Wikipedia (read the first paragraph to find out which states are included in this region) There's a lot of good information here, but you need to read the section headings to decide which parts you need to read more. There are good sections on climate, vegetation, and landforms (they call that "Regional Geography."
World Book Kids (This site gives information about each state. If you read about the different states in this region, you can start to put together the puzzle of what the region is like.)
Scroll down to find Britannica School Elementary Near the bottom tap "Tour the USA." (Double click on a state to get more info. about it. Then tap "summary" and "read article" to get some more tabs with more great information.) Easy to read! If you need even more information, change the reading level to 2 to get more (tap on the purple square with a 2 in it near the top-center of the screen).
Scroll down to find Kid InfoBits Tap on Geography. Tap on U.S. States. (Use this site to research individual states.)
The Southwest (this site tells about a few different regions, so be sure you are reading the section about the Southwest)
The Southwest Border Area (this site is quite advanced level reading, but if you scroll down to the Population Growth section, you can find out why so many people move there)
US Regions and States This site tells about a variety of regions, so be sure you find the section about the Southwest.