You are not getting the point. I get it just fine. In any given year, a finite amount of rain and snow fall on the western slopes of the Rocky Mountains and the continental divide, this precipitation being the inflow to these reservoirs, installed at a time of plenty to harvest Mother Nature's bounty.
Physics, even at the rudimentary, non-measured by the casual observer level, will note, a vessel being filled at a declining rate, while emptied from the bottom at an ever-increasing rate, will inevitably reach a point of un-sustainability to meet the demand rate, causing water levels to drop. If it happens due to changes in climate (man-made or natural) Mother Nature and the sweet cool rains to quench your thirst, will not be there to save you. If you believe in a deity, you may remember man was given dominion over animals to suit it's needs, but not over mother nature, if she decides to share her bounties elsewhere or not at all. Indeed, geologic history is replete with examples of rainforests turned to desert or even ice fields to remain thousands of years. If depending on the laws of man (at this point) or looking for Mother Nature to get us out of this, it may indeed be in vain or just plain too late. It looks like she's not there.
Here is fantastic piece of music and one of my favorite (and possibly a prophetic truism) ecology songs relating to Mother Nature. I hope you enjoy.