A triple rainbow?!? Photographer gets three bows in one shot
It turns out, Teren was in the right geographical spot at the right time. The double rainbows are fairly common, but the vertical rainbow that connects from the base of the primary rainbow to the second rainbow is called a "reflection rainbow" -- caused simply by sunlight reflecting off the body of water and, in essence, making a second sun to create a new rainbow.
According to Crystallinks.com, "these rainbows share the same endpoints as a normal rainbow but encompass a far greater arc when all of it is visible. Both primary and secondary reflection rainbows can be observed."