There are so many factors affecting water. One is global warming. The EPA has an interesting page siting some of the challenges of water in a warming climate.
“Changes in the amount of rain falling during storms provide evidence that the water cycle is already changing. Over the past 50 years, the amount of rain falling during the most intense 1% of storms increased by almost 20%.  Warming winter temperatures cause more precipitation to fall as rain rather than snow. Furthermore, rising temperatures cause snow to begin melting earlier in the year. This alters the timing of streamflow in rivers that have their sources in mountainous areas.” Go to the Environmental Protection Agency to learn more: