I am constantly amazed by the power of water to sculpt the landscape. From glacially carved canyons and deep V-shaped ravines to massive floods capable of eroding and depositing sediment over 100s of miles—water in its various forms has shaped the Earth in profound ways. The impact of water on the landscape can be seen all around us. If we know where to look.
Luckily for me, I arranged to meet up with Hal Wershow, a geologist and expert on reading the landscape, to help me better see and understand water’s influence in the Pacific Northwest.
Naturally, we headed to the Cascades to a popular hiking spot in Three Sisters Wilderness called Green Lakes.
The Hike
- Trailhead: Green Lakes Trailhead
- Distance: 9 miles round trip to first two Green Lakes
- Elevation Gain: 1,100 ft
- Details: This trail is very popular and was heavily trafficked until permits were put in place in 2021. A Central Oregon Cascades Wilderness is required from May to September. The trailhead is easily accessible and there is ample parking. A pit toilet is available at the trailhead.
Opening the Flood Gates
The path hastens along next to a fresh flowing creek lined with conifers and dotted with colorful wildflowers. A few puffy white clouds floated past us overhead as Hal and I began our hike from the Green Lakes Trailhead.
The ground was level with baseball-sized pieces of pumice and other volcanic rocks scattered between bunches of vegetation. “Fluvial is the term we use for sediment moved by water,” explained Hal. The rounded rock and flat ground are signs that water flooded the area in the past.
This, of course, begs the question—what happened? Short answer—No Name Lake.
You see, No Name Lake was formed by a glacial moraine, or an accumulation of unconsolidated rock, that was carried in and left behind by a receding glacier from the “Little Ice Age” of the 1800s. Then in the 1960s, an unexplained breach in the moraine occurred resulting in a flood. Perhaps some ice or rock had fallen in the lake generating waves that overtopped the dam causing it to fail.
Interestingly, the source of the flood was reported by Bruce Nolf, a geology professor at COCC at the time—a position Hal now occupies.
The waters from that flood would have washed into the area, Hal explained, carrying sediments and debris all the way across the highway we had just driven in on.
Snow Going
It wasn’t long before Hal and I, following the creek-side path, entered a more densely wooded area still blanked in snow. It was early summer and winter snow still lingered in large patches on the trail.
Snow accumulation in the Cascades is incredibly important in the Pacific Northwest. As snow melts it seeps into the ground slowly through pores in rock, becoming part of the groundwater. This water eventually escapes back to the surface through springs that feed our streams and rivers. The lag time between precipitation, snowmelt, and water resurfacing is important helping ensure water supply even in the drier parts of the year.
Spring Forward
Hal told me about a project he is doing with his students at CCOC where they are studying the time water spends underground—also called residence time. Referred to as the “Spring Monitoring Project,” Hal’s students are locating and gathering samples of water from springs in the Central Cascades near Bend. Then they are sending the samples to a lab for stable isotope dating to determine the residence time of each spring.
Stable isotope dating is used for a lot of applications—to date the age of fossils, archeological artifacts, etc. Elements, like carbon and hydrogen, have a different ratio of their respective isotopes depending on conditions and can change over time. For example, all living things contain a ratio of C-12 to C-14 that is constant, but once an organism dies, C-14 will decay predictably, changing the ratio. This change in ratio allows scientists to determine the age of tissue containing artifacts.
Spring water works in much the same way but uses different isotope tracers to figure out how long water has been underground. The time spent underground varies a lot. Water can remain underground for minutes to thousands of years.
“This research is important, especially in the light of climate change,” Hal explained. With increased drought conditions coupled with increasing demands on water resources, it is important that we understand how much water will be available each water year. Springs with long residence times may be more resilient to climate change.
Rushing Waters
Hal and I continued to crunch over frozen hills of snow, watching out for snow bridges, as we continued to pick our way alongside Fall Creek under a canopy of mountain hemlock and fir.
Eventually, we passed by Fall Creek Falls in just a little over half a mile and took a moment to appreciate the raging white waters as they rushed down a short rockface. Fall Creek and its falls are fed by the same waters that fill Green Lakes which, in turn, are fed by glacial and snowmelt from South Sister.
Waterfalls are another example of the force of water on the landscape. Water is an agent of erosion, but not all materials erode equally. For example, most sedimentary rock erodes easily, while others, like igneous rock, granite, are more resistant. Waterfalls, like Fall Creek Falls, form when there is a difference between the materials that make up the streambed. Essentially, the material below the waterfall eroded more easily than the material above it.
We continued to trace Fall Creek’s flow further upstream, the trail trending uphill through some switchbacks, eventually crossing the creek on a narrow log bridge.
Walk on, Rock On
As we walked along the path, Hal pointed out some of the different rocks found along the trail. All the rocks we saw were igneous rocks—formed from cooled magma.
In general, igneous rocks can be divided into two major groups based on their silica content—mafic rock and felsic rock. Mafic rock is low in silica (45-55% silica) and is generally darker in color. The lava is less viscous (due to its low silica content) and erupts smoothly, as gases readily escape and don’t build up generating the pressure needed for an explosive eruption. Dark grey basalt is a classic example of mafic rock.
Felsic rock on the other hand is high in silica (65% or higher silica) and tends to be lighter in color. The lava is much more viscous and stickier making it difficult for water and gases to escape. The result is a buildup of pressure and more explosive, violent eruptions. Pale tan or pink rhyolite is a classic example of felsic rock.
Light grey andesite is an intermediary (55-65% silica) between mafic and felsic. Andesite rock has enough silica to produce quartz crystals, so it often has a “salt and pepper” appearance.
Disorganized
However, the chemical composition of igneous rocks is not the only thing that determines their final structure. For example, rocks exposed to oxygen may become redder; rocks that form under the Earth’s surface grow larger crystals; and rocks formed during explosive eruptions may be more fragmented.
One of the most common rocks Hal pointed out on the trail was pumice. Chemically, pumice is like any other rhyolite rock, but because of the conditions it formed in, pumice has some unique qualities.
Pumice is formed during violent eruptions of very viscous rhyolite lava that is very high in water and gases. When ejected, the gases escape rapidly and the water evaporates and expands, causing the lava to become frothy. Pumice is a disorganized rock—formed so quickly that there was no time for it to crystalize. Hal called it “volcanic glass.”
The resulting rock is an incredibly light, vesicular rock with the reputation of being able to float in water.
Slow your Flow
However, one of the most striking rocks seen on the trail isn’t pumice, but obsidian—a shiny, (usually) black rock, generally known for its use in arrowheads and other edged tools. The cutting edge of an obsidian tool is sharper than a surgeon’s steel scalpel.
Not too long after crossing Fall Creek, part of the 2,000-year-old Newberry lava flow comes into view—a massive wall of rhyolite—much of it in the form of obsidian. The wall is a spectacular feature for the next few miles, hemming in Fall Creek on the opposite bank from the trail.
Hal explained that obsidian, like pumice, is also rhyolite. However, unlike pumice, obsidian is not the result of explosive eruptions, but rather viscous lava that exudes slowly from volcanic vents. Just like pumice and volcanic ash, obsidian has no crystalline structure and is also “volcanic glass.”
Hal described the lava flow as being so slow that the movement would have been imperceptible to the human eye—we are talking less than a few meters per hour. The flow would have also been cooler and not like the red-hot magma seen erupting from volcanos in Hawaii that tend to be mafic lava flows.
Lakes O’ Plenty
Hal and I continued to hike uphill through the forest, crossing several smaller creeks as we went. Eventually, we reached a sign with a map indicating we were about to enter the Green Lakes Basin.
Early in the hike, Hal told me that there were several ways lakes can form. A glacial moraine is one way, like the one that formed Broken Top’s No Name Lake. A lava flow dam is another. Green Lakes is an example of a lava-dammed lake. From the map you could see where the lava flow displaced the creek and cut off most of the area above, creating the basin.
Hal also pointed out the areas where water is flowing into Green Lakes. Not just water, but sediment too. They are being filled up, Hal explained. The addition of sediment means that Green Lakes will not be around forever.
“Another 1,000 years and they won’t be here,” Hal stated emphatically.
Composite
Past the sign, the first of the Green Lakes comes into view. Flanking the blue-green waters are two massive peaks—South Sister and Broken Top. Like sentinels, they tower above Hal and me. While at the same time, seemingly close enough to touch.
Both South Sister and Broken Top are stratovolcanoes, also called composite volcanoes—named for the varying nature of erupted materials that build their steep cones—anything from lava to ash. The formation of a stratovolcano is a process of building up and tearing down. They are known for violent eruptions where large amounts of their mass may be ejected into the air—sometimes leaving a large crater. Mount St. Helen’s is a composite volcano. Mt. Mazama, where Crater Lake now stands, is also a composite volcano that blew its top over 7,000 years ago.
Fire and Ice
However, as Hal reminded me, volcanism is not the only powerful force at work in the High Cascades. Ice—in the form of glaciers—is also a powerful agent of change in this volcanic landscape.
South Sister, with her tall dome shape retained, is still active—with recent eruptions dating back only a couple thousand years. In contrast, Broken Top is a long-extinct volcano—last active over 150,000 years ago. Since then, Broken Top has been roughly hewn by glaciers leaving its summit a jagged pile of rock and eruption crater exposed. Glaciers are moving ice, capable of abrading and polishing down rock, creating steep-sided hollows, and leaving behind sharp peaks and ridges. Hal pointed out some of the features formed by glaciers on Broken Top, including a cirque, horn, and arete.
Glaciers can still be seen on both Broken Top and South Sister—though they are much smaller and fewer than just a hundred years ago due to anthropogenic climate change. Staring up at South Sister, I asked Hal how to identify a glacier well enough to tell it apart from snowpack. “Crevasses—deep breaks in the ice formed as different parts of a glacier travel at different speeds—are one key difference,” Hal responded.
But Hal also noted that Glaciers can be very difficult to spot. So difficult, in fact, that only a month earlier, a “new” glacier was discovered on South Sister by Oregon Glacier Institute, an organization with the goal of identifying and monitoring Oregon’s glaciers. And by “new,” I mean new to science. “Glaciers tend to be in areas that aren’t very visible,” Hal warned, “making them difficult to locate.”
Alluvial Fans
Continuing our hike, Hal and I followed a trail that put us closer to the base of South Sister. Here we reached a deep water crossing and a view of one of the alluvial fans that South Sister’s meltwaters created stretching out in front of us.
An alluvial fan forms when terrain suddenly becomes less steep, like at the base of a mountain, and the water flow less restricted. As the gradient is lowered, the water flow slows and spreads out, dropping sediment in a fan or cone shape.
Earlier in the hike, Hal pointed out a “mini-version” of an alluvial fan where steep flowing drainage of water slowed near the trail as the path of the water flattened and the water was unconstrained. Though perhaps not as dramatic as the large alluvial fan in front of us, the principals are the same. When water slows, sediment drops out.
Hal and I considered crossing the creek to get a better look at the first fan, Hal even attempting to balance his way across some unstable logs, but instead opted for an adventure around the second Green Lake and past the third to the alluvial fan on the far side of Green Lakes.
Round We Go
As Hal and I made our way around the largest of the Green Lakes, we kept a lookout for more geological treasures.
The snow continued to be a bit challenging at times, but we treaded carefully along the narrow trail.
Before long we spotted signs of an ephemeral spring. Though no water was rushing forth from the Earth, Hal pointed out the eroded channels, changes in vegetation, and exposed roots—all indicators that water had flown forth at some point during the year.
A bit later, Hal spotted a perfect example of high silica, rhyolite, and low silica, basalt sitting side by side on the trail.
Breach
Eventually, we made it to the bottom of the alluvial fan. Hal explained that there was evidence, at least in part, that the fan was a result of a breach in a moraine-dammed lake further up the mountain. The plan was to head off-trail and follow the alluvium up to see if we could reach the moraine lake.
Almost immediately after heading off-trail, Hal started pointing out the changes in terrain. Like a kid-in-a-candy-store he had me looking at the rock that now littered the ground. “No pumice!” he exclaimed.
Instead of pumice, the area was filled with volcanic rock that looked speckled—with larger crystals embedded in a finer grain. A “porphyritic texture,” stated Hal—formed from lava that cooled slowly below the surface before rapidly cooling above the surface.
The “fresh rock” signaled to Hal that the lava bed we were walking in was from a different eruption than the pumice and lava flow from earlier.
Signs of a Flood
Hal’s excitement continued as we picked our way up the drainage—the area was literally awash in signs of past flooding.
For one, the size of the rocks changed—smaller rocks gave way to larger rocks—as we moved up. Hal explained that this was expected, as smaller rocks can be carried by the floodwaters farther than larger rocks, which would have been dropped closer to the breach.
Larger rocks also piled up along the edges of the now-empty flood channel—forming natural levees. Again, Hal explained how the energy of the floodwaters would have dissipated toward the edges, dropping these boulders into place.
Hal also noted how the forest looked different in the flood zone. Looking beyond, you could see a lot of taller trees, but within the flood zone, there were only small trees. Trees in the area would have been toppled by the floodwaters. The smaller trees, Hal explained, would have sprouted after the last big flood.
Flow Banding and Glacial Polish
Hal and I continued to pick our way over larger and larger rocks. Along the way, we saw some more fun geological features in the rock.
One such feature was a large rock near the edge of our flood channel that looked striped or banded. Hal explained that each band was really the result of different flow rates in the lava that cooled to form the rock—a phenomenon known as flow banding. Flow banding occurs because there is the shearing force between the layers of lava causing them to flow differently relative to one another.
A bit later, Hal pointed out another rock. This one was smooth with some well-defined grooves. Unlike the flow-banded rock, the lines in this rock were formed from a glacier. When glaciers pass over rock, Hal explained, they carry gritty sediments that will abrade the rock, polishing the rock smooth. If a larger rock is stuck in the glacier, it will carve deeper grooves in the rock as well. The overall effect is called glacial polish. Hal suggested thinking of it like sandpaper—different parts of the glacier may have a different grit resulting in differences in the polish.
Survivor
We continued heading up the rocky drainage, crossing several snowfields. The rock levees are now as much as 10 feet tall in places. Looking back, beautiful views of the Green Lakes Basin periodically caught my attention.
Apart from the snow, boulders made up most of the ground surface as we trekked upward. The young forest seen toward the base of the washout was nonexistent. But what we did find were remnants of a vegetative past.
At one point, Hal and I saw a log stuck in the sediment that sparked some interest. Organic material, like the log, can be dated using either radiocarbon dating or dendrochronology. Radiocarbon dating would provide the apparent age of the tree, a decent estimate of age as far as geological events go.
However, one of my favorite spots on our hike was where we passed a live tree that had somehow survived the floods. Though a bit disheveled, broken and stripped of bark on one side, it was beautiful in its own way. We stopped for a while by this tree, breaking for water. Standing there looking up at its worn trunk I was drawn to its ruggedness. It’s history. It’s story.
A Story
Hal and I never made it to the glacial lake to see the breach. Logistics didn’t allow for it. We did, however, see its effects.
The story of the Earth is one of constant change—often slow but punctuated by quick, sometimes devastating, alternations. Hiking with Hal reminded me of this.
Powerful natural forces that shape the planet, like water, make change inevitable, but also knowable. The story of our planet unfolds as we read the geology. And, like a tree battered by floodwaters, it is one of beauty and resilience.
Hal Wershow is an Assistant Professor of Geology at Central Oregon Community College. His prior experience includes work in the environmental services industry and geoscience education. Hal earned a Master’s in Geology from Western Washington University.