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Abstract. The width of valleys and channels affects the hydrology, ecology,and geomorphic functionality of drainage networks. In many studies, thewidth of valleys and/or channels (W) is estimated as a power-law function ofthe drainage area (A), W=kcAd. However, in fluvial systemsthat experience drainage reorganization, abrupt changes in drainage areadistribution can result in valley or channel widths that are disproportionalto their drainage areas. Such disproportionality may be more distinguishedin valleys than in channels due to a longer adjustment timescale forvalleys. Therefore, the valley width–area scaling in reorganized drainagesis expected to deviate from that of drainages that did not experiencereorganization. To explore the effect of reorganization on valley width–drainage areascaling, we studied 12 valley sections in the Negev desert, Israel,categorized into undisturbed, beheaded, and reversed valleys. We found thatthe values of the drainage area exponents, d, are lower in the beheadedvalleys relative to undisturbed valleys but remain positive. Reversedvalleys, in contrast, are characterized by negative d exponents, indicatingvalley narrowing with increasing drainage area. In the reversed category, wealso explored the independent effect of channel slope (S) through theequation W=kbAbSc, which yieldednegative and overall similar values for b and c. A detailed study in one reversed valley section shows that the valleynarrows downstream, whereas the channel widens, suggesting that, ashypothesized, the channel width adjusts faster to post-reorganizationdrainage area distribution. The adjusted narrow channel dictates the widthof formative flows in the reversed valley, which contrasts with the meaningfullywider formative flows of the beheaded valley across the divide. Thisdifference results in a step change in the unit stream power between thereversed and beheaded channels, potentially leading to a “width feedback”that promotes ongoing divide migration and reorganization. Our findings demonstrate that valley width–area scaling is a potential toolfor identifying landscapes influenced by drainage reorganization. Accountingfor reorganization-specific scaling can improve estimations of erosion ratedistributions in reorganized landscapes. 

Abstract. The location of drainage divides sets the distribution of discharge, erosion, and sediment flux between neighboring basins and may shift through time in response to changing tectonic and climatic conditions. Major divides commonly coincide with ridgelines, where the drainage area is small and increases gradually downstream. In such settings, divide migration is attributed to slope imbalance across the divide that induces erosion rate gradients. However, in some tectonically affected regions, low-relief divides, which are also called wind gaps, abound in elongated valleys whose drainage area distribution is set by the topology of large, potentially avulsing side tributaries. In this geometry, distinct dynamics and rates of along-valley wind-gap migration are expected, but this process remains largely unexplored. Inspired by field observations, we investigate along-valley wind-gap migration by simulating the evolution of synthetic and natural landscapes, and we show that confluences with large side tributaries influence migration rate and extent. Such confluences facilitate stable wind-gap locations that deviate from intuitive expectations based on symmetry considerations. Avulsions of side tributaries can perturb stable wind-gap positions, and avulsion frequency governs the velocity of wind-gap migration. Overall, our results suggest that tributaries and their avulsions may play a critical role in setting the rate and extent of wind-gap migration along valleys and thus the timescale of landscape adjustment to tectonic and climatic changes across some of the tectonically most affected regions of Earth, where wind gaps are common. 

Abstract Rapid ice loss is facilitated by sliding over beds consisting of reworked sediments and erosional products, commonly referred to as till. The dynamic interplay between ice and till reshapes the bed, creating landforms preserved from past glaciations. Leveraging the imprint left by past glaciations as constraints for projecting future deglaciation is hindered by our incomplete understanding of evolving basal slip. Here, we develop a continuum model of water-saturated, cohesive till to quantify the interplay between meltwater percolation and till mobilization that governs changes in the depth of basal slip under fast-moving ice. Our model explains the puzzling variability of observed slip depths by relating localized till deformation to perturbations in pore-water pressure. It demonstrates that variable slip depth is an inherent property of the ice-meltwater-till system, which could help understand why some paleo-landforms like grounding-zone wedges appear to have formed quickly relative to current till-transport rates. 

Many subglacial environments consist of a fine‐grained, deformable sediment bed, known as till, hosting an active hydrological system that routes meltwater. Observations show that the till undergoes substantial shear deformation as a result of the motion of the overlying ice. The deformation of the till, coupled with the dynamics of the hydrological system, is further affected by the substantial strain rate variability in subglacial conditions resulting from spatial heterogeneity at the bed. However, it is not clear if the relatively low magnitudes of strain rates affect the bed structure or its hydrology. We study how laterally varying shear along the ice‐bed interface alters sediment porosity and affects the flux of meltwater through the pore spaces. We use a discrete element model consisting of a collection of spherical, elasto‐frictional grains with water‐saturated pore spaces to simulate the deformation of the granular bed. Our results show that a deforming granular layer exhibits substantial spatial variability in porosity in the pseudo‐static shear regime, where shear strain rates are relatively low. In particular, laterally varying shear at the shearing interface creates a narrow zone of elevated porosity which has increased susceptibility to plastic failure. Despite the changes in porosity, our analysis suggests that the pore pressure equilibrates near‐instantaneously relative to the deformation at critical state, inhibiting potential strain rate dependence of the deformation caused by bed hardening or weakening resulting from pore pressure changes. We relate shear variation to porosity evolution and drainage element formation in actively deforming subglacial tills.