For the casual observer the lower Ruby River may look lazy and calm, but it actually is a very dynamic river channel that continues to migrate from year to year, said a Bozeman scientist who recently concluded a channel migration zone mapping project on the river.
During the past 65 years, between the Ruby Dam and its confluence with the Beaverhead River, the Ruby River has made new channels, abandoned river bends and jumped back and forth between historic channels all in an effort to move sediment and water downstream, said Karin Boyd from Applied Geomorphology Inc. in Bozeman
But all this is quite normal for a river like the Ruby, Boyd said.
Boyd and her partner Tony Thatcher of DTM Consulting in Bozeman recently completed the channel migration zone mapping for the Ruby Conservation District with funding from Montana Department of Environmental Quality and Department of Natural Resources and Conservation, said Rebecca Ramsey, Ruby Watershed Coordinator.
The CMZ mapping will provide area landowners and conservation district supervisors another tool to better understand the lower Ruby River, Ramsey said.
“Basically the idea was that with channel migration zone mapping and other science-based tools that local decision makers, be it landowners to government, can make better land use decisions,” she said.
The CMZ mapping is part of several tools the conservation district has developed to help landowners. These include wetland and riparian area mapping and groundwater studies.
“This was sort of a natural progression to add this to our toolbox,” Ramsey said. “We wanted to have people be able to see for themselves where the river’s been and where it’s likely to go.”
Boyd, who has done CMZ mapping for other rivers around the Montana, sees the project as an educational tool for people in the communities that border the Ruby River.
“Probably the first application that we see is outreach and education – just getting the information to people regarding the behavior of the river,” she said. “You get people sitting around looking at maps and it’s a very good forum for collaborative learning about the river.”
To map the CMZ for the Ruby River, Boyd looked at aerial photos from 1955, 1995 and 2009 as well as historical maps.
In several locations between 1955 and 1995, the river abandoned broad river bends, cutting them off with straight channels, she said.
“There’s a lot of channel movement and a lot of cutoffs,” Boyd said. “There were a whole lot that happened between 1955 and 1995 and there’s been some discussion as to whether those are related to the ‘84 flood event.”
There is also evidence that the lower seven miles of the Ruby has moved substantially since the General Land Office first mapped it as the Stinking Water River in 1870.
She’s not sure if that channel switch was due to natural or man-made causes.
“It might have been in part engineered into a new channel or naturally shifted into a new channel,” Boyd said.
For a river like the Ruby, channel migration is healthy and natural. Rivers need to move over time in order to remain healthy, she said. The sinuous nature of the lower Ruby River means that over the course of a year or decade or several decades, the river will move sediments and organic material around during the high water events. This deposition and re-deposition of material can mean the river will expand its channel, change channels or even completely cutoff river channels.
The goal of the CMZ mapping project is to provide people with a 100-year zone in which the river is likely to migrate, Boyd said. This is different than a 100-year flood plain, which looks at where a one-time event is likely to happen.
The CMZ is where the river needs to be allowed to migrate to remain healthy, she said.
“Identifying a zone that you let the river work back and forth is critical to the health of that river,” she said.
Healthy rivers have seasonal high water that redistributes sediments, plant material and seeds that rejuvenate riparian vegetation and spawning beds crucial for wildlife and fish, Boyd said. In turn, healthy riparian habitats help with soil stabilization and erosion.
Knowing where a river is likely to migrate will provide landowners, whether they’re ranchers or homeowners, good information that will help them make decisions with their land, she said.
On the Yellowstone River, for example, she’s seen farmers adjust the location of center pivots to account for the natural migration of the river channel. In other places, she’s seen homeowners have to invest in many thousands of dollars worth of rip-rap and other bank armor to stabilize the river bank because the a river was threatening to under cut their home.
The good thing with the Ruby River is that landowners and community members are actively looking at things like channel migration and watershed health while the whole system is still in good shape, she said.
“One thing about the Ruby is that the Ruby is ahead of very many river corridors in that they have an engaged community that is really passionate about that river,” Boyd said.
Boyd and Thatcher will present their CMZ map and report to the Ruby Watershed Council and Conservation District Wednesday, Jan. 12 at the Moraine Center in the Philanthropy River Building in Sheridan at 7 p.m. The presentation is open to the public.