In all honesty, the principal reason would be the loss of wetlands. We've lost over 80% of the wetlands. Wetlands provide a very, very important function of retaining water just as a dam or a reservoir would. In the spring, wetlands retain the water and slowly discharge it. Through the course of the summer, the wetlands are gone for the most part. It's just a function of the land use, not necessarily because of water takings.
On March 4th, 2014. See this statement in context.