Unexpected turbulence

This year was one of the few mother’s days in the past decade with my mom and I in the same place, and to celebrate the occasion we went packrafting in the snow.  Our intention was the lower Dearborn, that is to say the final 20 miles down to the Missouri, which is engaging, scenic, and in a small boat straightforward.  Drizzle turned to snow going north out of town, and while descending towards Wolf Creek and seeing the snow line drop to only a few hundred feet above the creek, I rerouted.  We would paddle the next 10 miles upstream, from highway 200 to highway 287.  I had not done that stretch, but it promised few surprises, and halving our time on the water seemed a good way to ensure that even in drysuits, no one turned into a popsicle.

The float was chilly and beautiful.  It snowed lightly the entire time, only transitioning to drizzle perhaps 20 minutes before we took out.  That stretch of the Dearborn proved both predictable and surprising.  The prairie and mountain scenery was gorgeous.  The riverbed, cobbles representing the whole range of Bob Marshall geology, was enthralling.  And the floating threw out a few curveballs,  We had three barbed wire fence portages, two of which were rather hard to see from any distance.  I’ve never seen more intense beaver activity, and wood hazard, than we saw for a mile in the middle bit.  And that same flat middle section provided a lesson in surprise hazards on the river.

screenshot-map

Occasionally, rivers cut new channels.  You can see an excellent example above, from that stretch of the Dearborn.  The bands of riparian vegetation newly interrupted by the river is evidence of fairly recent turbulence, enough to cause the river to move right from the previous channel.  Anything like this is a reason to start paying extra attention, as new channels usually mean new wood and debris.  A new channel heading straight into mature forest, especially spruce, is almost always a reason to expect portages.

screenshot-2018

When to expect these, especially on a backcountry float without recent reports, is an important skill.  A simple big flow is generally not enough to reshape river channels, especially on a mountain stream or river which both have big seasonal variations and large variations of peak flows, year to year.  The Dearborn usually gets a sustained pulse in May and June.  In 2018, big snows that lingered in the prairie made that variation bigger than usual, and a sudden warm up in mid June pushed a pulse out of the high mountains that was 10 times the usual peak, and set a record for that time of year.  The same pulse drastically reshaped the lower 10 miles of Danaher Creek.

My thesis is that a pulse exponentially greater than the mean high stage introduces enough turbulence, crammed into the same space, that disruption of old channels can almost be expected.  This seems to happen more often on stretches with less gradient, which presumably have less latitude for absorbing the chaos.  Taking a deep dive on fluvial geomorphology can provide a lot of entertainment here.

1 Comment

  1. The other fascinating thing about that flood event is that it sent rivers out of their channels but came down too quickly to carve new channels in some instances. Deep Creek was almost entirely reshaped after leaving the nozzle of the canyon and entering the less constrained plains, with the creek running often overland through trees with no channel, in new channels, and at times in the old channel. In one case it flowed overland and was pouring over the old bank like a waterfall into the old channel, slowly head cutting a new channel through the shrubs. Fascinating to witness!

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