Water discharge and sediment flux variations are important parameters controlling the morphodynamic behaviour of rivers. Although quantitative estimates for water discharge and sediment flux variability are well‐constrained for modern rivers, far fewer assessments of flow and sediment flux intermittency in ancient fluvial systems from the rock record are available. In this study, a relationship between water discharge, sediment flux variability and patterns of changing fluvial stratigraphic architecture in the Middle Eocene Escanilla Formation, Spain, is explored. Water discharge intermittency factor (IWF), calculated as a ratio of the total water discharge (over the averaging time period) to the instantaneous channel‐forming water discharge if sustained for the same period, ranges from 0.03 to 0.11 in the high amalgamation intervals and from 0.10 to 0.32 in the low amalgamation intervals. Similarly, the sediment flux intermittency factor (ISF) is estimated to be in the range of 0.008 to 0.01 in the high amalgamation intervals and of 0.01 to 0.03 in the low amalgamation intervals. Consequently, high amalgamation intervals were most probably deposited under more intermittent and short‐lived intense precipitation events while low amalgamation intervals were the result of less intermittent flows spread throughout the year. Overall, these estimates are consistent with values from modern ephemeral rivers typically found in arid to semi‐arid climate and is in agreement with available proxy data for the Middle Eocene climatic context of the studied alluvial system. This highlights an important connection between hydroclimate, river morphodynamics and landscape evolution, and has implications to predict river flow and sediment transport across the Earth's surface in the geological past.

Water discharge and sediment flux intermittency in the fluvial Escanilla Formation, Spain: Implications for changes in stratigraphic architecture

Large wood drives both the form and function of gravel‐bed rivers draining forested basins. Previously overlooked benefits of wood in rivers are now widely recognized. Together with flow and sediment regimes, the wood regime controls rivers' physical and ecological integrity. Yet large quantities of wood transported during floods can pose additional hazards, potentially damaging infrastructures like bridges or dams and exacerbating flooding. However, unlike the water and sediment regimes intensively studied over the past decades, the instream wood regime or budgeting has been only recently defined and thus is still rarely quantified. The instream wood budget describes the cascading processes from supply or recruitment, entrainment, and transport to deposition, storage and decay (i.e., fragmentation or decomposition). These processes show high spatial and temporal variability but can be characterized by magnitude, frequency, timing, duration and mode. Instream wood budgeting is challenging, primarily because of the lack of observations, monitoring stations, and standardized protocols to acquire data. This contribution reviews the most recent advances to quantify the different instream wood budget components, notably the wood supply, and transfer. Case studies showing applications of biogeochemistry, videography, artificial intelligence, numerical modelling or tracking illustrate the current progress. Because critical challenges remain, we identify and describe some of them and discuss how the wood in riverine sciences may develop in the future.

Current progress in quantifying and monitoring instream large wood supply and transfer in rivers

Boulders are globally widespread and influence processes across many landscapes including hillslopes, coasts, rivers and extra‐terrestrial settings. Boulders are described as particles, sufficiently large, that they have a disproportionate effect on the surrounding landscape. Moving beyond this conceptual definition, however, requires a somewhat arbitrary decision of how to define a minimum boulder size. The implications of boulder definition on study findings are rarely considered. We investigate the suitability of five boulder definitions, two based on fixed sizes: (1) 0.26 m and (2) 1 m, and three definitions which vary based on system characteristics: (3) grain mobility, (4) grain protrusion and (5) surface grain size distribution (> 84th percentile, D84). We consider the impact of definition on calculated boulder metrics, and, for the >1m and >D84 definitions, their association with channel and catchment characteristics across 20 boulder‐bed streams in northern Sweden. We also surveyed river managers responsible for restoring these rivers, to gain a practitioner insight on boulder size definition. We found that boulder definition matters; for metrics relating to the number or density of boulders, the >D84 and >1m size definitions were negatively correlated. Surveys indicated the importance of communicating boulder definition. We conclude that, whilst the best choice of boulder size definition will vary based on the questions of interest and techniques employed, evaluating the implications of the chosen boulder size definition and communicating the reasoning behind boulder definition choice is crucial.

How big is a boulder? The importance of boulder definition choice in earth science research and river management

Boulders are globally widespread and influence landscape processes across hillslopes, coasts, rivers and extra-terrestrial settings. Boulders are described as particles, sufficiently large, that the movement of an individual grain promotes substantial geomorphic change. Moving beyond this conceptual definition, however, requires a somewhat arbitrary decision of how to define a minimum boulder size. Furthermore, the implications of boulder definition on study findings are rarely considered. We compare two lower thresholds for boulder size; a fixed boulder minimum diameter (&gt; 1 m) and a variable diameter relative to the surface grain size distribution (&gt; 84th percentile). We consider the impact of definition on measured boulder metrics, and their association with channel and catchment characteristics across 20 boulder-bed streams in northern Sweden. We also surveyed the river managers responsible for restoring these rivers, to gain a practitioner insight on boulder size definition. Definition choice resulted in fundamental differences in boulder metrics; metrics describing the number or density of boulders were negatively correlated. Using these two studies, we explore boulder definition in earth sciences, including the application of fixed definitions and those relative to grain size or system power. We emphasise the importance of evaluating the implications of the chosen boulder size definition, and communicating the reasoning behind boulder definitions and these implications. We discuss the implication of boulder size definition choice and provide guidelines for future studies seeking a process-based definition of boulders. 

pdf/how-big-is-a-boulder-evaluating-fixed-and-process-based-2y76lfclra.pdf

How big is a boulder? Evaluating fixed and process-based definitions for boulders

Water discharge and sediment flux variations are important parameters controlling the morphodynamical behavior of rivers. Although quantitative estimates for discharge and flux variability are well constrained for modern rivers, far fewer assessments of flow and sediment flux intermittency in ancient fluvial systems from the rock record are available. In this study we explore the relationship between water discharge, sediment flux variability and patterns of changing fluvial stratigraphic architecture in the Middle Eocene Escanilla Formation, Spain. We estimate water discharge intermittency factor (IWF) to range from 0.03 – 0.11 in the HA intervals and from 0.10 – 0.32 in the LA intervals. Sediment flux intermittency factor (ISF) is in the range of 0.008 – 0.01 in the HA intervals and from 0.01 – 0.03 in the LA intervals. Consequently, we suggest that high amalgamation (HA) intervals were most likely deposited under more intermittent and short-lived intense precipitation events, while low amalgamation (LA) intervals were the result of less intermittent flows spread throughout the year. Overall, our estimates are consistent with values from modern ephemeral rivers typically found in arid to semi-arid climatic conditions, which is in agreement with available proxy data for the Middle Eocene climatic context of the studied alluvial system. Our data highlights the important connection between hydroclimate, river morphodynamics and landscape evolution, and has implications to predict river flow and sediment transport across the Earth’s surface in the geological past. 

/pdf/water-discharge-and-sediment-flux-intermittency-in-the-1c6v2ct6.pdf

Abstract. Bedload transport, entrainment of coarse sediment by a river, is inherently a stochastic and intermittent process whose monitoring remains challenging. Here, we propose a new method to characterize bedload transport in the field. Using an uncrewed aerial vehicle (UAV) equipped with a high-resolution camera, we recorded yearly images of a bar of the Grande Rivière des Vieux-Habitants, a gravel bed river located on Basse-Terre Island (Guadeloupe, French West Indies). These images, combined with high-frequency measurements of the river discharge, allow us to monitor the evolution of the population of sediments of a diameter between 0.5 and 0.75 m on the riverbed. Based on this dataset, we estimate the smallest discharge that can move these boulders and calculate the duration of effective transport. We find that the transport of boulders occurs for approximately 10 h yr−1. When plotted as a function of the effective transport time, a given population of boulders decreases exponentially with an effective residence time of approximately 17 h. This exponential decay suggests that the probability of dislodging a grain from the bed is proportional to the number of grains at repose on the bed, an observation consistent with laboratory experiments. Finally, the residence time of bedload particles on a riverbed can be used to evaluate bedload discharge. 

Entrainment and deposition of boulders in a gravel bed river

&amp;lt;div&amp;gt; &amp;lt;div&amp;gt; &amp;lt;div&amp;gt; &amp;lt;div&amp;gt; &amp;lt;p&amp;gt;Understanding how rivers adjust to the sediment load they carry is critical to predicting the evolution of landscapes. Presently, however, no physically based model reliably captures the dependence of basic river properties, such as its shape or slope, on the discharge of sediment, even in the simple case of laboratory rivers. Here, we show how the balance between fluid stress and gravity acting on the sediment grains, along with cross-stream diffusion of sediment, determines the shape and sediment flux profile of laminar laboratory rivers that carry sediment as bedload. Using this model, which reliably reproduces the experiments without any tuning, we confirm the hypothesis, originally proposed by G. Parker (1978), that rivers are restricted to exist close to the threshold of sediment motion (within about 20%). This limit is set by the fluid&amp;amp;#8211;sediment interaction and is independent of the water and sediment load carried by the river. Thus, as the total sediment discharge increases, the intensity of sediment flux (sediment discharge per unit width) in a river saturates, and the river can transport more sediment only by widening. In this large discharge regime, the cross-stream diffusion of momentum in the flow permits sediment transport. Conversely, in the weak transport regime, the transported sediment concentrates around the river center without significantly altering the river shape. If this theory holds for natural rivers, the aspect ratio of a river could become a proxy for sediment discharge&amp;amp;#8212;a quantity notoriously difficult to measure in the field.&amp;lt;/p&amp;gt; &amp;lt;/div&amp;gt; &amp;lt;/div&amp;gt; &amp;lt;/div&amp;gt; &amp;lt;/div&amp;gt; 

Sediment load determines the shape of rivers&amp;#160;

Abstract. We develop a phenomenological model of suspended-sediment transport on the basis of data acquired in the Capesterre river, which drains a small tropical catchment in Guadeloupe. The model correctly represents the transport of suspended sediment during floods, provided that the relation between concentration and water-level forms a counterclockwise loop. In the model, the properties of the sediment and of the river are all lumped into four parameters: a settling velocity related to the size of the suspended sediment, a threshold water-level which acts as a proxy for the threshold shear stress, a characteristic erosion rate and a dimensionless exponent, both of which are related to the availability of fine sediment. The assimilation of field data to our model shows that the value of the parameters change from one flood to the next, probably reflecting changes in the characteristics of the river and the sediment. Finally, a test of the model against data acquired in a small catchment in the french Alps, suggests that the model is versatile enough to be used in diverse hydrological settings. 

/pdf/phenomenological-model-of-suspended-sediment-transport-in-a-2tjedh1u.pdf

Phenomenological model of suspended sediment transport in a small tropical catchment

&amp;lt;div&amp;gt; &amp;lt;div&amp;gt; &amp;lt;div&amp;gt;Seepage erosion occurs when groundwater emerges at the surface of a granular heap. A&amp;lt;/div&amp;gt; &amp;lt;div&amp;gt;spring forms and feeds a river which entrains sediments, thus changing the groundwater&amp;lt;/div&amp;gt; &amp;lt;div&amp;gt;flow.&amp;lt;/div&amp;gt; &amp;lt;div&amp;gt;&amp;amp;#160;&amp;lt;/div&amp;gt; &amp;lt;div&amp;gt;We reproduce this phenomenon in the laboratory using a quasi-2D aquifer filled with glass&amp;lt;/div&amp;gt; &amp;lt;div&amp;gt;beads, by imposing a water level at one end of the pile. Water flows through the aquifer and&amp;lt;/div&amp;gt; &amp;lt;div&amp;gt;emerges at the surface of the granular bed. For large enough water levels this river erodes&amp;lt;/div&amp;gt; &amp;lt;div&amp;gt;its bed and the spring progressively ascends the heap. We investigate its trajectory, the&amp;lt;/div&amp;gt; &amp;lt;div&amp;gt;evolution of the groundwater discharge and the river depth. Intriguingly, we find that after an&amp;lt;/div&amp;gt; &amp;lt;div&amp;gt;initial erosive period the river attains a new equilibrium profile, with an elevated spring.&amp;lt;/div&amp;gt; &amp;lt;div&amp;gt;&amp;amp;#160;&amp;lt;/div&amp;gt; &amp;lt;div&amp;gt;We model the flow in the aquifer using Darcy's law, predicting the shape of the water table,&amp;lt;/div&amp;gt; &amp;lt;div&amp;gt;the position of the spring and the groundwater discharge. By applying Coulomb&amp;amp;#8217;s frictional&amp;lt;/div&amp;gt; &amp;lt;div&amp;gt;law to the forces experienced by a grain we predict a threshold for the onset of erosion as a&amp;lt;/div&amp;gt; &amp;lt;div&amp;gt;function of reservoir height and aquifer length. This prediction provides a dynamical theory&amp;lt;/div&amp;gt; &amp;lt;div&amp;gt;for the erosional dynamics of the river. Our combined theoretical and experimental approach&amp;lt;/div&amp;gt; &amp;lt;div&amp;gt;thereby helps constrain the response of an idealized erosive river-catchment system to&amp;lt;/div&amp;gt; &amp;lt;div&amp;gt;steady forcing.&amp;lt;/div&amp;gt; &amp;lt;/div&amp;gt; &amp;lt;/div&amp;gt; 

Erosional dynamics of a river driven by groundwater seepage

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Entrainment and deposition of boulders in a gravel bed river

Sediment load determines the shape of rivers&#160;

Phenomenological model of suspended sediment transport in a small tropical catchment

Erosional dynamics of a river driven by groundwater seepage

M. A. Chemakos

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Entrainment and deposition of boulders in a gravel bed river

Sediment load determines the shape of rivers&amp;#160;

Phenomenological model of suspended sediment transport in a small tropical catchment

Erosional dynamics of a river driven by groundwater seepage

Sediment load determines the shape of rivers 