Idaho’s Big Lost River emerges for first time in five years

The Big Lost River near Arco percolated through the ground last week, something it hasn’t done since 2012.

A wet fall season paired with a large snowpack and above-normal water levels behind the Mackay Reservoir allowed the river to spring up this year, the Post-Register reports.

U.S. Geological Survey Project Manager Roy Bartholomay said the river last had water in June 2012. Bartholomay said the river sometimes shows up in the Arco area, but it rarely flows for more than four or five months.

He predicts the Big Lost could maintain flows into the summer, though its discharge levels have fallen since it began March 22, according to USGS data. The river filled its banks March 23, hitting more than 100 cubic feet per second but flows have dropped to about 30 cfs Thursday.

Bartholomay said a variety of factors contributed to allowing the Big Lost to start flowing again. He said flows have been spotty but growing in the past years since efforts have been underway to recharge the aquifer between Mackay and Arco for agricultural purposes.

Whether its flows will stick around through the summer remains to be seen.

“The Big Lost is defined as a lost river since it readily infiltrates into the ground and loses a lot of its water to the ground,” Bartholomay said. “It takes a bit more water to keep it running. It will depend on how much water comes off” in the spring runoff.

This story was originally published March 31, 2017 at 7:42 AM with the headline "Idaho’s Big Lost River emerges for first time in five years."