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• 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]

Uranium (U) isotopes can be used to estimate the comminution age of sediments , i.e. the time elapsed from sediment production on continents, via weathering and physical erosion, to deposition in the sedimentary record. The calculation of this comminution age is based on measured (234U/238U) activity ratios in river sediments, and inferred time-dependent recoil effect, which leads to the preferential release of 234U from mineral lattices during erosion processes. In this study, we report on a large-scale (234U/238U) investigation of modern river sediments worldwide, with the aim to determine the extent to which parameters such as grain size, lithology, weathering, climate and geomorphology may influence the distribution of U isotopes in fine-grained sediments. Our extensive dataset (N = 64) includes U isotopic measurements for many of the world's largest rivers, but also rivers draining particular climatic and geological settings. Our results indicate that sediments collected from river basins draining mostly igneous, metamorphic or volcanic rocks often display (234U/238U) ratios > 1, with clay-size fractions (<4 µm) being less depleted in 234U (higher 234U/238U) than corresponding silt-size fractions (4–63 µm). In contrast, sediments derived from multi-lithological basins or draining sedimentary rocks are typically characterized by (234U/238U) ratios <1, with clays generally exhibiting more depleted 234U signatures than silts. Taken together, these observations suggest that the formation of secondary clay minerals in soils from basins draining mostly igneous, metamorphic is accompanied by partial incorporation by recoil injection of 234U initially released during weathering processes, possibly from U-rich minerals, such as sphene or apatite. Instead, in multi-lithological catchments draining sedimentary rocks, we propose that the erosion of recycled sediments having experienced several cycles of weathering, possibly over glacial-interglacial timescales, could explain the much lower (234U/238U) ratios observed in clay-size fractions. While no direct relationships can be identified between sediment (234U/238U) ratios and lithology, weathering intensity, climatic or geomorphic parameters in corresponding river basins, we show that the catchment size probably plays an important role in controlling the distribution of (234U/238U) in river sediments, through its direct influence on the sediment residence time. Finally, a multiple regression analysis of our data, combining various environmental parameters for the lithology, climate and geomorphology of studied river basins, indicates predicted (234U/238U) values that are very similar to measured values (with R2 ∼ 0.8). This finding provides further support for the usefulness of (234U/238U) ratios in the sedimentary record for reconstructing past landscape changes and their effect on sediment transport and residence time in river basins. [ABSTRACT FROM AUTHOR]

Reservoirs behind dams act as deposition sites for much of the sediment being transported by rivers. As a result, the downstream river flow can be well below the transport capacity for bed-material. This promotes bed erosion and other geomorphic changes over some length of river located immediately downstream from a dam. These adjustments have been characterized for the Trinity River, TX, downstream of Livingston Dam. Field measurements and results from a 1D numerical model define a 50–60 river kilometer segment of river undergoing bed erosion as the transport capacity for bed material is reestablished. Consequences of this erosion include lowering of the channel bed, reduction in the sediment volume of channel bars, coarsening of sediment on bar tops, steepening of channel banks, and reduction in lateral migration rates of river bends. Repeat surveys of the river long profile reveals that 40 yr of dam closure has produced up to seven meters of channel-bottom incision downstream of the dam, transforming an initially linear profile into a convex-up long profile. The model output matches this observed change, providing confidence that calculated estimates for spatial and temporal changes in bed-material sediment flux can be used to explore the long-term signature of dam influence on the geomorphology of a sand-bed channel. Measurements of channel geometry, profile, lateral migration, and grain size of the lower Trinity River with distance downstream define both the trend and expected variability about the trend associated with the disruption to the bed-material load. [ABSTRACT FROM AUTHOR]

This study describes channel changes following completion of the Provo River Restoration Project (PRRP), the largest stream restoration project in Utah and one of the largest projects in the United States in which a gravel-bed river was fully reconstructed. We summarize project objectives and the design process, and we analyze monitoring data collected during the first 7 years after project completion. Post-project channel adjustment during the study period included two phases: (i) an initial phase of rapid, but small-scale, adjustment during the first years after stream flow was introduced to the newly constructed channel and (ii) a subsequent period of more gradual topographic adjustment and channel migration. Analysis of aerial imagery and ground-survey data demonstrate that the channel has been more dynamic in the downstream 4 km where a local source contributes a significant annual supply of bed material. Here, the channel migrates and exhibits channel adjustments that are more consistent with project objectives. The upstream 12 km of the PRRP are sediment starved, the channel has been laterally stable, and this condition may not be consistent with large-scale project objectives. [ABSTRACT FROM AUTHOR]

The Atchafalaya River in south-central Louisiana is of geomorphic interest because of the large volumes of flow and sediment transported and its role as a major floodway for the lower Mississippi River, notwithstanding its potential to become an avulsion pathway at Old River. Since its origin in the sixteenth century, it has undergone many transformations and currently receives flow from the Red River and partial flow of the Mississippi River through multiple pathways. Flow increased progressively beginning in the mid-1800s and continued until concerns of diversion spurred placement of engineering structures to regulate flows at the juncture between the Mississippi and Atchafalaya rivers in 1963. Prior studies, that looked at reaches or six to seven rangelines or cross sections on hydrographic surveys document areal, width, and depth increases in the uppermost 80 km (50 mi) of the Atchafalaya River from 1880 to 1974. This study assesses how channel geometry of the Atchafalaya River has changed with flow variations since the 1880s by evaluating instantaneous discharge measurements that document change at superior temporal resolution and differences in thalweg values from coupled modern hydrographic surveys which document change at better spatial resolution than prior works. While structures have regulated and limited flow into the Atchafalaya since 1963, the highest values of discharge, cross-sectional area, and width at Simmesport (upstream) follow river regulation. Associated geometry data at Simmesport and Morgan City (downstream) document that mean bed and thalweg elevations have risen. Comparison of thalweg elevations on hydrographic surveys from 1963 and 2006 show pronounced local variability in bed elevation change but also reaches dominated by degradation (including the Red River and uppermost section of the Atchafalaya) near the Old River diversion. Aggradation occurs in the lower Atchafalaya, a zone where along-channel lakes are filling, flow and sediment quantities are changing in part caused by the constructed Wax Lake Outlet, and deltas have been growing in Atchafalaya Bay. Findings are relevant to river engineering and flood management, especially navigation, and help in better understanding the evolution of large distributaries and the role of human impacts. [ABSTRACT FROM AUTHOR]

• An efficient 2D shallow water hydro-sediment-morphodynamic model is presented. • Morphodynamic processes under the impact of cascade movable weirs are studied. • Disparate morphological changes may result under different operation schemes. • An alternating operation scheme is proposed to constrain morphological changes. Cascade movable weirs have been increasingly widely used for river landscaping, water storage and ecological wellbeing. The weirs disturb the prior equilibrium in line with fluvial hydrology, sediment transport and geomorphology, and may lead to profound changes in the river, of which, however, the understanding has so far remained poor. Here, the first of its kind, a computational modelling study is presented on the morphodynamic processes under the impact of cascade hydraulic lifting dams, a type of movable weirs, as to the middle Fen River, China. A two-dimensional shallow water hydro-sediment-morphodynamic model is applied to resolve the fluvial processes, which is physically coupled and computationally efficient based on Finite Volume Method, unstructured mesh, local time stepping and parallel computing. Computational results show that the cascade dams alter the flow and sediment transport locally and on the reach scale in a distinct manner under different operation schemes, which in turn incur disparate morphological changes. Collapse of all the dams all year round would lead to net degradation in the study reach, with the floodplains aggrading and the main channel degrading. Operation of all the dams all year round may induce an overall aggradation both on the floodplains and in the main channel. If all the dams collapse in flood seasons and operate in dry seasons (CF), degradation occurs in the main channel whereas the floodplains experience aggradation, with a net degradation. When only the dams in the main channel operate all year round (OM), the floodplains degrade while the main channel aggrades, resulting in a net aggradation in the study reach. Either aggradation or degradation may prevail locally on floodplains, in main channel and individual subreaches. The present findings lead us to propose a complex operation scheme of annually alternating CF and OM to constrain significant morphological changes. [ABSTRACT FROM AUTHOR]

A landscape's sediment grain size distribution is the product of, and an important influence on, earth surface processes and landscape evolution. Grains can be large enough that the motion of a single grain, infrequently mobile in size-selective transport systems, constitutes or triggers significant geomorphic change. We define these grains as boulders. Boulders affect landscape evolution; their dynamics and effects on landscape form have been the focus of substantial recent community effort. We review progress on five key questions related to how boulders influence the evolution of unglaciated, eroding landscapes: 1) What factors control boulder production on eroding hillslopes and the subsequent downslope evolution of the boulder size distribution? 2) How do boulders influence hillslope processes and long-term hillslope evolution? 3) How do boulders influence fluvial processes and river channel shape? 4) How do boulder-mantled channels and hillslopes interact to set the long-term form and evolution of boulder-influenced landscapes? 5) How do boulders contribute to geomorphic hazards, and how might improved understanding of boulder dynamics be used for geohazard mitigation? Boulders are produced on eroding hillslopes by landsliding, rockfall, and/or exhumation through the critical zone. On hillslopes dominated by local sediment transport, boulders affect hillslope soil production and transport processes such that the downslope boulder size distribution sets the form of steady-state hillslopes. Hillslopes dominated by nonlocal sediment transport are less likely to exhibit boulder controls on hillslope morphology as boulders are rapidly transported to the hillslope toe. Downslope transport delivers boulders to eroding rivers where the boulders act as large roughness elements that change flow hydraulics and the efficiency of erosion and sediment transport. Over longer timescales, river channels adjust their geometry to accommodate the boulders supplied from adjacent hillslopes such that rivers can erode at the baselevel fall rate given their boulder size distribution. The delivery of boulders from hillslopes to channels, paired with the channel response to boulder delivery, drives channel-hillslope feedbacks that affect the transient evolution and steady-state form of boulder-influenced landscapes. At the event scale, boulder dynamics in eroding landscapes represent a component of geomorphic hazards that can be mitigated with an improved understanding of the rates and processes associated with boulder production and mobility. Opportunities for future work primarily entail field-focused data collection across gradients in landscape boundary conditions (tectonics, climate, and lithology) with the goal of understanding boulder dynamics as one component of landscape self-organization. • We synthesize work to date exploring how boulders affect the evolution of nonglaciated, eroding hillslopes and channels. • Boulder dynamics are one facet of landscape self-organization, influencingtransient adjustment and steady-state form. • The effects of boulders on landscape shape can be quantified by combining field data with simplified theoretical models. • The key research need is for more field data on boulder dynamics spanning gradients in boundary conditions. [ABSTRACT FROM AUTHOR]

Two areas on Mars in the old Thaumasia region were analysed, where interaction between fluvial and tectonic landforms provided conditions to improve age estimation for valley network formation. An Earth analog terrain was also involved in this work at Villány Hills (Hungary) for reference and to see the relations in tectonic-fluvial interaction. On Mars such interaction was observed by morphology (parallel curvatures of neighbour valleys) and by the comparison of statistical analyses of the fluvial and tectonic lineaments' azimuthal orientation. While in the case of Mars the fluvial network elements' distribution closely follows that of the tectonic elements' distribution, in the case of the Earth the connection is weaker. The comparison gives an interesting example for tectonic landforms controlling fluvial valley orientations, occasionally producing resembling features in valley tracks, influencing the sediment transport and the locations of deposits. Because the tectonic faults crossed a much larger area than the fluvial valleys they influenced, crater size frequency distribution based ages could be better measured with them. Using this approach, we found the minimal age of the fluvial network formation to be around 3.7 Ga (or up to 3.9 Ga). This result demonstrates that the small, scattered, and poorly integrated valleys might also have formed at the same period as the larger, branching systems with well determined ages: around the Noachian/Hesperian transition. Thus valley network formation even in the case of small, non-integrated systems might not necessarily extend into the early or middle Noachian ages and might have happened at a potential climatic optimum around 3.7 Ga ago. However the most difficult task remains the exact dating of old and small valleys at other locations on Mars, to confirm this possibility. • Fluvial-tectonic interaction was analysed on Mars and Earth. • Characteristic lineament orientation distribution was found at both sample sites. • Interaction between tectonic faults and fluvial channels were identified on Mars. • Channels followed more accurately the preformed tectonic directions on Mars. • Calculated age for fluvial activity was 3.7 Ga. [ABSTRACT FROM AUTHOR]