Significance: The bends of alluvial meandering rivers often double back on themselves, showing skewing. This skewing may be directed upstream or downstream. How skewing evolves as bends develop remains incompletely understood. Our analysis shows that, on 20 reaches of nearly pristine alluvial meandering rivers, downstream skewing dominates when the bends are relatively straight, but upstream skewing increasingly dominates as bend sinuosity increases. This provides a guide for interpreting bend evolution and offers a useful comparison with numerical models. The results suggest that rivers often carry an imprint of the direction of the flow that created them, through the shape of their high-amplitude bends and neck cutoffs. This provides a reference tool for estimating paleoflow direction on Earth and other planets.

Floodplains of meandering rivers are colonized with various plant species that differ in how they stabilize streambanks, modulating sinuosity evolution. Here, we compile observations of meander migration from North and South America, categorizing channels based on the riparian vegetation as cropland, forest, grassland, and rainforest. Our analysis reveals that the most stable meanders are those developed in rainforests and the reason is likely related to their established root systems and clay‐rich soils. The most unstable meanders were found in cropland areas, which is explained by the lack of vegetation cover and the frequent land disturbances associated with cultivation. Rivers in grassland and North American forest environments have intermediate migration rates. We found that meanders in grasslands have lower migration rates than those in North American forests. The reason for this may lie in the fact that grassland meanders in general have higher sinuosity, lower gradients, and layered banks.

The spatio-temporal distribution of geological hazards, including collapses, landslides, and debris flows, in Shaanxi province, China was studied based on data from 1951 to 2018. The potential impact factors, including the geomorphologic types, rivers, roads, rainfall, and earthquakes, were analyzed using Random Forests. The results indicated that most hazards occurred in summer (i.e., July–September) and were triggered by rainstorms. The freeze–thaw effect had a considerable contribution to hazards in the north. Spatially, most hazards in the north occurred in valley terraces of the Loess Plateau, while medium-relief terrane (relief ranged from 500 to 1000 m) in the southern Qinling Mountains were hazard-prone areas. The collapses and landslides were mainly affected by human factors in Northern Shaanxi, whereas in Southern Shaanxi geomorphology was the primary factor. Permeability was a dominant factor for debris flows. In addition, the 2008 Wenchuan earthquake had a remarkable influence on the spatial distribution of hazards. In contrast, for the situation in the Sichuan province, which was close to the earthquake epicenter, the Wenchuan earthquake triggered many collapse and landslide events in the southwest regions of Shaanxi province only on 12 May 2008. The thresholds for the three hazard types in the north and south regions were almost the same despite their distinctly different geologic characteristics. Through a sensitivity analysis, we found an appropriate dry period of 12 h for the area.

This paper performs a numerical investigation on the kinetic behaviors of dry granular flows generated by the collapse of granular columns with different fractal particle size distributions (FPSDs) using the discrete element method. The FPSD has a remarkable impact on the regime and mobility of a granular flow. As the fractal dimension of a granular assembly increases, the translational velocities of the particles increase in the horizontal direction, whereas the rotational velocities decrease. The number of particles involved in the horizontal spreading increases due to the decreasing intensity of the contact shearing particle behavior. These increased translational velocities in the horizontal direction effectively facilitate particle spreading. A boundary layer is formed at the front bottom of the granular body in which the clockwise rotational velocities of particles are significantly increased, thereby benefiting particle spreading. The whole front of the granular flow obtains an immense spreading velocity. Moreover, the translational and rotational characteristics are dependent on the particle size. Smaller particles tend to translate and rotate faster than larger particles because of the greater particle contact forces of the former. As the fractal dimension increases, medium- and large-size particles obtain less kinetic energy than do small-size particles. Therefore, small-size particles play a crucial role in the mobility of granular flows in a granular assembly with a high fractal dimension.