Top scientists are warning: A water crisis is fast approaching if there is continued draining of the country's two largest reservoirs at the current pace.

It's hard to fill a bathtub when the drain is open; it's even harder when the tub is shaped like a bowl said Doug Kenney, who led the Colorado River Research Group study.

He said visitors to Lake Mead and Powell who measure the water levels by the bath-tub-like rings on the canyon walls see only part of the problem.

"As the reservoir gets lower and lower, it gets narrower at the bottom," he explained. "So, what looked like a 10-foot decline one year, might look like a 30-foot decline the year after."

Lake Mead is currently only 38-percent full, but draws its water from Lake Powell, which was last measured as 48-percent full.

If Lake Mead relied solely on its own winter runoff, Kenney said, "It would be a dry river bed now."

RELATED: Lake Mead's Imminent Shortage May Result In Cutbacks By Late 2019

The fact Lake Mead has avoided shortage these last three or four years, he said, "Only happened because we've been pulling water out of Lake Powell to prevent that crisis. But, we can't do that forever."

Wyoming, Colorado, Utah and New Mexico belong to Lake Powell's upper basin reservoir, while Arizona, California and Nevada, as well as Mexico draw from Lake Mead, the lower basin.

"There's no silver bullet," but individual conservation helps he said. "Those add up. The question is, are we going to need something bigger than that."

In Colorado, his state has a separate court regulating water use.

"Most of this isn't rocket science," he suggested adopting rebate programs that give consumers and businesses a reason to reduce water use, "the challenge is not so much about technology, you've got to incentivize."

Beyond that, he recommended, all seven states no longer separate the issue as an upper and lower basin shortage, but as one enormous crisis impacting any community relying on Colorado River water.