The Teton Conservation District Board on Feb. 9 received comments from two dozen people, both in the audience at the Stage Stop Inn in Choteau and via videoconference, regarding a 72-page report on recent erosion and flood damage on the Teton River.
Mike Sanctuary of Confluence Consulting Inc. (a 12-employee firm) and Applied Geomorphology Inc. with one employee, Karin Boyd, both based in Bozeman, performed the assessment over a three-day period with TCD Chairman Ross Salmond as a guide. The two companies finished the report in November and both people presented the information at the Feb. 9 meeting.
Although challenged by technical problems (reported as a bad computer cable), Sanctuary got through the two-hour meeting. Salmond, four district board members and those in attendance discussed what steps to take to follow up on Confluence’s recommendations.
The Department of Natural Resources and Conservation provided a $35,000 Watershed Management Grant Program grant to the conservation district. DNRC employees Michael Downey and Jorri Dyer attended the meeting to explain how the agency might assist the conservation district in the future.
The evaluation was a description of the damage to infrastructure, including roads and diversions that occurred in the 2018 and 2019 high water events from the South Fork Bridge to where the river flows under the Bellview Road west of the city of Choteau.
Boyd reminded everyone that the Teton River would always be a challenge for irrigators because the river has no true valley similar to the Sun River, for example, but a broad floodplain. Sanctuary said the upper watershed in the mountains 20 miles west of Choteau has a 15% slope which in spring runoff transports a “tremendous amount” of gravel that is a perennial problem for irrigators wanting to keep more than a dozen diversions flowing.
Sanctuary added that in looking at the river’s flood history, there has been a re-think of the 1964 flood, which volume was only estimated at the time because it blew out the flow gauge. The old estimate might have been inflated because of “backwater in play,” he said, making the 64,000 cubic feet per second flow estimate inaccurate.
In any event, all other floods since then can be labeled 10- to 50-year events, and before the 2018 and 2019 floods, there had been no major floods for 50 years, he said. He said the 1975 flood at 16,000 cfs was only slightly above a 50-year event (15,300 cfs).
As a result of two flood years in a row, the river has changed the landscape. “The river is not done changing. It will be in a period of adjustment for years to get to a stable alignment and slope. … Typically, we try not to recommend major structural changes when it is in a dynamic state, in an adjusting phase. So, no rebuilding major structures,” Sanctuary cautioned.
Rancher Dusty Crary asked whether there is funding for more permanent road protection near his place. The county has put in riprap in two places where the river eroded very close to the Teton Canyon Road. “Can we get ahead of this? Where do you go from there?” he asked. Sanctuary’s report had several recommendations for those sites.
“I don’t see TCD spearheading any of this,” Salmond said, of the road projects. Downey offered that partnerships would be advisable to make the projects happen.
South Fork Road area homeowner Terry Barch expressed concern about the state of the South Fork Bridge, whether anyone knew its future for repairs or replacement. Sanctuary had recommendations for addressing the river flow at the bridge abutments, but no one could say anything about its disposition.
However, former Teton County Commissioner Mary Sexton told the group that she had learned that day that a replacement might cost $1 million, putting it off limits for the county. “Replacing the bridge is beyond question,” she said.
Barch said the decking might be replaced in the interim and he said he would follow-up with the commissioners. Salmond said, “You live there, you need to be the big push.”
Sanctuary said culverts and flow redirection around the abutments could mitigate the problems that occur with high water. He reminded everyone that the report was a great opportunity to consider how it prioritized projects.
Bottom line, Downey said, is that Confluence’s assessment gives the group some authority behind any suggestions. “This report will give you more credibility,” he said.
Sanctuary fielded questions about the completion of a new state-sponsored LIDAR data assemblage for the Teton River watershed, but he said it would best be used by engineers and consultants in future flood studies.
Dyer said there was money left in the grant to keep the momentum going to get “more on the ground.”
“We had to start somewhere,” Salmond said.