of the river. In order for the development of these structures to occur the erosion rate of the river must exceed both the average erosional rate of the area, and the rate of uplift of the orogen. The two factors influencing the development of these structures are stream power of the associated river and the flexural rigidity of the crust in the area. The combination of increased stream power with decreased flexural rigidity results in the system's progression from a transverse anticline to a river anticline. Channel flow Channel flow describes the process through which hot, viscous crustal material flows horizontally between the upper crust and lithospheric mantle, and is eventually pushed to the surface. This model aims to explain features common to metamorphic hinterlands of some collisional orogens, most notably the Himalaya–Tibetan Plateau system. In mountainous areas with heavy rainfall (thus, high erosion rates) deeply incising rivers will form. As these rivers wear away the Earth's surface two things occur: (1) pressure is reduced on the underlying rocks effectively making them weaker and (2) the underlying material moves closer to the surface. This reduction of crustal strength, coupled with the erosional exhumation, allows for the diversion of the underlying channel flow toward Earth's surface. Erosional processes The term erosion refers to the group of natural processes, including weathering, dissolution, abrasion, corrosion, and transportation, by which material is worn away from Earth's surface to be transported and deposited in other locations. Differential erosion – Erosion that occurs at irregular or varying rates, caused by the differences in the resistance and hardness of surface materials; softer and weaker rocks are rapidly worn away, whereas harder and more resistant rocks remain to form ridges, hills, or mountains. Differential erosion, along with the tectonic setting, are two of the most important controls on the evolution of continental landscapes on Earth. The feedback of erosion on tectonics is given by the transportation of surface, or near-surface, mass (rock, soil, sand, regolith, etc.) to a new location. This redistribution of material can have profound effects on the state of gravitational stresses in the area, dependent on the magnitude of mass transported. Because tectonic processes are highly dependent on the current state of gravitational stresses, redistribution of surface material can lead to tectonic activity. While erosion in all of its forms, by definition, wears away material from the Earth's surface, the process of mass wasting as