Hi Sis, How far away is the river from the hot springs? Reason why I say methane. When we go fishing her in the river and the water is down and no rain, we get a lot of methane bubbles, which do occur naturally. When you walk in the muddy river, along the edge, in the water, you will get releases similar to that. This being an eq. that would shake the mud up intensively and you would have methane release look like boiling water...