One obvious reason is that rivers provide pleasant places for people to relax and enjoy themselves. They want to have a nice place to go. We've already ruined the Salt River in Phoenix and the Santa Cruz River in Tucson. But human effects on rivers go beyond aesthetics. For example, agriculture, cattle grazing, and damming have led to a rise in the population of salt cedar, a non-native plant imported from Eurasia.
Salt cedar makes the soil more saline, which impedes the growth of many native plants.
In addition, salt cedar is highly flammable, making it hazardous to have around during seasonal Arizona wildfires. Many dollars have been spent in search of means to eliminate salt cedar. Salt cedar is a new piece of the puzzle. It does not fit well into our riparian picture. As Stromberg and other scientists learn more about what should exist in a natural system, they may find even more benefits in preserving existing systems and repairing those that have been altered.