Yes, Meng is correct. The tannic river water "intrudes" on the crystal clear spring water and then swirls and mixes to create the "water on fire" If there is too much tannic water, you get nothing but darkness and if there is not enough, you get a dull effect. We were lucky that the river water had not receded this past weekend, so we got a good effect. If the sun had been out, it would have been even better, but I'll take what I can get. These photos don't compare to some of the spectacular ones from Jill Heinerth, Wes Skiles, Becky Kagan, but I took them, so they are special to me.