Rain....and from under the earth.
Scientists have discovered enough water under the earth that IF it were on the surface would cover the mountains.
There is a theory that the Grand Canyon was not formed over millions of years but in just a few weeks.
According to the theory there was a huge ice dam that gave way sending billions of acre feet of water down through Arizona and gouged out the canyon very rapidly.
During dry times here in central Indiana, I can dig down about 4 feet and find water in my backyard!