An old diversion dam on the Gros Ventre River south of Kelly will probably come down next spring.

The Newbold Dam impedes natural movement of native cutthroat trout and bluehead sucker, a state-listed sensitive species, according to studies by Wyoming Game and Fish Department and Trout Unlimited. Both species have declined across their range on the Gros Ventre.

Collapsing and no longer used to divert water, the dam’s age is unknown. The water it once diverted helped farmers and ranchers grow crops and hay on Mormon Row and west of the highway. If it’s found to be older than 50 years, the dam would warrant an environmental assessment because it would be considered a historic resource.

“We know that it was built between 1927 and 1949, and we also know it was modified in the ’70s,” Grand Teton National Park spokeswoman Jackie Skaggs said. “If it’s younger than 50 years, then we would be able to use a categorical exclusion to remove it. If it is a historic structure, that doesn’t necessarily mean we can’t remove it.”

The dam could fall apart during next spring’s runoff, Skaggs said. That could endanger people fishing or rafting downstream and it also could threaten a bridge about 650 feet upstream because of increased river flows, she said.

“This is a very crude log dam,” Skaggs said. “It’s all being eroded by the river channel. We are convinced that with the spring runoff, it’s going to fail.”

Recent photos of the dam show that even at low water, the flow of the Gros Ventre crests the structure.

Trout Unlimited proposes to raise funds and contract for the removal of Newbold Dam during spring 2013.

In 2010, a 13-foot-tall, 125-foot-long concrete and metal dam on Spread Creek was removed. That dam blocked fish migration, Skaggs said.