Episode 12 - Channel Migration

Channel Migration on the Bitterroot

by Marisa Sowles, Geum Environmental Consulting, Inc.

February 2022

The Bitterroot River moves, it migrates, and this a natural and important process. When you walk, run, float, swim, wade, waddle, or fish the Bitterroot River, you probably notice differences from year to year. Maybe a familiar gravel bar extends further out toward the river or you need to re-route a well-trodden path because, well, it’s not there anymore; it’s been replaced by the river. The deposition of cobble that builds a bigger point bar, and the recruitment of streambank material that swallows your trail are exactly the processes that define channel migration. These are natural processes you see in many channels and rivers. Eroded floodplain material is deposited downstream, building more floodplain. In all reaches of a river, the floodplain grows in some locations and is lost in others.

Lateral channel migration patterns are driven by normal high flows and the erosion and deposition that happen in most years. Large, periodic flood events such as the 50 or 100-year flow can disrupt migration patterns and cause large, unpredictable changes in channel alignments (avulsions) due to a chaotic mix of high water, sediment movement and large wood that can redirect flows in unexpected directions.

The Bitterroot River from Darby to Florence includes reaches with a single main channel and other sections with two or more main channels referred to as braids. The river likes to meander across the valley bottom, but rarely reaches a sinuosity greater than 1.3 (a ratio of the channel length to the linear valley length). This means the river maintains a straight-ish channel alignment by re-routing water across the point bars of tight channel bends, or creating avulsions when water spills over outer banks, cuts across the floodplain, and returns to the river forming a new side channel or main channel.  The river likes to take the path of least resistance and that path follows the steepest slope.

While you and I might not think of the Bitterroot River as stable in a static sense, it isconsidered stable geomorphically. This means that the river is not recruiting too much sediment that would cause the channel bed to rise and river to widen (aggrade), and it is also not moving sediment too quickly which would cause the channel bed to scour (degrade) and the channel to carve itself more deeply into the floodplain over time. The ability of the river to transport flows and sediment, at a reach scale, is in equilibrium.

As noted above, channel migration is a natural process. On the other hand, unnaturally high rates of local migration along an individual bank may be the result of a lack of woody riparian vegetation, localized sediment or large wood deposition, or most likely both. Woody riparian plants such as willows and cottonwoods, and their root systems, help bind soils along channel margins and slow erosion of riverbanks. Channel migration rates of the Bitterroot River have likely increased in the past century and a half due to landscape changes that have removed some of this woody vegetation. Infrastructure changes such as roads and bridges have also added limitations to river movement.  To compensate for these restrictions, the river flows faster and accelerated erosion can happen upstream or downstream of bridges or any banks where riprap has been placed.

According to a hand drawn map at the Ravalli County Museum from the late 1800s, the Bitterroot River near Hamilton was bordered by a mix of timber, willows, and meadows. Timber likely means large cottonwood trees because willows and cottonwood trees both like to be close to water sources, but it is also likely that on higher terraces, timber represents pines that prefer drier conditions than those found adjacent to a river. Today, we still see timber, willows, and meadows along the river, but we also see agricultural fields, infrastructure, and residential and commercial development and these new landscapes influence channel migration.

There are many opportunities to observe the Bitterroot River throughout the valley and I’m sure you have a favorite spot. What channel migration patterns do you see?

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