The semi-arid and arid areas in the western part of the United States are harsh environments, and only specialized plants for these conditions can survive. The climate is hot and dry, the precipitation very low, and the soil is poor. The plants that survive in these places have adapted well to survive with very little water, to manage to extract water from the air, to preserve it, and being able to sustain very high temperatures. The short grasses are the dominant plants int he semi arid regions. They do not require rich soil, nor lot of water, so they manage to survive on the bear minimum that is available. The trademark plants of the arid areas are the cacti. The cacti are able to store large amounts of water in them, have very long roots, are able to extract water from the air through their needles, and also have skin that doesn't allow any water to come out of it.
<span>Erosion by rivers is a main valley-forming
process; other processes, such as movement of the earth's crust and
glaciers, also have an important part in some cases. The rate at which a
river deepens its valley depends on several factors.</span>
