|Location:||40km northwest of Sister. Follow US-20 going west and turn right onto NF-835 that wraps around the lake. NF-835, Sisters, Oregon (44.4340, -121.9052)|
Best time to see the vanishing lake is early spring.
|Classification:||Karst Lake lava tube|
|As far as we know this information was accurate when it was published (see years in brackets), but may have changed since then.
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A Lost Lake is a rather common thing in a karst area, where the underground is riddled with caves. Such lakes are connected with the karst groundwater, they rise and fall with the groundwater. But in Oregon we have volcanic rocks, and a vanishing lake is actually quite exceptional. The Lost Lake is a normal lake, surrounded by trees and with many fish. There is a river flowing into the lake at one side and flowing out at the other side.
But at a certain time in the year the water diminishes and the lake vanishes. The water level gets lower and lower, the outflow is empty, and at some point the water flows on the ground of the lake like a river and vanishes into a hole in the ground. It actually looks like somebody pulled the plug. But obviously that's not what happened. So if there is no plug and no karst, what did actually happen?
This part of Oregon is of volcanic origin. The rocks are actually a stack of basaltic lava flows which were produced by volcanoes, lava flowed over the surface until the eruption stopped and then formed a layer of basalt rocks. Those rocks are water tight and so the rain water flows on the surface in rivers and lakes. But sometime a lava tube is formed inside the lava flow (see lava tube) And such lava tubes have vents or other collapses of the ceiling, in this case it is most likely a vent. What happened now is that a lake formed on the surface right on top of this cave. The water is obviously flowing into the lava tube beneath, but the diameter of the vent is limited and so is the amount of water flowing through. The area is humid and most of the year there is enough inflow to exceed the capacity of the lava tube. The excess water fills the lake and the flows over at the outflow.
But every year in early spring the amount of water is lowest, and normally it is lower than the capacity of the lava tube. All inflow flows out underground, the surface outflow is dry, the level of the lake sinks until it reaches the ground. And finally we have the situation with a few pools of water left but the river flowing into the depression of the lake and to the hole in the ground where it vanishes.
We classified this lake as a karst lake, because there is no page for lava tube drained lakes, and karst lakes work similar. There is not only no explanation page on showcaves.com, actually there is no geologic term to name it. As far as we know there are other similar but less spectacular lakes in the area, but we never heard about this from anywhere else. And a last remark: the lava tube, although completely unexplored due to the water inside, at least has a name, which is Lost Lake Lava Tube for obvious reasons.