediment. We analyzed the data collected in a stream located in southern Brazil, which has a 2.3% gradient and mixed bed conditions. Optimal results in estimating the bedload transport rate were obtained when using the reduced shear stress τ ' and reduction factor of the available bed material (Frm) together in the five tested equations. Analyzing the two approaches separately, the implementation of τ ' proved to be more critical for improving the performance of the equations than using Frm alone. Among all the equations, Recking (2013) presented the best result for the case in which 85.2% of the estimates fell within one order of magnitude of the measured transport rates (0.1 < r > 10.0); this was followed by the Recking (2010) (70.4% of estimates within one order of magnitude) and Meyer-Peter and Müller (1948) (37% of estimates within one order of magnitude) equations. In conclusion, we observed that even by implementing flow resistance and with the limited availability of mobile sediment, the equations overestimated the bedload transport rate in the studied river reach, indicating an underestimation of energy loss in the flow resistance equations of mixed bedrock–alluvial channels. [ABSTRACT FROM AUTHOR]