But first, I needed to see a future hole in the ground.

White Sulphur Springs, population 970, is a ranching town unburdened by stoplights at the foot of the Castle Mountains, 27 miles south of where boaters put in their rafts. The town has been in decline ever since the timber mill, a main source of jobs, was shuttered in the 1980s, residents say. Main Street is gaptoothed with empty storefronts.

Jerry Zieg, vice president for exploration at Tintina Resources, a Vancouver, British Columbia, company, wants to turn around his hometown’s fortunes. Tintina has proposed an underground mine to tap the nearby Johnny Lee Deposit, which is thought to contain more than $2 billion worth of copper. Tintina hopes to apply for a permit this fall. After state review the mine could open in early 2020. Once operating, the mine would employ about 200 people and perhaps spur other development.

On a warm morning in May, I met up with Mr. Zieg, a 60-year-old geologist who grew up on a ranch on the banks of the Smith. We drove outside of town, to the rolling green ranching country where the mine would appear. When the rock that holds copper is exposed to air and water, it often sets off a nasty chain reaction known as acid mine drainage leakage, which can kill waterways’ insect and fish life.

Mr. Zieg said the mine would use the latest techniques to prevent this drainage, including taking about half of the crushed waste rock created by the mining process, mixing it into a cement-like paste and returning it to the tunnels, rendering it inert. The rest would be contained in a way yet to be determined, perhaps a tailings pond. Mr. Zieg also noted that the ore Tintina wants to remove is surrounded by limestone and dolomite that can help buffer any acid that’s created.

We drove on, past Sheep Creek, a major tributary of the Smith. Mr. Zieg scoffed at concerns that the mine, which would sit about one mile away, would reduce the creek’s flow, or taint its water.