Floating the Skagit River’s Geologic History

Guest post by Dr. Jon Riedel

Greetings from soggy Marblemount, where the cool, wet spring weather delayed the spring flood on Skagit River and led to a remarkable explosion of slugs in my garden [note: Jon wrote this blog post in the springtime]. The annual snowmelt flood can occur anytime in May or June, and persists for many days, if not weeks. Before construction of the dams, it was in many ways the most important annual event for the river. Fall floods are often larger, but less frequent and short-lived. The annual spring flood shapes the channel and gravel bars of mountain rivers, and delivers massive quantities of sediment to the sea. The floods also clear important cutbanks along the river, providing exposure of the river’s history.

I’m looking forward to exploring the Skagit River’s patterns and history on a one-day raft trip with North Cascades Institute participants – ‘Floating the Skagit River’s Geologic History’ on August 22. This Field Excursion will provide the best avenue for examining the river’s past as it gives us access to several important cutbanks along the river where sediments recording the river’s history accumulated over the past 12,000 years. We’ll stop to see evidence of ice age lakes that filled the valley, late ice age floods from Canada, and deposits from eruptions of Glacier Peak volcano. Along the way, we’ll examine other features of the river including changes in river pattern and activity.

I have been examining the Skagit River Valley’s geologic history for most of my professional career, including over 40 years with the National Park Service at North Cascades National Park. My early efforts were focused on the upper watershed, while more recently I have studied the river history from Gorge Dam to Skagit Bay. 

For this outing, we will explore evidence of several important events, and discuss the impacts of several others (including the slug outbreak).

The first event of interest occurred when a valley glacier emanating from the Baker River Valley blocked the Skagit River at Concrete from about 30,000 to 21,000 years ago, diverting it to flow to the sea via the Stillaguamish River.  We will stop at a spectacular outcrop above Concrete that exposes sediments deposited in the lake, including a rich collection of macrofossils that allowed us to reconstruct the climate of the last ice age.

Massive glacial floods flowed through Skagit Valley as the continental ice sheet retreated into Canada at the end of the last ice age between 13,600 and 11,700 years ago. One of these floods carried the flow of the Fraser River through Skagit Gorge. Imagine the sound and fury of that much water surging through the 18 ft. wide canyon at the base of Diablo Dam! These floods had several important impacts on the watershed, including returning the river to its former course into lower Skagit Valley and slicing deeply into the glacial deposits. 

Two other major events we will ponder include eruptions of Glacier Peak volcano about 6,000 and 2,000 years ago. The eruptions melted glaciers on the peak, sending huge floods of sand and gravel down the Sauk River to the Skagit River. These events led to rapid growth of Skagit River delta into Skagit Bay, forming the extensive and agriculturally productive Skagit Flats farmland.

I’m looking forward to exploring the Skagit River from the unique vantage point of a river raft and hope to meet some of you out there!

Jon L. Riedel holds a M.S. degree in Geography from the University of Wisconsin and a Ph.D. in Earth Science from Simon Fraser University (2007). His academic and professional career have focused on understanding the glacial and fluvial history of Skagit Valley, and he has published more than a dozen peer-reviewed studies. Jon retired from the U.S. National Park Service at North Cascades National Park after a 40-year career as an interpreter, ranger, wilderness steward, and geologist. Jon continues to be active professionally as a licensed geologist with his consulting business, Skagit Quaternary. He is currently leading a multi-year effort to map the surficial geology of the Skagit Valley floor from Gorge Dam to Puget Sound and is investigating groundwater resources in lower Skagit Valley.