The Hohokams who settled in what is today Phoenix area had probably the largest canal system in Native North America. Their culture and spread is more and more documented as more and more people settle in the Phoenix region and they keep digging up evidence of Hohokam settlement and culture. They used the Canals to irrigate vast fields where they grew corn, beans, tobacco and etc.. There are various claims to their population as well, some reach up to 300,000 while more moderate estimation is around 130,000 But most archaeologists today agree on around 30-45,000. We have hundreds of thousands of artifacts left behind by the Hohokams as well as new archaeological spots are constantly being found with the development of Phoenix area. Whats personally most amazing to me is that a group of people lived in what is today one of the hottest (and probably unlivable without air conditioning), long before us and even flourished culturally. I'm not sure how the Hohokams compare to mound builders of Mississippi, but I would say that the Hohokams had a pretty sophisticated culture as with Anasazi.

Source: The Ecological Indian