The Sauk River flows from melting snow and ice off the flanks of one of the most isolated of the ice shrouded Cascade volcanoes – aptly named Glacier Peak. From lowland van tage points, Glacier Peak barely stands above the surrounding peaks unlike the bold profiles of the other Cascade arc volcanoes. Yet its eruptive history and the danger it poses to the lowlands ranks among the highest of any in the chain. The calm and quiet of a forest walk along the banks of the Sauk ameliorate the danger. The moss draped trees and lush forest floor muffle the sound of the nearby rushing river. But the evidence of of a rushing lahar is here if you know what your eyes are seeing.
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The forest along the Old Sauk Trail I learned was logged in 1930, 90 years ago from this year. Over that timeframe, the firs and hemlocks have reached a considerable size. Only the occasional cedar stump with springboard notches provide a clue of past logging.
The Sauk River is a National Wild and Scenic River by being a tributary to the Skagit River. Congress established the Skagit Wild and Scenic River System in 1978 that included the Skagit and its tributaries – the Sauk, Suiattle, and Cascade Rivers. The National Wild and Scenic Rivers System was created by the Wild and Scenic Rivers Act of 1968 and signed into law by President Lyndon Johnson.
The Old Sauk Trail follows a level strip of land that is at a level somewhat higher than the active riverbed and channel. This level strip of land is a fluvial terrace. In places you can discern broad steps in the terrace where the trail steps up or down a few feet to another level terrace. You can see how this might look in the exaggerated cross-section.
Here’s a little fluvial terrace jibber jabber from Wikipedia.
Fluvial terraces are elongated terraces that flank the sides of floodplains and fluvial valleys all over the world. They consist of a relatively level strip of land, called a “tread,” separated from either an adjacent floodplain, other fluvial terraces, or uplands by distinctly steeper strips of land called “risers.” These terraces lie parallel to and above the river channel and its floodplain. Because of the manner in which they form, fluvial terraces are underlain by fluvial sediments of highly variable thickness.
Fluvial terraces are the remnants of earlier floodplains that existed at a time when either a stream or river was flowing at a higher elevation before its channel downcut to create a new floodplain at a lower elevation. Changes in elevation can be due to changes in the base level (elevation of the lowest point in the fluvial system, usually the drainage basin) of the fluvial system, which leads to headward erosion along the length of either a stream or river, gradually lowering its elevation. For example, downcutting by a river can lead to increased velocity of a tributary, causing that tributary to erode toward its headwaters. Terraces can also be left behind when the volume of the fluvial flow declines due to changes in climate, typical of areas which were covered by ice during periods of glaciation, and their adjacent drainage basins.
The map shows the flat terrace with the Old Sauk Trail following along the river and the Mountain Loop Highway on the other side of the terrace at the base of the slope. The contour interval is 80 feet, too great to show any subtle details of the terrace topography. For reference, Old Sauk North and Old Sauk South mark the two access points for the trail. We hiked this trail at the end of a long Thanksgiving weekend.
Summers are dry here in western Washington. The onset of rains in November breathe life into the forest floor. Mushrooms, lichens, and moss awaken in an astounding variety of shapes and colors.
Mosses are non-vascular plants. They can dry out and go dormant. Then, with some humidity and rain, they spring back to life.
Mosses and mushrooms.
A great variety of lichens are found here.
As mentioned in the opening, Glacier Peak is a very dangerous volcano. Because it’s so remote, there’s only one seismometer on it, where other Cascade volcanoes like Mount St. Helens and Mount Rainier are heavily instrumented. The big danger with Cascade volcanoes is lahars.
A lahar is a violent type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along a river valley.
Past eruptions from Glacier Peak are dated. Lahars that emanated from Glacier Peak are well documented and mapped.
Past eruptions have severely affected river valleys that head on Glacier Peak. Pyroclastic flows mixed with melted snow and glacial ice to form rapidly flowing slurries of rock and mud known as lahars.
About 13,100 years ago, dozens of eruption-generated lahars churned down the White Chuck, Suiattle, and Sauk Rivers, inundating valley floors. Lahars then flowed down both the North Fork Stillaguamish (then an outlet of the upper Sauk River) and Skagit Rivers to the sea. In the Stillaguamish River valley at Arlington, more than 60 miles downstream from Glacier Peak, lahars deposited more than seven feet of sediment. Shortly after the eruptions ended, the upper Sauk ’s course via the Stillaguamish was abandoned and the Sauk River began to drain only into the Skagit River, as it does today.
About 5,900 years ago and 1,800 years ago, dome-building eruptions generated lahars that extended once again to the sea, this time only along the Skagit River. In small eruptions since 1,800 years ago, lahars have extended the entire length of the White Chuck River and part way down the Suiattle.
pubs.usgs.gov/...
The vulnerable river valleys and lowlands are shown on the map. Our hike along the Old Sauk River Trail was just south of the town of Darrington.
This next photograph shows a lahar deposit on the other side of the river, visible from the Old Sauk River Trail. The course layer with boulders and big cobbles was likely the initial rapidly moving lahar flow. Lots of energy required to move those rocks plus they literally float on the moving mud flow which is denser than water, almost as dense as the rocks themselves. As the flow slowed down, there was less energy to carry the big cobbles, and so the finer material was carried and deposited above the coarse layer.
I enjoy looking at all these pieces, from the geology, to the trees, down to the little mosses, and trying to understand how it all fits together.
Sidenote: The Burger Barn in Darrington makes for a nice post-hike meal.
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Now It's Your Turn
What have you noted happening in your area or travels? As usual post your observations as well as their general location in the comments.