Scientific curiosity

My question about the natural world is, in the mountains where there are huge waterfalls that have been flowing for years and years how did they start and how do they not run out if they just keep flowing for years on end. This question doesn't bug me so much when I'm in San Diego but I go camping or backpacking or whatever it always makes me wonder when I walk by a huge waterfall. I think that every couple of seconds hundreds of gallons of water go crashing out and falling a few stories into another body of water that never seems to fill up. How does the body of water catching the waterfall not overflow if it constantly has thousands of gallons of water poured into it. Its not like the water can just float back up to the top for another go so how does the cycle all work?