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A History of Jökulhlaups from Strandline Lake, Alaska, U.S.A.

  • Matthew Sturm (a1) and Carl S. Benson (a1)

Abstract

Jökulhlaups, also known as outburst floods, have occurred every 1 to 5 years from Strandline Lake, one of the largest glacier-dammed lakes in North America. The development of a distinct calving embayment in the lobe of Triumvirate Glacier which dams the lake, as well as the filling of a number of supraglacier pools, appear to be reliable precursors to a jökulhlaup. Analysis of contour maps made from photographs taken immediately before and after the jökulhlaup of 17 September 1982 indicate that over 95% of the lake drains, a volume of approximately 7 × 108 m3 of water. The glacier lobe which dams the lake fractures and subsides during a jökulhlaup, indicating that the release mechanism is hydrostatic lifting of the ice off of a sub-glacial spillway. Evidence from the ice-free margins of the glacier suggests that the spillway may be controlled by bedrock. Large variation in the refilling period of Strandline Lake indicates that the subglacial drainage tunnels can remain open for as much as a few years after a jökulhlaup, before they become sealed by sediments and/or glacier ice.

Résumé

Des vidanges brutales du Strandline Lake, un des plus grands lacs de barrage glaciaire à vidange périodique et brutale d’Amerique du Nord, se produisent tous les 1 à 5 ans. La vidange catastrophique est précédée par le développement d’un golfe de vêlage dans le lobe du Triumvirate Glacier qui barre le lac ainsi que par le remplissage d’un grand nombre de petits lacs sur le glacier. L’analyse des cartes dressées par photogrammétrie immédiatement avant et après la vidange du 17 septembre 1982 montre que plus de 95% du volume du lac est évacué, soit environ 7 × 108 m3 d’eau. Le lobe du glacier qui barre le lac se fracture et s’affaisse pendant la vidange ce qui signifie que cette dernière est due à un soulèvement hydrostatique des 180 m de glace libérant un déversoir sous-glaciaire. Des évidences apparentes sur les bords libres de glace suggèrent que le déversoir peut être contrôlé par le lit rocheux. De grandes variations dans la durée de remplissage du Strandline Lake indique que les tunnels de drainage sous glaciaires peuvent rester ouverts pendant quelques années à la suite d’une vidange, avant d’être obstrués par des sédiments et/ou de la glace provenant du glacier.

Zusammenfassung

Gletscherläufe, auch als Ausbruchsfluten bekannt, traten aus dem Strandline Lake, einem der grössten Gletscherlaufseen in Nordamerika, alle 1–5 Jahre auf. Die Entwicklung einer bestimmten Einbuchtung durch Kalbung in der Zunge des Triumvirate Glacier, der den See abdämmt, und die Auffüllung einer Anzahl von Wannen auf der Gletscheroberfläche scheinen die untrüglichen Vorläufer eines Gletscherlaufes zu sein. Die Analyse von Höhenlinienkarten, hergestellt aus photographischen Aufnahmen unmittelbar vor und nach dem Gletscherlauf vom 17. September 1982, zeigt, dass über 95% des Sees auslaufen, eine Wassermenge von ungefähr 7 × 108 m3. Die Gletscherzunge, die den See abdämmt, bricht während eines Gletscherlaufes auseinander und sinkt zusammen, woraus sich schliessen lässt, dass der Auslaufmechanismus in der hydrostatischen Hebung des Eises aus einer subglazialen Abflussrinne besteht. Anzeichen am eisfreien Gletscherrand lassen vermuten, dass dieser Führungskanal im Felsbett ausgebildet ist. Grosse Schwankungen in der Periode der Wiederauffüllung des Strandline Lake lassen annehmen, dass die subglazialen Abflusstunnels bis zu einigen Jahren nach einem Gletscherlauf offen bleiben können, bevor sie durch Sedimente und/oder Gletschereis wieder verschlossen werden.

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References

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Sturm, M., and Benson, C.S. Unpublished. Report on Strandline Lake flooding hazard. Report submitted to Chugach Electric Company, Anchorage, Alaska. Fairbanks, Alaska, University of Alaska. Geophysical Institute.
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Journal of Glaciology
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