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Movement of Water in Glaciers *

  • R. L. Shreve (a1)

Abstract

A network of passages situated along three-grain intersections enables water to percolate through temperate glacier ice. The deformability of the ice allows the passages to expand and contract in response to changes in pressure, and melting of the passage walls by heat generated by viscous dissipation and carried by above-freezing water causes the larger passages gradually to increase in size at the expense of the smaller ones. Thus, the behavior of the passages is primarily the result of three basic characteristics: (1) the capacity of the system continually adjusts, though not instantly, to fluctuations in the supply of melt water; (2) the direction of movement of the water is determined mainly by the ambient pressure in the ice, which in turn is governed primarily by the slope of the ice surface and secondarily by the local topography of the glacier bed; and, most important, (3) the network of passages tends in time to become arborescent, with a superglacial part much like an ordinary river system in a karst region, an englacial part comprised of tree-like systems of passages penetrating the ice from bed to surface, and a subglacial part consisting of tunnels in the ice carrying water and sediment along the glacier bed. These characteristics indicate that a sheet-like basal water layer under a glacier would normally be unstable, the stable form being tunnels; and they explain, among other things, why ice-marginal melt-water streams and lakes are so common, why eskers, which are generally considered to have formed in subglacial passages, trend in the general direction of ice flow with a tendency to follow valley floors and to cross divides at their lowest points, why they are typically discontinuous where they cross ridge crests, why they sometimes contain fragments from bedrock outcrops near the esker but not actually crossed by it, and why they seem to be formed mostly during the later stages of glaciation.

Résumé

Un réseau de canaux situés le long des lignes de contact entre trois grains permet à l’eau de percoler à travers la glace des glaciers tempérés. Grâce à la capacité de déformation de la glace, ces canaux peuvent se dilater ou se contracter selon les variations de pression; la chaleur engendrée par l’écoulement visqueux et transportée par l’eau au-dessus de son point de fusion entraine la fusion des parois des canaux et aboutit à accroître les dimensions des plus grands canaux aux dépens des plus petits. Ainsi, le comportement des canaux est en premier lieu le résultat de trois caractéristiques fondamentales; (1) la capacité pour le système de s’ajuster continuellement, quoique non instantanément, aux fluctuations dans les apports d’eau de fusion; (2) la direction du mouvement de l’eau est déterminée surtout par la pression ambiante dans la glace qui à son tour est gouvernée d’abord par la pente de la surface de la glace puis par la topographie locale du lit glaciaire; enfin, et c’est le plus important. (3) le réseau des canaux tend, avec le temps, à devenir arborescent avec une partie superficielle ressemblant beaucoup à un réseau hydrographique dans une région karstique, une partie intraglaciaire composée d’un système de canaux en forme d’arbre pénétrant la glace du fond vers la surface, et une partie sous-glaciaire consistant en tunnels dans la glace charriant de l’eau et des sédiments le long du lit glaciaire. Ces caractéristiques indiquent qu’un niveau d’eau étendu comme un drap sur le lit d’un glacier devrait normalement être une formation instable, la forme stable étant celle des tunnels; ceci explique, entre autres, pourquoi les torrents et lacs d’eau de fusion sont si communs le long des rives des glaciers, pourquoi les eskers, qui sont généralement considérés comme ayant été formés par des canaux sous-glaciaires, tendent à s’aligner dans la direction générale de l’écoulement de la glace avec une tendance à suivre le fond de la vallée et à traverser les obstacles en leur point le plus bas, pourquoi ils sont typiquement interrompus lorsqu’ils traversent la crête d’une ondulation, pourquoi ils contiennent parfois des fragments prélevés sur le lit près de l’esker mais non exactement sur sa trajectoire et pourquoi ils semblent se former surtout au cours des derniers stades d’une glaciation.

Zusammenfassung

Ein Netzwerk von Durchlässen entlang von Dreifach-Kornverschneidungen ermöglicht das Durchsickern von Wasser durch das Eis temperierter Gletscher. Das Deformationsvermögen des Eises erlaubt in Abhängigkeit von Druckänderungen eine Erweiterung oder Verengung der Durchlässe; Schmelzen der Durchlasswände durch Wärme, die durch viskose Dissipation erzeugt und durch auffrierendes Wasser geleitet wird, verursacht ein stetiges Wachsen der grösseren Durchlässe auf Kosten der kleineren. Somit ist das Verhalten der Durchlässe im wesentlichen durch drei Grundtatsachen bestimmt: (1) Die Kapazität des Systems passt sich ständig, wenn auch nicht sofort, den Schwankungen im Schmelzwassernachschub an; (2) Die Richtung der Wasserbewegung wird vornehmlich durch den umgebenden Druck im Eis bestimmt, der seinerseits primär von der Neigung der Eisoberfläche und sekundär von der lokalen Topographie des Gletscherbettes abhängt; und vor allem (3) Das Netzwerk der Durchlässe neigt dazu, sich mit der Zeit baumartig zu verzweigen, mit einem Teil an der Gletscheroberfläche, der sehr einem gewöhnlichen Flusssystem in einer Karstregion ähnelt, einem innerglazialen Teil, der baumartige Durchlasssysteme enthält, die das Eis vom Grund bis zur Oberfläche durchsetzen, und einem subglazialen Teil, bestehend aus Tunneln im Eis, in denen Wasser und Sedimente am Gletscherbett entlanggeführt werden. Diese Tatsachen zeigen, dass eine blattförmige Wasserschicht am Grunde eines Gletschers normalerweise instabil ist; die stabile Form sind Tunnels. Darüber hinaus erklären sie neben anderen Erscheinungen, warum Schmelzwasserströme und Seen am Gletscherrand so häufig auftreten; warum Esker, deren Entstehung allgemein in subglazialen Kanälen angenommen wird, in der Hauptfliessrichtung des Eises verlaufen mit einer Tendenz, Talböden zu folgen und Wasserscheiden an ihren niedrigsten Stellen zu überqueren; warum sie beim Queren von Rücken so typisch unzusammenhängend sind; warum sie manchmal Bruchstücke von Gestein des Untergrundes enthalten, das zwar nahe am Esker ansteht, aber nicht von ihm erfasst wird, und warum sie sich anscheinend meistens während der Spätstadien einer Vergletscherung gebildet haben.

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Copyright

Footnotes

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Publication 956 of the Institute of Geophysics and Planetary Physics, University of California, Los Angeles.

Footnotes

References

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Journal of Glaciology
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