An eater, or swallowhole, is a reach of stream or a tidal area given to violent currents and waves that often upset and/or suck under boats and kayaks and the like as they are attempting passage. When tremendous volumes of water flow over a ledge or plunge over the downstream edge of a boulder in the bed of a channel, eaters can drive boats down to the streambed before spitting them out again, farther downstream. Large rapids in the Grand Canyon, especially during high water, are for good reason known as eaters because of the frequency with which boats disappear into them. The term swallowhole also refers to the place where streams disappear into rock faults, lava breaks, and tubes. Lost rivers are well known in active volcanic regions such as the Craters of the Moon National Monument in Idaho. The Williamson River in Oregon, below ancient Mount Mazama, which exploded to form Crater Lake, vanishes into a rocky ridge only to resurface again as a full-blown river a mile or so below. Such lost waters have been used as a literary metaphor for disappearing values.

William Kittredge