Flaming clouds of airborne gases, ash, and fine sediment rush down Mount Hood at 100 miles per hour, like an incinerator in flight. A slurry of hot water and sediment, in some cases 100 meters high, and the consistency of cement, follow—crashing down Mount Hood’s rivers and valleys; rocking and rolling between ridges; decimating everything.
This is Mount Hood 1,500 years ago. This is Mount Hood at various points during its geological history. Heck! As an active volcano, this is Mount Hood in the future.
Massive amounts of sediments were redistributed down the mountainside with each eruptive period. Sediments filled in valleys and creating an eerie lifeless landscape—in effect, a clean slate.
The Beginning
Which brings me to where our story begins…
I met up with hydrologist James (Dar) Crammond at the junction of Road 39 and Highway 26 to explore the Zigzag River Valleys.
Little Zigzag River and Big Zigzag River are fed by a glacier near the base of Mt. Hood’s crater, converging to become the Zigzag River further down the mountainside. They also sit precariously in the path of destruction described above.
However, despite this, Dar and I did not find ourselves hiking through a dry, flat moonscape, but a deep valley and forested oasis. The clean slate from 1,500 years ago was not clean anymore. It had been written upon by the very substance we had met up to talk about—water!
The Hike
- Trailhead: Unmarked trailhead off of Road 39 at the gate for Forest Service Road 2639-021 where Paradise Park Trail Begins.
- Distance: 2.5 miles
- Details: Recreation Pass for US Forest Service Trails may be required. Limited parking and no parking at the trailhead. Little Zigzag Falls Trailhead is at the end of Road 39 and is a great add on to this hike.
A Giant Reset
Before we hit the trail, Dar took me to an overlook of Mount Hood a little further east up 26 from our meeting point. As I stood there marveling at Mount Hood, Oregon’s tallest and most well-known stratovolcano, Dar explained Mt. Hood’s recent eruptive history.
In addition to the eruptive event 1,500 years ago (the Timberline eruptive period), the Zigzag episode (500 years ago) and the “Old Maid” episode (200 years ago) also sent pyroclastic flows (airborne debris flows) and lahars (water and sediment flows) down Mount Hood. In fact, in 1804-05 Lewis and Clark observed the remnants of debris flows in rivers coming from the Mountain into the Columbia. Consequently, this is how the Sandy River got its name.
The Sequence
Dar also pointed to the horseshoe-shaped crater on Mount Hood with a tooth in the middle, called crater rock. He explained that each time an eruption would occur the dome would collapse leaving a crater, but then the dome would grow and the volcano would erupt again. Crater rock is a remnant of one of these collapsed domes. Hot spots around crater rock signify the potential for a new dome to build.
In addition, the heat energy from each eruption would liquefy all of the ice, snow, and glaciers on Mount Hood. The superheated water would flow down the mountain at high speed, collecting material along the way. This “mudflow” is what is known as a lahar. Unlike pyroclastic flows, which are airborne, lahars flow down the mountainsides a bit slower, but much farther. This is why Lewis and Clark were able to observe debris from Mount Hood in the Columbia River many years ago. There is even evidence that the Columbia was temporarily dammed by lahar debris at least once following an eruptive episode.
Dar called this whole sequence “a giant reset”— as it flattens the terrain with loose sandy material and rocks—setting the stage for a new force to come in and shape the landscape—water!
Loose Landscapes
Leaving the viewpoint, Dar and I headed back to our meeting spot and drove up Road 39. At the end of the road is a parking lot and trailhead, as well a section of old Route 26 that was decommissioned in the 1960s. However, that is not why we stopped here. Instead, Dar wanted to show me an exposure that would provide some insight into the aftermath of Mt. Hood’s eruptions.
The exposure was probably 25 to 30 meters high and made up of fine textured sand. Growing along the exposure were red alder trees. Dar said, “Alders love loose landscapes” When you see red alders in an area it suggests disturbance.
Dar explained that during the 1550 eruption that a big lahar, with a peak 30% to 50% higher than what we could see, dropped down into the area where it would have been constrained as it moved down the canyon, causing it to ricochet from cliff to cliff. Eventually, the slurry of water and sediment would meet a constriction point downstream where the Little and Big Zigzag meet– blocking sediment transport and causing loose sediment to pile up. Hence, the alder trees.
Sediment Stratigraphy
This exposure was one of many Dar and I observed, as we moved downstream along road 39 to begin our hike through the woods.
Another exposure that was particularly interesting was near the pinch point where the Zigzag River tributaries meet and the canyon narrows (just above the trailhead on road 39). Here you could see horizons, or layers, of sediment from different eruptive events.
Dar explained how scientists can use organic bits found in the horizons, like a fragment of charred wood, to date each layer.
He also explained how sediment size and mixing within a horizon, is evidence for the origin of each layer. Fine, consistently sized grains of sediments signal the normal hydrology of rain and snow. While jumbled sediments of variable size and shape are characteristic of lahar deposition.
Of course, even between different eruptive events, lahar depositional characteristics will differ depending on the stage of dome-building in which the eruption occurred. Fine material is more predominant in layers from early-stage eruptions, while large angular rocks are found in late-stage eruptions that follow dome-building.
A Reckoning
Either way, we are talking about a lot of loose sediment! This is where hydrology comes into play, explained Dar. The powerful forces of big disasters often capture the imagination, but it is during the aftermath of these moments, where the real work begins. It is with the power of a raindrop and the force of a river that water reshapes the landscape—tearing down what plate tectonics builds up. In this case, a forested canyon just waiting to be explored.
A Giant Sandbox
When I was a kid I loved playing in the sand at the beach—digging holes, building sandcastles, and watching the waves wash it all away. Now that I have my own children—I am fascinated by how many hours they can spend playing in the sand.
For hydrologists, this fascination doesn’t stop at childhood. Hydrologists “play in the sand” all the time. In fact, many hydrologists work with small-scale “sandbox” models. Provided enough sediment and a continuous supply of water, these models help hydrologists better understand the large-scale ways water shapes the Earth.
The Zigzag River system is important to hydrologists because like a sandbox model, it too has a continuous supply of water and plenty of sediment—but it can be studied on a real-world scale. As Dar put it—it is a “giant sandbox.”
Let’s go play!
Hydrology Basics
Just a little past the confluence of Big Zigzag and Little Zigzag, Dar and I headed into the woods near the Paradise Park Trailhead. Here we followed the Zigzag River downstream along a lovely forested trail.
Stream morphology is influenced by a lot of different factors which makes interpreting a river’s path challenging for hydrologists. Unless you can directly observe the river as it takes shape, you must rely a lot on inferences.
However, according to Dar, there are still some basic principles and observations that offer a good starting point for understanding river dynamics.
Gradient
The first of these being steepness. Steep rivers tend to be more straight—water energy is directed downward resulting in deep, narrow channels. Flat rivers tend to meander or curve—water energy is directed unevenly, cutting one bank, while slowing and dropping sediment on the opposite bank.
Streamflow
The second principle involves streamflow. Streamflow or discharge is a measure of the volume of water flowing through a channel at a given point and at a given moment. Dar explained to understand streamflow you want to consider its velocity, or speed, as well as the cross-sectional area of the river. Knowing streamflow is important because, it not only tells you how much water is available, but it correlates with the kinetic energy of the stream. High flows will have a greater amount of energy, than low flows.
Streamflow is also dynamic. Thus, depending on how much the discharge fluctuates during a day or a year, the energy of the flow and the morphology of a stream may depend heavily on the time of day and/or seasonality. Even within a channel, streamflow can vary as water tends to follow the path of least resistance- resulting in more complex stream channels, with features like meanders, pools, gravel bars, etc.
Play Pooh Sticks
So next time you pass by a river or stream, take some mental measurements of all of that water rushing by—is the terrain steep? How much water is there? Throw a couple of leaves or sticks in the water and see how long it takes them to get from point A to point B. A quick game of “Pooh Sticks” and you can consider yourself an honorary hydrologist.
Riffle-riffle-riffle
Walking in the shade of the forest, we passed a turbulent section of the Zigzag River with impressive white water. While I was admiring it and snapping pictures, Dar explained what was going on.
“This is a riffle-riffle-riffle morphology,” he said. “It is fast because of the high gradient.” In a youthful stream, like the Zigzag River, water tends to follow the quickest path downhill. This generates a lot of erosive power and downcutting. Therefore, even though it was hard to see through all the white water, the loose sediment that makes up the Zigzag river bed was moving—transported downstream.
In contrast, streams with different flow regimes or sediment supplies have very different morphologies. For instance, if we were looking at a stream with no sediment load or an older stream where the stream bed was eroded to bedrock, we would be looking at a “pool-drop-pool-drop” morphology. Or if we were looking at the Zigzag River when the eruptions smoothed everything out, a single channel would have yet to be established. Instead many small, braided channels would make up the landscape.
Wood is a Wildcard
As important as gradient, streamflow, and sediment supply are to the morphology of a river, there is another factor of often equal importance. Dar described it as “a wildcard” when it comes to morphology—and that is wood!
As we continued following the trail downstream, we began to notice places where wood had fallen in the Zigzag River and altered its morphology.
Small Jam
One of the first examples we took note of was a small log jam. One end of a log had fallen into the stream and was still sticking out of the water on the other end—what Dar called a subhorizontal arrangement.
“There are only four or five ways a tree can interact in the water,” explained Dar. It can stick straight up and down, stick out from the bank, create a perfect dam across, or be subhorizontal in the water. Each of these creates different eddy patterns that accelerate the water in some places, scouring away sediments; while slowing down water in others, allowing sediments to accumulate creating bars or other depositional features.
With our small log jam, it was easy to see this lopsided pattern of stream erosion and deposition—there was erosion on the bank nearest to us and deposition on the opposite bank. In fact, some of the small boulders on the depositional side had been sitting in place long enough for moss to grow.
Big Jam
As we walked further along the tree-lined trail, we saw more examples of how wood was altering the morphology of the Zigzag River, changing it from a narrow, straight channel to one with increasing complexity.
Eventually, we ran into what Dar described as a “classic logjam.” The logjam was elaborate with two piers produced from tree fall on each bank. These piers slowed the water upstream, allowing for some pooling and deposition especially during high flows. In addition, the piers constricted the current—sending it through the middle of the river. The energy from the constriction was enough to scour the bottom of the stream, removing sediment, and creating a large scoop pool in the middle of the jam.
Dar also explained how logjams—like the one in front of us—form and are naturally maintained. When trees growing along a bank are undercut, they will fall into the river where they will collect sediment. If enough sediment is collected, another tree may grow in the sediment and eventually fall. So it is the repeated falling in of trees that creates and perpetuates logjams in a river.
Restoration in Reverse
Of course, one might wonder why logjams even matter. According to Dar, “wood is critical” in the Pacific Northwest. Wood naturally alters forested streams and has been doing do so long before humans arrived on the scene. Fish and other aquatic life have evolved in these wood enhanced streams. Thus, complex stream systems are essential for the survival of many of our culturally and ecologically important species, like salmon.
Unfortunately, when Europeans arrived on the scene, rivers were seen as a resource for commerce and transport. So wood, which interfered with these goals, was cleared out. Dar talked about how rivers like the Alsea and Nestucca were once wood-choked. However, with the removal of wood, they lost their complexity and their gravel. Now they are armored streams with hard rock and boulder bottoms. Dar called it, “restoration in reverse.”
Now, we know better. And we have been trying to get wood back in the rivers to restore their lost functions. The Zigzag River serves as an important model for how a forested stream develops without human intervention; providing information for restoration work now and in the future.
Lost in Time
As the trail directed us away from the Zigzag River and back toward road 39, Dar’s and my conversation began to meander. I brought up a topic that seemed important to the Zigzag River story and hydrology in general—the concept of time.
“Time is the 4th dimension of hydrology,” Dar said, “it is as big a parameter as anything else.” Even 100s of years of stream data and observation only provides a snapshot of the “life of a stream.”
In hydrology, change is relatively slow. It takes time for rocks to weather and erosion to occur; for banks to undercut and trees to fall; and for sediment to accumulate. Even faster processes like streamflow are restricted by time-bound processes like snowmelt and groundwater flow. Just like it is difficult to deduce the plot of a movie from one scene, our understanding of hydrology is time-bound and limited.
As Dar and I ended our hike on the Zigzag River, I reflected on all of this.
In only a few hours, Dar shared with me a fascinating story of a river—a story fashioned from a science that is only about 100 years old. Yet it is a story that has been playing for literally thousands of years and will play for thousands more. We are just getting started.
James “Dar” Crammond is the director of the USGS Water Science Center in Portland, Oregon. He also worked as the Chief of the Water Research Branch for USFWS and began his career with the Bureau of Reclamation in 1997, where he was a water rights expert. Dar has a B.S. in hydrology and J.D. from the University of Arizona, and is a member of the Arizona and Oregon State Bar Associations.