I thought I'd seen this loop effect somewhere before. It's (probably)
the effect of an open enclosed system when the input from one
reservoir is slightly greater than from the other(s).

No doubt the loop has a slightly different lien when the chief input
source is the stuff coming up from Mexico.
http://en.wikipedia.org/wiki/Fluidics

That would (obviously) occur in summer.

In a closed system, this principal becomes a vacuum pump rather than a
whistle. Further reduction in its intricacy and it reverts to a spigot
or (as in this case) a river mouth.