Out of an intensive mapping project begun in the early 1990s, a new and revealing picture has emerged: Much of the lower Roaring Fork Valley and western Eagle County has collapsed over the last several million years.

As the picture has come together, signs supporting the collapse theory are being found everywhere, and are easy to see, particularly from a high point or airplane.

You can taste the evidence as well. Anyone who has soaked in the Hot Springs Pool knows the saltiness of its water.

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That salinity is directly linked to a massive underground excavation project that has lowered a 1,400-square-mile area by as much as 4,000 vertical feet, according to Bob Kirkham, an Alamosa geologist who has been instrumental in studying the phenomenon. He retired earlier this year from the Colorado Geological Survey.

Over the last 10 million years or so, and particularly over the last 3 million years, much of the lower Roaring Fork Valley has sunk as groundwater dissolved deep deposits of evaporite, consisting of salt and gypsum, and shipped them away via the Roaring Fork and Colorado rivers.

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The process left a void that upper layers of earth settled into, and also is evident in numerous sinkholes and other depressions in the area.

The same thing is happening in western Eagle County and west of Glenwood Springs.

“I guess that’s why we’re a valley,” said Dave Merritt, chief engineer for the Colorado River Water Conservation District, and a river salinity expert.

“The valleys we live in are valleys partly because of the collapse,” confirmed Kirkham.

Garry Zabel, who teaches geology at Colorado Mountain College’s Spring Valley Campus near Glenwood Springs, said the collapse theory has quickly become standard thinking on what shaped the region.

“It’s pretty well accepted from the data he has presented,” Zabel said of Kirkham. “Once you kind of know where to look, it really makes a lot of sense.”

Areas such as Spring Valley and what is called the Sopris Bowl, a depression taking in much of the area between Carbondale, El Jebel and Mount Sopris, are cited as examples in research done by Kirkham and others.

Recent findings about the collapse are contained in a 234-page book, “Special Paper 266: Late Cenozoic Evaporite Tectonism and Volcanism in West-Central Colorado,” published in January by the Geological Society of America. Kirkham helped edit the book and also authored some of its chapters.

Kirkham said it’s rare to see such a large area collapsing as a result of salt and gypsum dissolving deep underground.

“This one certainly is world-class. It’s one of the largest ones in the world that’s known today,” he said.

Vince Matthews, senior science advisor with the Colorado Geological Survey, said evaporite dissolution is happening on a “massive scale” in the Glenwood Springs area.

The reason, he said, is the area is underlain by a “tremendous thickness of salt.” This salt was deposited about 200 million years ago, when the ancestral Rocky Mountains rose up and isolated an ancient ocean.

Merritt has been dealing with the end result of dissolving salts through the seven-state Salinity Control Forum, which is trying to reduce salt content in the Colorado River. But he gave little thought to the void left behind until he went on a recent tour with geologists, who pointed out signs of collapse.

“At first I thought that some of the quantities that they had come up with were too large to be believable,” he said, referring to the volumes of dissolved evaporite and the size of the collapsed area.

But as he looked at the theory more closely, he became a believer.

“If you get a chance to fly over the area, you can start to pick up the depressions better. It’s pretty interesting,” he said.

The combined discharge of hot springs from Glenwood Springs to Dotsero, and associated groundwater seepage, adds an estimated 440,000 tons per year of dissolved solids, mostly salt and gypsum, to the river.

“That would make a pretty big pile in your backyard,” said Merritt.

The source spring for Glenwood’s Hot Springs Pool alone discharges about 260 tons of dissolved salt and gypsum into the Colorado River each day.

That’s enough to create a new sinkhole of 120 cubic yards.

Kirkham said evaporite formations still waiting to be washed away are at least hundreds of feet deep, and maybe 1,000 to 2,000 feet deep.

“This process hasn’t ended. We’re well into it, but it’s a long ways away from being over,” he said.

It could be enough to give you a sinking feeling. But the good news about this ongoing collapse, said Matthews, is that “you can plan for it.”

Regional collapse takes place over such a long period of time that it is of concern mostly along margins of the collapse area. It otherwise isn’t a threat over the lifetime of most buildings.

More widespread concern focuses on collapse byproducts such as sinkholes, and the general danger that evaporite soil compaction can damage building foundations.

Maps can help community planners decide where construction should and shouldn’t occur, and engineers can mitigate collapse risks in construction projects.

As the collapse continues, Merritt, for one, isn’t worried about the region sinking to ocean level any time soon.

“We’re not going to get hit by a tsunami or anything in the near future,” he said.

On Sunday: How dating of volcanic outcrops played a crucial role in supporting the collapse theory – and also gave a better idea of the age of Glenwood Canyon.

Contact Dennis Webb: 945-8515, ext. 516

dwebb@postindependent.com