Which describes how alluvial fans form? Group of answer choices After exiting mountain canyons streams speed up and deposit alluvium as they flow out onto a valley After exiting mountain canyons streams slow down and erode alluvium as they flow out onto a valley After exiting mountain canyons streams speed up and erode alluvium as they flow out onto a valley After exiting mountain canyons streams slow down as they spread out and flow onto a relatively flat valley floor, depositing alluvium