It is, perhaps, artistic license. This pool is filled by gravity feed from the French Broad River, and drains back into it. The French Broad River is considered to be one of the oldest in the world.
https://en.wikipedia.org/wiki/French_Broad_River
A pipe is put into the river a few miles upstream which provides plenty of headpressure. The drainpipe runs back downstream a long ways. A set of valves controls either filling or emptying the pool.