Spring Hole

The emergence of the aquifer at the surface of the benthic or hyporheic zone of the river, out of which flows cold, un-oxygenated water. Spring holes are refugia for trout and macro invertebrates during periods of drought, floods, and extreme cold when abundant anchor ice forms.

Links

http://wrgis.wr.usgs.gov/docs/parks/noca/sb16river.html