I believe the answer you're looking for is Southwest Asia, looking at a climate map, the drier areas are located to the southwest.
A river's width and depth increases as more water is added from tributaries.
<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>
I would have to go with Sewage mostly because it smells and because it can really hurt the environment. Plants can die and it just is gross to smell. So this would definitely pollute the earth. I hope this helped! :D
