Sediment supply is a fundamental control on the stratigraphic record. However, a key question is the extent to which climate affects sediment fluxes in time and space. To address this question, estimates of sediment fluxes can be compared with measured sediment volumes within a closed basin that has wellconstrained tectonic boundary conditions and well-documented climate variability. The Corinth rift, central Greece, is one of the most actively extending basins on Earth, with modern-day GPS extension rates of up to 15 mm/yr. The Gulf of Corinth forms a closed system, and since ca. 600 ka, the gulf has fluctuated between marine and lacustrine. We esti mated suspended sediment fluxes for rivers draining into the Gulf of Corinth using the empirically derived BQART method over the last interglacial-glacial-interglacial cycle (0-130 k.y.). Modern temperature and precipitation data sets, Last Glacial Maximum reconstructions, and paleoclimate proxy insights were used to constrain model inputs. Simultaneously, we exploited high-resolution two-dimensional seismic surveys to interpret three seismic units from 130 ka to present, and we used these data to derive an independent time series of basin sedimentary volumes to compare with our sediment input flux estimates. Our results predict total Holocene sediment fluxes into the Gulf of Corinth of between 19.2 km 3 and 23.4 km 3 , with a preferred estimate of 21.3 km 3. This value is a factor of 1.6 less than the measured Holocene sediment volume in the central depocenters, even without taking lithological factors into account, suggesting that the BQART method provides plausible estimates. Sediment fluxes vary spatially around the gulf, and we used them to derive minimum catchment-averaged denudation rates of 0.18-0.55 mm/yr. Significantly, our time series of basin sedimentary volumes demonstrate a clear reduction in sediment accumulation rates during the last glacial period compared to the current interglacial. This implies that Holocene sediment fluxes must have increased relative to Late Pleistocene times. Furthermore, BQART-derived sediment flux predictions indicate a 28% reduction in supply during the last glacial period compared to the Holocene; likewise, seismic sediment accumulation rate estimates indicate a similar magnitude of reduction (32%). At the Last Glacial Maximum, mean annual temperatures in the region were lower by 5 °C, but mean annual precipitation rates were broadly similar. We hypothesize that although weathering rates might be greater under glacial conditions, warmer interglacial temperatures may be more conducive to generating larger storms, which do more geomorphic work, driving greater sediment fluxes. Our results demonstrate that sediment export to the basin is sensitive to glacial-interglacial cycles, and we explore the potential mechanisms behind this sensitivity.

Considerable progress has been made in modelling the response of rivers to tectonic perturbation in order to decode the tectonic signals embedded in river long profiles and planform geometry. Whilst studies have showed the importance of rock type on the morphology of rivers responding to tectonics on a local scale, these effects are often not captured in landscape evolution models. In fact, current models of fluvial response to tectonic perturbation such as active faulting require carefully collected data sets to fully constrain or calibrate key parameters, including the effect of bedrock lithology on substrate erodibility and timescales for tectonic signal propagation in bedrock river systems. Here we constrain the role of bedrock in controlling fluvial incision for a 240 km 2 catchment draining into the Gulf of Corinth, which has excellent tectonic constraints and a variety of bedrock lithologies. An active normal fault at the downstream end of the catchment (the East Eliki Fault) is known to have initiated at 0.7 Ma, with average Quaternary uplift and incision rates of 1.00-1.25 mm/yr. The initiation of the East Eliki Fault is recorded in the river as a prominent knickzone 7-16 km upstream of the fault. Detailed field data collected within this catchment at 500 m intervals along the main channel length at this tectonically wellconstrained field site show an order of magnitude increase in river channel slopes (y/x = 0.02 to 0.18) and stream powers (1 kWm -2 to 27 kWm -2 ) at the lithological boundary between weak and resistant bedrock. The weak conglomerates and strong limestones have Schmidt hammer compressive strengths of ca. 30 and 50 respectively, based on in-situ rock strength measurements. Consequent erodibility values of 1.8 ± 0.3 × 10 -14 and 5 -6 ± 2 × 10 -15 ms 2 kg -1 show a factor of three to four decrease in erodibility for a two-fold increase in rock strength. A simple simulation of tectonic signal propagation through the river catchment based on these erodibility values indicates that the observed plan view position of the knickpoint is consistent with our calculated knickpoint positions based on erodibility values derived from incision rate and stream power data. However, the tectonic signal associated with faulting would have propagated completely through the catchment within 1 Ma if it were entirely composed of weak rock for similar climatic and tectonic conditions, indicating that lithology has a major control on landscape response times.Our results help constrain the effect of lithology on river channel geometry and allow erosional parameters to be derived that are crucial for effective modelling of tectonic rates from topography.

Self-similar downstream grain-size fining trends in fluvial deposits are being increasingly used to simplify equilibrium sediment transport dynamics in numerical models. Their ability to collapse time-averaged behavior of a depositional system into a simple mass balance framework makes them ideal for exploring the sensitivity of sediment routing systems to their climatic and tectonic boundary conditions. This is important if we want to better understand the sensitivity of landscapes to environmental change over timescales >10 2 years. However, the extent to which self-similarity is detectable in the deposits of natural rivers is not fully constrained. In transport-limited rivers, stored sediment can be remobilized or "recycled" and this behavior has been highlighted as a mechanism by which externally forced grain-size fining trends are distorted. Here we evaluate evidence of self-similarity in surface gravel-size distributions on three geomorphically diverse alluvial fans in the Iglesia basin, south Central Argentine Andes. We find that size distributions are self-similar, deviating from that condition only when significant variability occurs in the coarse tails of the distributions. Our analysis indicates a strong correlation between the degree of sediment recycling and the proportion of coarse clasts present on the bed surface. However, by fitting a relative mobility transfer function, we demonstrate that size-selectivity alone can explain the bulk size distributions observed. This strengthens the application of self-similar grain size fining models to solving problems of mass balance in a range of geomorphic settings, with an aim for reconstructing environmental boundary conditions from stratigraphy.Plain Language Summary To date, little research has been conducted into how sensitively rivers respond to changes in their sediment and water supplies. This is because river processes that control how and when sediment moves are complex and evolve in response to landscape change over a wide range of time and spatial scales. Using a unique field site in the Argentine Andes, we find evidence for an important river behavior that can simplify the complexity of physical river processes in space and time. Each of three rivers investigated had sorted the gravel they transport by size, both locally on the river bed and along their downstream length. We found that at any site along the river, the degree to which the gravel is sorted is the same. This means that taking into account the change in sediment size downstream, the underlying grain size distributions are all the same. As size sorting is related to the efficiency of the river to transport its sediment load, we can use this finding to reconstruct the mobility of sediment anywhere along the river. We do this by fitting a simple and elegant transfer function to our data, which can be incorporated into models of long-term river evolution.

The volume and grain-size of sediment supplied from catchments fundamentally control basin stratigraphy. Despite their importance, few studies have constrained sediment budgets and grain-size exported into an active rift at the basin scale. Here, we used the Corinth Rift as a natural laboratory to quantify the controls on sediment export within an active rift. In the field, we measured the hydraulic geometries, surface grain-sizes of channel bars and full-weighted grain-size distributions of river sediment at the mouths of 47 catchments draining the rift (constituting 83% of the areal extent). Results show that the sediment grain-size increases westward along the southern coast of the Gulf of Corinth, with the coarse-fraction grain-sizes (84 th percentile of weighted grain-size distribution) ranging from approximately 19 to 91 mm. We find that the median and coarse-fraction of the sieved grain-size distribution are primarily controlled by bedrock lithology, with late Quaternary uplift rates exerting a secondary control. Our results indicate that grain-size export is primarily controlled by the input grain-size within the catchment and subsequent abrasion during fluvial transport, both quantities that are sensitive to catchment lithology. We also demonstrate that the median and coarse-fraction of the grain-size distribution are predominantly transported in bedload; however, typical sand-grade particles are transported as suspended load at bankfull conditions, suggesting disparate source-to-sink transit timescales for sand and gravel. Finally, we derive both a full Holocene sediment budget and a grain-size-specific bedload discharged into the Gulf of Corinth using the grain-size measurements and previously published estimates of sediment fluxes and volumes. Results show that the bedload sediment budget is primarily comprised (~79%) of pebble to cobble grade (0.475-16 cm). Our results suggest that the grainsize of sediment export at the rift scale is particularly sensitive to catchment lithology | 1601 EAGE WATKINS eT Al.

How information about sediment transport processes is transmitted to the sedimentary record remains a complex problem for the interpretation of fluvial stratigraphy. Alluvial fan deposits represent the condensed archive of sediment transport, which is at least partly controlled by tectonics and climate. For three coupled catchment‐fan systems in northern Death Valley, California, we measure grain size across 12 well‐preserved Holocene and late‐Pleistocene surfaces, mapped in detail from field observations and remote sensing. Our results show that fan surfaces correlated to the late Pleistocene are, on average, 30–50% coarser than active or Holocene fan surfaces. We adopt a self‐similar form of grain size distribution based on the observed stability of the ratio between mean grain size and standard deviation downstream. Using statistical analysis, we show that fan surface grain size distributions are self‐similar. We derive a relative mobility function using our self‐similar grain size distributions, which describes the relative probability of a given grain size being transported. We show that the largest mobile grain sizes are between 20 and 35 mm, a value that varies over time and is clearly lower in the Holocene than in the Pleistocene; a change we suggest is due to a drier climate in the Holocene. These results support recent findings that alluvial fan sedimentology can record past environmental change and that these landscapes are potentially sensitive to climatic change over a glacial‐interglacial cycle. We demonstrate that the self‐similarity methodology offers a means to explore changes in relative mobility of grain sizes from preserved fluvial deposits.

Rivers can abruptly shift pathways in rare events called avulsions, which cause devastating floods. The controls on avulsion locations are poorly understood as a result of sparse data on such features. We analyzed nearly 50 years of satellite imagery and documented 113 avulsions across the globe that indicate three distinct controls on avulsion location. Avulsions on fans coincide with valley-confinement change, whereas avulsions on deltas are primarily clustered within the backwater zone, indicating a control by spatial flow deceleration or acceleration during floods. However, 38% of avulsions on deltas occurred upstream of backwater effects. These events occurred in steep, sediment-rich rivers in tropical and desert environments. Our results indicate that avulsion location on deltas is set by the upstream extent of flood-driven erosion, which is typically limited to the backwater zone but can extend far upstream in steep, sediment-laden rivers. Our findings elucidate how avulsion hazards might respond to land use and climate change.

River deltas grow through repeated lobe-scale avulsions, which often occur at a location that correlates with the backwater lengthscale. Competing hypotheses attribute the avulsion node origin to either the morphodynamic feedbacks caused by natural flood discharge variability (backwater hypothesis) or to the prograding delta lobe geometry (geometric hypothesis). Here, using theory, historical flood records, and remotely sensed elevation data, we analyzed five lobe-scale delta avulsions in Madagascar, captured by Landsat imagery. Avulsion lengths were 5-55 km, distances significantly longer than the backwater lengthscale and inconsistent with the geometric hypothesis. We show that the steep, silt-bedded rivers of Madagascar have flood-induced bed scour, driven by backwater hydrodynamics, that propagates farther upstream than the backwater lengthscale. The avulsion lengths are 3.1 ± 1.5 times the predicted flood scour lengths, similar to low-gradient deltas, and consistent with backwater hypothesis. Results demonstrate that erosion initiated by nonuniform flows in the backwater zone is a primary control on delta avulsion locations. Plain Language Summary River deltas grow through abrupt channel shifts, called avulsions, which pose a threat to life and property, but we do not understand why avulsions occur where they do. One hypothesis is that flood discharge variability creates a preferential zone of sediment accumulation within the so-called backwater zone of coastal rivers, which becomes the locus of avulsion. The rationale is that the flow acceleration and deceleration within the backwater zone during different sized floods creates nonoverlapping patterns of erosion and deposition resulting in a peak in sediment accumulation. If this hypothesis is correct, then avulsion sites should be farther upstream, relative to the backwater lengthscale, in steep rivers with fine-grained sediment beds that have more significant bed scour during large floods. By analyzing historical avulsions on Madagascar, we found that avulsions occurred far upstream due to the large floods and fine-grained sediment, in support of the flood discharge variability hypothesis. Our work highlights that changes in flood regime and sediment grain size of coastal rivers due to land use and climate changes can influence avulsion location.

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