Portage Glacier – a Century of Retreat EarthCache

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Portage Glacier – a century of retreat

Welcome to an earthcache, where there is no container to find or logbook to sign. Instead, your task is to locate answers on-site to six questions about the geology of this location, and to email your answers to the cache owner to demonstrate what you’ve learned here during your visit.

Located at the eastern end of Portage Valley, Portage Glacier and its associated US Forest Service Begich-Boggs Visitor Center has been identified as southcentral Alaska’s most popular tourism attraction. Archaeological evidence (reinforced by discoveries made during beach cleaning activities in the aftermath of the Exxon Valdez oil spill) shows the route from western Prince William Sound across Portage Pass and Portage Glacier was popular for trade and hunting purposes long before European visitors reached this region. All early European explorers of this region make written note of this route as well,  confirming its enduring popularity as a place to visit for many different reasons.

A methodical survey of Portage Valley and Portage Pass was made in 1911, as part of the first effort to link the ice-free waters of Passage Canal and Prince William Sound with what would later become the Alaska Railroad. The tunnel connecting Whittier (at the head of Passage Canal) with the main line of the Alaska Railroad finally went into construction early in WWII, and survey parties in the area in 1939 as part of the tunnel construction planning were quick to note how the face of Portage Glacier had receded eastward away from its 1911 location. In fact, Portage Glacier had receded so far that the route across the glacier from Whittier to Portage Valley was no longer usable. The surveyors had to travel over the very top of the mountain along the north side of the glacier and Portage Lake at an elevation of 3,000 feet.

When the Begich-Boggs Visitor Center was constructed alongside the outlet of Portage Lake in 1986, the face of Portage Glacier was clearly visible, and icebergs calved from the glacier would often jam the western outlet of the lake right in front of the Visitor Center. Today Portage Glacier is not visible at all from the Visitors Center (only Burns Glacier can be seen - refer to map below), and what icebergs arrive at the western end of the lake where Portage River spills out into Portage Valley are much smaller than those seen over two decades ago.  The glacier has retreated so far east up the valley that its face has partly drawn out of the lake, exposing rock at the foot of the face, above the water level of the lake. Falling ice still drops directly into the lake below, but the height of the ice is almost all above water. When the glacier was in the lake, much of the height of the face was unseen below water.

You’re standing on the terminal moraine of Portage Glacier, estimated to have been established some time in the 1880s. This mound of gravel and rock was pushed up by the face of the glacier (or deposited by melting) at its maximum extent of travel down the valley, during the end of its period of growth in the mid-late 1800s.  Like other Alaskan glaciers that terminate in water, such as Columbia Glacier near Valdez or Mendenhall Glacier near Juneau, Portage Glacier has experienced accelerated retreats in recent decades that likely were initially triggered by climate change begun at the end of the “Little Ice Age” in the mid-1800s.

Examine the interpretive sign in front of you here at the high point of the moraine. It uses a conversational style of communication to impart information, and contains an error which leaves a false impression of just how far away the glacier’s face is from this viewpoint. You’ll need the information presented on this panel to answer part of the questions below. Note the distance to the face of the glacier is given in miles – it should have been listed as kilometers. The number is accurate, but only if it is read as X kilometers. This error is on the sign is confirmed by a close study of current maps and the US Forest Service’s own technical reports and public reports regarding how far away the face of Portage Glacier has retreated in the last century. The interpretive sign at the foot of the stairway to the south contains anecdotal information needed to answer your learning questions as well.

Please email your answers to the cache owner – do not include them in your on-line log.

1. According to the interpretive sign, about how far away is the face of the glacier from here?  (remember, the sign should have read X kilometers, not X miles)
   
2. Based on this distance to the face of the glacier, what is the approximate average retreat per year in meters since 1890 (assuming the glacier’s face was here where you stand in 1890)?
   
3. According to the interpretive sign, how deep is the lake’s deepest point?
   
4. According to the southernmost interpretive sign, what is the primary tool of erosion in this glacial valley?
   
5. Why is the water in most of the nearby streams and Portage River gray in color?
   
6. How high above the lake are you where you’re standing now atop the moraine? Estimate the distance of your location from the lakeshore. These two measurements will help you imagine how the face of the glacier may have looked from this viewpoint at its maximum point of advance. Do you think the face of the glacier may have been taller than the height of this viewpoint on the moraine at its point of furthest advance? (Be sure to examine the first photograph on the cache page itself to help understand elevation of the face of the glacier - the head of the man sitting in the boat is perhaps 4' above the lake's surface. Helpful hint – you can take an elevation reading and mark a waypoint with your GPS at the lakeshore to help determine elevation above and distance to the lake)
   
7. Although not required, please contribute to the photographic record of this location by uploading a photographic record of what you saw looking east over Portage Lake during your visit.

U.S. Geological Survey Hydrologic Investigations Atlas HA-583
Recession of Portage Glacier from 1880 to 2020
By C.R. Mayo, Chester Zenone, and D.C. Trabant, 1977

U.S. Geological Survey Bulletin 926-D
Geology of the Portage Pass Area, Alaska
By F.F. Barnes, 1943