Missoula County looks to river mapping project in wake of ’18 floods

Posted at 6:53 AM, Jan 24, 2019
and last updated 2019-01-24 08:53:19-05

MISSOULA – Missoula County officials will press forward with a plan for the first update of maps of the Clark Fork and Bitterroot rivers in a decade, hoping to use the information to better understand the potential for flooding.

The move comes after last year’s record flooding which put some neighborhoods underwater for weeks and even destroyed some mobile homes.

When the Clark Fork River went on a rampage last spring it brought some major changes along the river’s course below downtown Missoula.

The massive snowmelt, combined with a dramatic rise in groundwater levels, inundated some properties in the Orchard Homes neighborhood — and some mobile homes were swept away downstream near Kelly Island and the confluence with the Bitterroot.

The record flood levels are prompting county, state and federal officials to take another look at what happened — and especially the changes that were left behind. So, the county is planning an extensive river mapping project this spring.

“The last time the study was completed was in 2009. And since then we’ve had a couple of higher water events. Of course 2018 and also 2011. So what this model will do is, first of all, will capture new aerial photography of the river channel itself,” explained Missoula County Environmental Health Specialist Travis Ross.

Those aerial photos can then be combined with maps — some dating back to 1937 and even before — to show how the river’s channel has changed over the years.

“This type of information helps us in planning and making better decisions and helping homeowners make decisions too — and prepare,” Ross said. “A significant piece of information to know where on your property a river might migrate to.”

The Clark Fork River and the lower Bitterroot River have historically changed course many times across the valley. In fact, engineers are learning rivers often do better when they are allowed to naturally “meander”.

Ross says the new maps, funded in part by The Missoula Conservation District, should also help show whether past flood control measures are actually causing problems.

“We know that when we place rip-rap, or rock or armoring on river channels that that changes the dynamics as well. And past channel movement, the geology — all of that goes together to form a really well-informed model of what we think may happen,” Ross said.

He told MTN News that the mapping could be finished as early summer. At the same time, the US Army Corps of Engineers is also studying the river dynamics and what happened last year.