(birds chirping, water rushing)
All right, so let's dive into Glenwood Canyon.
First of all, where is Glenwood Canyon?
Glenwood Canyon is in the western part of Colorado,
in the Rocky Mountains.
Probably most people listening are familiar with this,
they've probably driven down I-70 here.
Yeah, it's a steep canyon.
There's granite kind of at the bottom of the canyon
by the Colorado River,
and then it goes into a sequence of sedimentary layers
kind of at the top.
There's some really interesting geology there.
♪
Probably what you haven't seen,
I know you probably haven't seen lately,
is these new big fans that are in the Colorado River.
Those all happened this summer.
You can see where it's squeezing the river to,
you know, sometimes less than half of the original width.
So that's new.
♪
I want to introduce you to the Grizzly Creek Fire
that happened in 2020 in the Glenwood Canyon.
The Grizzly Creek Fire started on the 10th of August.
It ended up burning 13,000 hectares.
It wasn't fully contained until December 18th.
Luckily, nothing happened in the fall following
the Grizzly Creek Fire,
so we sort of dodged a bullet,
there wasn't a big rainstorm.
In winter, it was pretty quiet.
But then we had the summer monsoon in 2021.
That resulted in really big debris flows.
These wildfire disturbances create ideal conditions
for landscape change,
particularly debris flows,
and Glenwood Canyon was hit hard by a lot of debris flows
that following summer in 2021.
(water rushing)
(helicopter blades whirring)
♪
All right, so, Alex, you know,
we've been looking for a source of initiation
for these debris flows.
We've come up from the channel,
we're here on the hillslope.
We're about 400 meters below the ridge,
and we haven't seen any kind of distinct initiation.
But we have seen some signs of overland flow.
So how would you describe this overland flow
that we see here?
So, I see a series of rills right about here, um,
going across the slope,
maybe a group of five or so,
about 10 centimeters deep,
10 to 20 centimeters wide.
Pretty subtle,
you really have to be looking for them.
But, yeah, nothing distinct.
Yeah, these are pretty subtle rills.
Obviously, there's been some regrowth since last summer
when we had the debris flows.
But there's still some obvious rills where--
yeah, we can see there was overland flow.
That's probably because after the fire,
there was less infiltration into the ground.
So this runoff that's creating these rills,
it's more of a distributed source of material,
unlike a discrete landslide.
♪
This is French Creek.
This is in the Glenwood Canyon watershed.
This is part of the Grizzly Creek Fire
that burned in 2020.
French Creek had a small, little debris flow,
it wasn't gigantic.
It came down and it scoured, looks like,
some of the valley walls.
So you kind of see over to the left
that you've scoured and you left behind
some exposed tree roots where the bank used to be.
By the end of the summer of 2021,
after a debris flow came down,
this had filled in pretty much completely
with sediment.
It's like a ramp.
And now, it looks like this summer,
there's no debris flows,
just water flows that have been cutting down,
reworking this,
leaving behind some terraces.
Yeah, maybe we should look at the sediment here
to look at what the debris flow sediment looks like.
(sediment softly pattering)
All right, so let's look at this sediment.
What do you think?
Do you believe that it's matrix-supported?
That's what we usually see here at debris flows.
I do. If you pull clasts out of the deposit,
you can see that there's this kind of fine-grained sediment
that's on all the preserved edges,
kinda separating the larger clasts, so...
Yeah, okay.
That's matrix-supported.
Okay, yeah, so matrix-supported fine-grained sediment
supporting the clasts.
So this is different than what we would see
if this was a fluvial deposit where it'd be sorted.
We might see some, like, imbrication,
and it'd be clast-supported.
But this is more matrix-supported,
unsorted sediment.
There's definitely boulder-sized clasts
but also a lot of gravel and sand in here.
Nothing's gigantic in this section of deposit.
We don't see any really, really, large, like,
-meter-sized boulders here. -Right.
Some of the rocks are very angular,
which is, they haven't gone very far.
They've broken off of cliffs.
This is, like, a limestone that's probably been
chipped off a cliff pretty recently
and it's still retaining almost a rectangular kind of shape.
Pretty sharp edges for a rock that you'd find in a deposit,
so it's certainly not a fluvially turbulent flow.
This seems like it came off of a cliff or something,
some of the limestone that's really local here
and hasn't been transported very far.
Just got entrained in this debris flow
and was moving down with that matrix.
♪
(clicking)
♪
All right, so I'm here with Alex,
and we were just looking at some splash marks on a tree
from the debris flow.
These splash marks have the mud that was being transported
in the flow.
So, Alex, about how tall would you say
the splash marks are on the back of that tree?
Uh, a little more than two meters.
Wow, over two meters.
What about on the front here?
Probably closer to about 50 centimeters.
Okay, so the flow that was coming from upstream
is splashing up.
That's why it's making that two-meter height upstream,
but the flow depth on the front,
it cuts down to about 50,
which is probably more reflective
of the actual flow depth.
Anything else on the back of this tree,
any kind of impacts or anything like that?
Yeah. On the upstream side,
we see about 75 centimeters of debarking.
Wow, okay, 75 centimeters of debarking.
So, rocks and debris that's in the flow
is actually hitting the tree
and it's taking the bark right off the tree,
it's impacting it.
Okay. And it looks like, yeah,
this sediment has actually stuck on there
for over a year,
since the last debris flow.
So, that's kind of a long time for that sediment
to just be stuck on the back of a tree.
♪
(footsteps crunching)
So this is a pretty small debris flow here,
but it illustrates some interesting features--
in particular, levies that are off to either side
of the debris flow.
So, levies develop as you have a surge front coming down
and it's moving sediment off to either side
as the front comes down the channel.
So here, this initiated on a hillslope.
There's a burned area above, and also,
probably the cliffs that you can see
helped to create a lot of runoff
that helped to kick off this debris flow.
♪
There are rock falls every winter here.
Pretty big rock falls that come down on the road.
So, Alex, we're down here at the bottom of French Creek
by the bike path,
looking at the debris flow deposit.
It looks a lot like machines have gotten in
and kind of cleaned out this portion of the debris flow.
From your perspective,
what are some of the things that you're seeing
about how the debris flow was moving and accumulating
in this fan here?
Yeah, so, right in front of us,
we have this concrete bike path,
and it looks to me like the bike path caught up
the majority of the flow and kind of acted as a dam
and then routed the flow to either side.
Down to our left over here,
we have some damage to the bike path,
it looks like it's recently been repaired.
That suggests to me that maybe some flow
overtopped it in that location
after being rerouted when it first hit the bike path.
Yeah, it absolutely looks like that.
I mean, the freeboard underneath the bike path
at this point is maybe 50 centimeters
and it's completely buried in a few places.
So, this very sturdy concrete bike path
has kind of operated as a dam.
Luckily, downstream from here,
the highway, I-70, is actually elevated.
And so, luckily, this debris flow was
caught up here mostly is this fan,
and then, any of the sediment that made it downstream
is not hitting the highway because it's superelevated.
Yeah, what about some of the vegetation
and things that we see buried up here?
Yeah, so it looks like we have some trees on the fan
right down here
as well as further upstream,
and it looks like those trees have been buried
by a good bit of sediment.
Yeah, yeah, they definitely seem to be buried by sediment,
and I can even see some woody debris here
that looks like it's come down in the debris flow itself.
So this is maybe somewhat of a low-energy environment
by the time it was making it down to here.
It seems like a few of the boards were broken
on the bike path,
but clearly not all of the boards were broken,
so, yeah, maybe we were starting to really enter
the deposition phase of this debris flow here.
All right, so here we are on the bike path.
We can definitely see that flow has made it underneath
and deposited a lot of sediment.
There's pile that seems to have been reworked
by heavy machinery.
What about boards that have broken?
Can you kind of identify a few of those for us, Alex?
Yeah, so it does look like flow went over the bike path.
You can see right over here.
Yep, that board definitely seems new,
so that broke, yep.
And then, there's a few more just off to our right here,
suggesting that the flow did make it over the bike path.
But like we said earlier,
it does look like the vast majority of the sediment
was caught upstream of the path,
and relatively little,
while some did deposit downstream,
I'd say most got caught up just upstream.
Yeah, absolutely.
What about the gap underneath the bike path?
What are we looking at here?
♪
Yeah, so, if we look at the freeboard,
like Francis said earlier,
only maybe about 50 centimeters of space
between the flow surface and the bike path.
So we're actively working with the data from Glenwood--
specifically, rainfall, field observations, lidar,
and aerial imagery--
to try to better understand the debris flow processes
that happened in Glenwood Canyon.
♪
(birds chirping, water rushing)