What if the Colorado River was Navigable?

What if the Colorado River was able to be navigated by large river barges that could carry the produce of the southwest to the pacific ocean?

This could probably have been accomplished through a series of locks and dams. If railroads had never been invented, it might well have been done.

However, what produce are we talking about here? Most of the produce of the Southwest is actually grown outside the Colorado Basin. There is a relatively significant stone-fruit industry around Grand Junction, Colorado (as far as I'm aware), and I suppose these would have no market without railroads (and eventually highways) to take them to Denver, Salt Lake City, Phoenix, Las Vegas, etc., so... yes, they would have been able to develop without railroads. Also, there's quite of bit of irrigated farming done far downstream, in the lowlands around Yuma and into southeastern California. I believe this part of the river is not navigable today simply because so much of the water is taken out for urban use before it reaches the lower portion of the river. So, that area would have developed as well, but conflict would have arisen with the needs of Las Vegas and etc. Railroads would have made the barges anachronistic (or at least superfluous), and the expensive water would have been bought up by the thirsty urban areas just like in OTL.

Hmmm... do you mean maybe further back? Would the Spanish have exploited these areas during the colonial period? I don't know... they had a lot of space to work with and not enough manpower to do it, which is why California was so thinly developed by the 1840s (the inland mineral wealth, gold in particular, hadn't even been noticed yet, despite the unending Spanish appetite for it).

No... let's start even further back... pre-Columbian times:
A major river valley civilization, like that of the Mound Builders (Kahokia, anyone?) in the Mississippi valley develops. A Native American Empire develops, and grows to rival those of the Valley of Mexico (Toltecs, later Aztecs). Cities of 10,000+ dot the Colorado Valley all the way from the Rockies to the Gulf of California. The people develop their own great boats like the giant canoes the Great Lakes peoples developed during the fur trade. Based on this efficient transportation of beans, corn, and etc, a capital city of 50,000 to 100,000 inhabitants can be sustained. The mineral wealth of the southwest is exploited, vast quantities of turquoise, gold, silver and copper jewelry are exported throughout the trade routes of pre-Columbian America.

Then comes the great 30-year drought of the 13th Century. A smaller population might have survived, but this civilization of several millions is starved out. Hungry masses fan out over the Southwest creating a refugee crisis that none of its neighbors can deal with. The starving hordes turn to barbarism and ultimately cannibalism. Within a generation the land is laid waste and one percent of the population remains by the end of the drought. The once great cities stand empty. 3 centuries later, Coronado's expedition discovers these monumental ruins, still filled with gold, silver and gems because there simply are not enough people left in the region to loot the vast stores of wealth that were left behind. The 7 Cities of Cibola turn out to be REAL after all!