A losing stream is a stream or river that loses water as it flows downstream. The water infiltrates into the ground recharging the local groundwater, because the water table is below the bottom of the stream channel. This is the opposite of a more usual gaining stream which increases in water volume farther down stream as it gains water from the local aquifer.
There are two rivers in Idaho, the Big Lost River and the Little Lost River, which both flow into the same depression and become subterranean, feeding the Snake River Plain Aquifer. Via the aquifer and numerous springs, they are tributaries of the Snake River.