Water Flow Through Temperate Glaciers

Portland State University PDXScholar Geography Faculty Publications and Presentations Geography 8-1-1998 Water Flow through Temperate Glaciers Andrew G. Fountain Portland State University Joseph S. Walder Follow this and additional works at: https://pdxscholar.library.pdx.edu/geog_fac Part of the Environmental Sciences Commons Let us know how access to this document benefits ou.y Citation Details Fountain, Andrew G., and Joseph S. Walder. "Water flow through temperate glaciers." Reviews of Geophysics 36.3 (1998): 299-328. This Article is brought to you for free and open access. It has been accepted for inclusion in Geography Faculty Publications and Presentations by an authorized administrator of PDXScholar. Please contact us if we can make this document more accessible: [email protected]. WATER FLOW THROUGH TEMPERATE GLACIERS Andrew G. Fountain1 Joseph S. Walder Department of Geology U.S. Geological Survey Portland State University Cascades Volcano Observatory Portland, Oregon Vancouver, Washington Abstract. Understanding water movement through a are usually full of water and flow is pressurized. In glacier is fundamental to several critical issues in glaci- contrast, water flow in englacial conduits supplied from ology, including glacier dynamics, glacier-induced the ablation area is pressurized only near times of peak floods, and the prediction of runoff from glacierized daily flow or during rainstorms; flow is otherwise in an drainage basins. To this end we have synthesized a open-channel configuration. The subglacial drainage conceptual model of water movement through a temper- system typically consists of several elements that are ate glacier from the surface to the outlet stream. Pro- distinct both morphologically and hydrologically. An up- cesses that regulate the rate and distribution of water glacier branching, arborescent network of channels in- input at the glacier surface and that regulate water cised into the basal ice conveys water rapidly. Much of movement from the surface to the bed play important the water flux to the bed probably enters directly into the but commonly neglected roles in glacier hydrology. arborescent channel network, which covers only a small Where a glacier is covered by a layer of porous, perme- fraction of the glacier bed. More extensive spatially is a able firn (the accumulation zone), the flux of water to nonarborescent network, which commonly includes cav- the glacier interior varies slowly because the firn tempo- ities (gaps between the glacier sole and bed), channels rarily stores water and thereby smooths out variations in incised into the bed, and a layer of permeable sediment. the supply rate. In the firn-free ablation zone, in con- The nonarborescent network conveys water slowly and is trast, the flux of water into the glacier depends directly usually poorly connected to the arborescent system. The on the rate of surface melt or rainfall and therefore arborescent channel network largely collapses during varies greatly in time. Water moves from the surface to winter but reforms in the spring as the first flush of the bed through an upward branching arborescent net- meltwater to the bed destabilizes the cavities within the work consisting of both steeply inclined conduits, nonarborescent network. The volume of water stored by formed by the enlargement of intergranular veins, and a glacier varies diurnally and seasonally. Small, temper- gently inclined conduits, spawned by water flow along ate alpine glaciers seem to attain a maximum seasonal the bottoms of near-surface fractures (crevasses). Engla- water storage of ;200 mm of water averaged over the cial drainage conduits deliver water to the glacier bed at area of the glacier bed, with daily fluctuations of as much a limited number of points, probably a long distance as 20–30 mm. The likely storage capacity of subglacial downglacier of where water enters the glacier. Englacial cavities is insufficient to account for estimated stored conduits supplied from the accumulation zone are quasi water volumes, so most water storage may actually occur steady state features that convey the slowly varying water englacially. Stored water may also be released abruptly flux delivered via the firn. Their size adjusts so that they and catastrophically in the form of outburst floods. 1. INTRODUCTION dently due to temporal changes in subglacial hydrology [Kamb et al., 1985]. Glacial outburst floods, a common The movement of water through glaciers is important hazard in mountainous regions, result from the rapid for scientific understanding and for immediate practical release of large volumes of water stored either within a applications. Water in glaciers profoundly affects glacier glacier or in a glacier-dammed lake [Bjo¨rnsson, 1992; movement by influencing the stress distribution at the Haeberli, 1983; Walder and Costa, 1996]. Water from glacier bed and thereby the rate at which the ice slides glaciers is becoming increasingly important for hydro- over the bed. This process is important for both alpine electric power generation [Benson et al., 1986; Lang and glaciers [e.g., Iken and Bindschadler, 1986] and polar ice Dyer, 1985]. Some hydropower projects in France [Hantz streams [e.g., Alley et al., 1987; Echelmeyer and Harrison, and Lliboutry, 1983], Norway [Hooke et al., 1984], and 1990; Kamb, 1991]. The episodic surging (orders-of- Be´zinge magnitude increase in speed) of some glaciers is evi- Switzerland [ , 1981] have involved tapping water from directly beneath a glacier. The purpose of this paper is to present a conceptual 1Formerly at U.S. Geological Survey, Denver, Colorado. model of water flow through a glacier based on a syn- Copyright 1998 by the American Geophysical Union. Reviews of Geophysics, 36, 3 / August 1998 pages 299–328 8755-1209/98/97RG-03579$15.00 Paper number 97RG03579 c 299 c 300 c Fountain and Walder: WATER FLOW THROUGH GLACIERS 36, 3 / REVIEWS OF GEOPHYSICS Figure 1. Idealized longitudinal cross section of a temperate alpine glacier showing the important hydrological components. In the accumulation zone, water percolates downward through snow and firn to form a perched water layer on top of the nearly impermeable ice, and then flows from the perched water layer in crevasses (open fractures). In the ablation zone, once the seasonal snow has melted, water flows directly across the glacier surface into crevasses and moulins (nearly vertical shafts). Based on Figure 10.11 of Ro¨thlisberger and Lang [1987]; copyright John Wiley and Sons Ltd.; reproduced with permission. thesis of our current understanding. We have extended partially water saturated. The rate of water movement the scope of previous reviews [Ro¨thlisberger and Lang, through unsaturated firn depends on the firn’s perme- 1987; Lawson, 1993] by focusing on ways in which the ability and the degree of saturation [Ambach et al., various components of the drainage system interact. As 1981], similar to percolation through unsaturated snow part of the conclusions, we outline subjects that need [Colbeck and Anderson, 1982] and soil [e.g., Domenico further investigation. This paper emphasizes temperate and Schwartz, 1990]. The near-impermeable glacier ice alpine glaciers (glaciers at their melting point), but re- beneath promotes the formation of a saturated water sults from and implications for ice sheets are included layer at the base of the firn. Such water layers are where appropriate. We do not discuss the hydrological common in temperate glaciers [Schneider, 1994]. The role of the seasonal snowpack, as there is a comparative depth to water generally increases with distance upgla- wealth of literature on the subject [e.g., Male and Gray, cier [Ambach et al., 1978; Fountain, 1989], as can be 1981; Bales and Harrington, 1995] and because the effect expected from the general increase in snow accumula- of snow on glacier hydrology has recently been reviewed tion with elevation. High in the accumulation zone, the [Fountain, 1996]. water table may be as much as 40 m below the glacier surface [Lang et al., 1977; Schommer, 1977; Fountain, 1989]. 2. HYDROLOGY OF THE FIRN The hydrological characteristics of firn are fairly uni- AND NEAR-SURFACE ICE form between glaciers. Field tests of the hydraulic con- ductivity (permeability with respect to water) of the firn At the end of the melt season the surface of a glacier at five different glaciers [Schommer, 1978; Behrens et al., consists of ice at lower elevations in the ablation zone, 1979; Oerter and Moser, 1982; Fountain, 1989; Schneider, 2 where yearly mass loss exceeds mass gain, and snow and 1994] indicate a surprisingly narrow range of 1–5 3 10 5 firn at upper elevations in the accumulation zone, where m/s. This may reflect a uniform firn structure resulting yearly mass gain exceeds mass loss (Figure 1). Firn is a from a common rate of metamorphism of firn to ice. transitional material in the metamorphosis of seasonal Firn samples from South Cascade Glacier, Washington snow to glacier ice. As we will discuss in section 2.1, the State, had a porosity of 0.08–0.25 with an average of presence or absence of firn has important implications 0.15 [Fountain, 1989]. This average value is equal to the for subglacial water flow and for variations in glacial value that Oerter and Moser [1982] found to be most runoff. appropriate for their calculations of water flow through the firn. Within the water layer, ;40% of the void space 2.1. Accumulation Zone is occupied by entrapped air [Fountain, 1989]. The accumulation zone typically covers ;50–80% of The depth to the water layer depends on the rate of an alpine glacier in equilibrium with the local climate water input, the hydrological characteristics of the firn, [Meier and Post, 1962]. The near surface of the firn is and the distance between crevasses, which drain the 36, 3 / REVIEWS OF GEOPHYSICS Fountain and Walder: WATER FLOW THROUGH GLACIERS c 301 Figure 2. Photomicrograph showing the veins in glacier ice formed at junctions where three grains of ice are in contact.

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