Fan deposition at mountain fronts, avulsion, and lobe switching
Alluvial fans form where sediment-laden flows emerge from mountain canyons onto a piedmont. As the channel aggrades and its gradient steepens relative to adjacent areas, it eventually avulses — jumping to a new position on the fan surface. This lobe-switching creates the characteristic semicircular fan shape, with older lobes building the outer arc while the active lobe receives all deposition. Death Valley, Owens Valley, and the Himalayan piedmont are classic examples.