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The ratio of channel width to channel depth (α) is widely used to quantify the valley geometry of natural alluvial river channels. However, rare studies discussed the feasibility of α to constrain the changes of tectonic and climatic forcing in landscape evolution. To reveal the implication of alluvial valley α values on the effect of rock uplift, we selected an ideal site of the Rumei catchment (RMC) in the mountains of southeast Tibet, where is crossed by the active Lancang River Fault (LCRF) with different spatial characteristics. We used remote sensing, topographic analysis, and grain-size data to analyze channel valley geometry (e.g. valley width, depth, gradient, and α), valley geometry relations and grain-size distribution. Based on the present study, we found that there are three definite groups of α values (α ≤ 4, 4 < α ≤ 6, and α > 6) among the 13 channels in the catchment. The low α group corresponds to alluvial channels at the downstream end of the catchment, where the channels are steep and are controlled by rock uplift driven by thrusting. The medium α group includes channels in the headwaters of the catchment of RMC. High α channels are found in the mid-catchment location of RMC. The time of sediment transport in the mid-catchment has been constrained using radiocarbon ages on organic sediments in alluvial terrace deposits, whose results indicate that the high α channels there were disturbed by a major sediment transport event (debris flows or flood deposits) sometime between 630 and 1991 years ago. We observe that the grain size of deposits is not well related with variation of α values in the study area. We interpreted that the spatial difference of α values is dominantly controlled by the thrusting of LCRF. Hence, we concluded that α is a good indicator of the effect of rock uplift on channel morphology, which could be used to constrain the changes of tectonics in tectonically active mountains. [ABSTRACT FROM AUTHOR]

There is abundant evidence from fluvial landforms and deposits that Mars had rivers that actively transported sediment and shaped its surface. Sediment transport equations are playing a key role in quantifying river processes from these observations, which continue to increase in quality and quantity. In this study, we review sediment transport equations developed on Earth and isolate the effect of gravity for the case of an alluvial channel. We compare 33 formulas used to calculate the sediment transport rate, under transport‐limited conditions, for grain sizes that range from silt to boulders and a lognormal sediment distribution. Results indicate that for a given discharge, channel morphology and grain size, the lower gravity on Mars compared to Earth results in: (a) larger grains mobilized on Mars and transported in suspension, and (b) larger suspended sediment transport rates on Mars and therefore larger total transport rates. Importantly, the effect of gravity is different for bed load and suspended load, with nonlinearity at the bed load‐suspended load transition zone. Therefore, typical total‐load transport relations that do not distinguish between bedload and suspended load are not appropriate for other planets as they simplify the effect of gravity. Gravity‐driven differences in fluvial sediment transport should produce differences in sediment sorting, morphology and stratigraphy between Earth and Mars. Additionally, our results show how Earth‐derived fluvial sediment transport theory can be applied beyond Mars to other planets and moons. Plain Language Summary: There is much evidence that Mars had rivers that actively transported sediment and shaped its surface. Preserved ancient landscapes altered by water provide valuable insights into past processes on the planet's surface and the presence of water. To better understand these landforms, we rely on knowledge gained from systems on Earth. However, is it fair to do so when the gravity on Mars is much lower? How does gravity affect sediment transport and the landforms created by water? In this study, we isolate the effect of gravity on sediment transport by water with an analytical river model. We used 32 sediment transport formulas to compare sediment transport rates on Earth and Mars for the same conditions except gravity. The results show that larger grains are picked up by the flow on Mars and the transport rate of sediment traveling in suspension is higher, and therefore total transport as well. Because grains transported near and on the bed are less affected than the grains in suspension, the effect of gravity varies with the way of transport and hence grain size. Therefore, gravity‐driven differences in sediment transport by water should produce differences in sediment sorting, morphology and stratigraphy between Earth and Mars. Key Points: Fluvial sediment transport rates are higher on Mars than on Earth for the same discharge and channel geometryGravity affects suspended sediment transport more than bed load transport and could hence lead to differences in morphology and stratigraphyTotal load sediment transport equations should be avoided on other planets and moons due to simplified attribution of gravity [ABSTRACT FROM AUTHOR]

Understanding the development and spatial distribution of alluvial patches in mixed bedrock‐alluvial rivers is necessary to predict the mechanisms of the interactions between sediment transport, alluvial cover, and bedrock erosion. This study aims to analyze bedrock alluviation patterns using a 2D morphodynamic model, and to use the model results to better understand the mechanisms responsible for alluvial patterns observed experimentally. A series of simulations are conducted to explore how alluvial patterns in mixed bedrock‐alluvial channels form and evolve for different channel slopes and antecedent sediment layer thicknesses. In initially bare bedrock low‐slope channels, sediment cover increases with increasing sediment supply because areas of Froude‐subcritical flow enable sediment deposition, while in steep‐slope channels the flow remains fully supercritical and the model predicts so‐called runaway alluviation. For channels initially covered with sediment, the model predicts a slope‐dependent sediment supply threshold above which sediment cover increases with increasing sediment supply, and below which the bedrock becomes fully exposed. For a given sediment supply, the fraction of bedrock exposure and average alluvial thickness converge toward the equilibrium value regardless of the initial cover thickness as long as it exceeds a minimum threshold. Steep channels are able to maintain a continuous strip of sediment under sub‐capacity sediment supply conditions by achieving a balance between increased form drag as bedforms develop and reduced surface roughness as the portion of alluvial cover decreases. In lower‐slope channels, alluvial patches are distributed sporadically in regions of the subcritical flow. Plain Language Summary: Bedrock rivers may have patches of alluvial sediment that cover some or all of the underlying bedrock. The amount of this sediment cover can change dynamically over time depending on the flow, upstream sediment supply, channel morphology, and antecedent sediment conditions. Here, we use a numerical model to simulate flow and sediment transport so that we may better understand what controls sediment cover in mixed bedrock‐alluvial rivers. We used the model to simulate channels of varying slopes, sediment supplies, and initial sediment cover, and we analyze the model's output to gain insight into how alluvial patches form and what controls their extent and dynamics. Our numerical model results produce phenomena that have been observed in physical experiments, and they show that the channel slope and initial sediment thickness play an important role in determining whether and how much sediment can be deposited. Our results also show that flow transitions provide critical locations where sediment deposits can start to form. Persistent sediment cover in bedrock channels can develop when a delicate equilibrium is reached between sediment roughness and the flow field. Key Points: Numerical simulations are conducted in mixed bedrock‐alluvial channels to analyze controls on alluvial patternsPartial alluvial cover over steep slopes requires roughness from bedforms and sediment transport to prevent sediment washoutTranscritical flow produces zones of deposition over otherwise exposed bedrock beds [ABSTRACT FROM AUTHOR]

Mixed bedrock‐alluvial rivers can exhibit partial alluvial cover, which plays an important role in controlling bedrock erosion rates and landscape evolution. However, numerical morphodynamic models are generally unable to predict quasi‐steady persistent patches of depositional alluvial features under conditions where the sediment supply is less than the sediment transport capacity of the channel. Hence, we present a new two‐dimensional depth‐averaged morphodynamic model that can be applied to both fully alluvial and mixed bedrock‐alluvial channels, and we use the model to gain insight into the mechanisms responsible for the development of sediment patches and patterns of bedrock alluviation. The model computes hydrodynamics, sediment transport, and bed evolution using a roughness partitioning that accounts for differential roughness of sediment and bedrock, roughness due to sediment transport, and form drag. The model successfully replicates observations of bar development and migration from a fully alluvial flume experiment, and it models persistent sediment patches observed in a mixed bedrock‐alluvial flume experiment. Numerical experiments in which the form drag, sediment transport roughness, and bedform stress correction were neglected did not successfully reproduce the observed persistent sediment cover in the mixed bedrock‐alluvial case, suggesting that accounting for these different roughness components is critical to successfully model sediment dynamics in bedrock channels. Plain Language Summary: Some rivers have patches of sediment intermittently distributed among areas of bare bedrock. These sediment patches can influence flow conditions and patterns of bedrock erosion in these channels. Flume experiments and field studies have shown that the amount of sediment cover and the pattern of sediment patches in mixed bedrock‐alluvial rivers can sometimes stabilize over time, but numerical models have been largely unable to replicate those observations. Here, we present a new numerical model for mixed bedrock‐alluvial rivers that explicitly accounts for how the bedrock and sediment affect the flow. We show that the model can successfully simulate observations from flume experiments conducted with and without exposed bedrock, and it is able to simulate persistent partial sediment cover. These conditions develop because of the way the bedrock and sediment interact with the flow, which we demonstrate through simulations where some of those feedback are "turned off," resulting in completely exposed bedrock without sediment cover. Key Points: A new model is developed to predict the dynamics of alluvial patterns in mixed bedrock‐alluvial channelsModel‐predicted alluvial patterns are similar to those observed in fully alluvial and semi‐alluvial flume experimentsAccounting for differential roughness and shear stress correction are important for model prediction of the degree of exposed bedrock [ABSTRACT FROM AUTHOR]

The depositional history of alluvial fans on Mars provides insight into the climatic conditions during the time of fan formation in the late Hesperian to early Amazonian. However, traditional stratigraphic analysis of the alluvial fan deposits is not possible across most of Mars. This study assesses the use of thermal inertia data as a tool for sedimentologic interpretation of 437 Mars alluvial fans. Based on previous work demonstrating the relationship between depositional style, grain size, and thermophysical properties, this study uses analysis of the thermal inertia of alluvial fan surfaces across the global population of fans on Mars to make an assessment of depositional styles that built the alluvial fans. The thermal inertia values across the global population of fans are indicative of sand- to pebble-sized sediment. The variability of grain sizes across the global population is more homogenous than expected based on comparisons to terrestrial alluvial fans. Nearly all Mars alluvial fans have an average thermal inertia that corresponds to pebble and smaller grain size, and < 1% of Mars alluvial fans have an average thermal inertia that corresponds to cobble-sized grains. Spatial patterns of thermal inertia variability on alluvial fan surfaces show a small number of fans with evidence for either downslope fining or channelization, but the majority of fans show no recognizable geologic patterns in surface thermal inertia. The interpretation of the thermal inertia-derived grain size suggests that either there is widespread mantling of unconsolidated sand across the surface, or that Mars alluvial fans were built by primarily sand-sized sediment, which may be indicative of lower energy sediment transport events. • Thermal inertia of alluvial fan surfaces indicates primarily sand-sized sediment. • Thermal inertia of Mars alluvial fan surfaces is surprisingly homogeneous globally. • Alluvial fans built by sand-sized sediment suggest low energy fluvial processes. • Fluvial depositional processes may correlate to lack of clay in catchments. [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]

The Paleozoic arkoses of the Inkisi Group in both the Republic and the Democratic Republic of the Congo has been investigated in order to examine whether they can represent an alluvial fan originating from the Gondwana supermountains in Northern and East Africa. This study focuses on outcrops' description, lithofacies, bounding surfaces, architectural and sequential organization of sedimentary bodies and paleocurrents. The aim is to determine the nature of the Inkisi sandstones, the provenance of its sediments and the depositional setting, in order to test whether the East African orogen was also the sediments source for the formation of sedimentary fan in the western part of Central Africa. The study shows that the Inkisi Group consists of various sandstone lithofacies, amalgamated by conglomerates. The sediments are immature composed of quartz and feldspars associate with lithic fragments and micas. The sediments originated from the erosion of the West-Congolian belt and the Congo Craton, and were transported by braided fluvial system under mainly upper flow regime and secondarily lower flow regime. Sediments were deposited in the Inkisi basin, which was extensional and affected by burial. The Inkisi sediments setting up in a proximal to medial distributary zone of an alluvial megafan. The flows were directed southward, as opposed to paleofans originating from the Gondwana supermountain in Northern and East Africa. Thus, the Inkisi sandstone constitutes a new alluvial megafan distinguished from the Gondwana superfan derived from the Gondwana supermountain. It originated from the Pan-African belt of Mayombe and the Congo Craton. We name this megafan " the Cambro-Ordovician alluvial megafan of Central Africa ". • The Inkisi Group are arkoses and sub-arkoses associated with conglomerates. Conglomerates are gravel bars and gravity flows. • The Inkisi sediments originated from the peneplanation of the Pan-African West-Congolian/Mayombe and the Congo craton. • The Inkisi sediments were transported by a braided fluvial system and deposited in an extensional basin affected by burial. • The Inkisi Group is an alluvial megafan deposits. • The Inkisi Groupe constitutes a Lower Paleozoic alluvial megafan in Central Africa. [ABSTRACT FROM AUTHOR]