The Cryosphere, 8, 537–546, 2014 Open Access www.the-cryosphere.net/8/537/2014/ doi:10.5194/tc-8-537-2014 The Cryosphere © Author(s) 2014. CC Attribution 3.0 License. Near-surface permeability in a supraglacial drainage basin on the Llewellyn Glacier, Juneau Icefield, British Columbia L. Karlstrom1, A. Zok2, and M. Manga2 1Department of Geophysics, Stanford University, 397 Panama Mall, Stanford, CA 94305, USA 2Department of Earth and Planetary Science, 307 McCone Hall, University of California at Berkeley, Berkeley, CA 94720, USA Correspondence to: L. Karlstrom ([email protected]) Received: 10 October 2013 – Published in The Cryosphere Discuss.: 4 November 2013 Revised: 4 February 2014 – Accepted: 13 February 2014 – Published: 27 March 2014 Abstract. Supraglacial channel networks link time vary- 1 Introduction ing melt production and meltwater routing on temperate glaciers. Such channel networks often include components of both surface transport in streams and subsurface porous In the ablation zone of glaciers and ice sheets, surface flow through near-surface ice, firn or snowpack. Although meltwater channelizes and thermally erodes the icy sub- subsurface transport if present will likely control network strate. This flowing meltwater localizes to form supraglacial transport efficacy, it is the most poorly characterized compo- streams if melt production exceeds the transport capacity of nent of the system. We present measurements of supraglacial near-surface ice, firn or snowpack. Supraglacial melt trans- channel spacing and network properties on the Juneau Ice- port links surface melt to subglacial hydrologic systems field, subsurface water table height, and time variation of through englacial drainage (Fountain and Walder, 1998), so hydraulic characteristics including diurnal variability in wa- temporal variations in the rate and volume of supraglacial ter temperature. We combine these data with modeling of meltwater production have the potential to influence the basal porous flow in weathered ice to infer near-surface permeabil- environment and hence bulk ice movement (e.g., Iken, 1972; ity. Estimates are based on an observed phase lag between Müller and Iken, 1973). Large-amplitude englacial drainage diurnal water temperature variations and discharge, and in- events, (e.g., Das et al., 2008) as well as diurnal (e.g., Kamb dependently on measurement of water table surface eleva- et al., 1994) and seasonal (e.g., Iken and Truffer, 1997) vari- tion away from a stream. Both methods predict ice perme- ability in surface melting, all influence large-scale motion of ability on a 1–10 m scale in the range of 10−10–10−11 m2. ice masses. These estimates are considerably smaller than common pa- A critical parameter to both developing and sustaining rameterizations of surface water flow on bare ice in the lit- streams is the permeability of near-surface ice that con- erature, as well as smaller than most estimates of snow- trols transport of meltwater into channels. Over bare and pack permeability. For supraglacial environments in which impermeable ice or in channels, flow timescales are com- porosity/permeability creation in the subsurface is balanced monly parameterized according to an effective friction pa- by porous flow of meltwater, our methods provide an esti- rameter (Manning’s equation, e.g., Arnold et al., 1998). mate of microscale hydraulic properties from observations However, in some settings there is also a layer of frac- of supraglacial channel spacing. tured, partially melted or otherwise weathered bare ice in the near surface through which porous flow of melt may oc- cur (Fountain and Walder, 1998). Because channelized flow moves quite rapidly (typical velocities up to ∼ 1 m s−1), sub- surface porous flow if present will be the limiting factor for propagation of diurnal signals into the near-surface glacier hydrologic system. Permeability varies as a function of depth Published by Copernicus Publications on behalf of the European Geosciences Union. Karlstrom, Zok, Manga: Near–surface permeability 9 538 L. Karlstrom et al.: Near-surface permeability because melting attenuates with the decay of solar radia- tion in the subsurface and percolating meltwater can refreeze (Pfeffer et al., 1991), in addition to possible compaction ef- fects. Permeability will also vary with altitude and season as surface snow transitions to bare ice down glacier (Braith- waite et al., 1994). Ice permeability sets the transport efficiency of the supraglacial system, a quantity of interest in large-scale and short-time water budgets for glaciers and ice sheets (Ren- nermalm et al., 2013). Measurements of firn and weathered ice permeability have been conducted on O(0.1 − 1) m scale samples (e.g., Fountain, 1989; Schneider, 1999; Albert et al., 2000). But because permeability is often a scale-dependent material property (e.g., Schulze-Makuch et al., 1999), mea- surements on larger scales should provide better character- ization of hydraulic transport relevant to supraglacial chan- nels. Such measurements may also be more relevant for relating permeability to porosity measurements taken on larger (100–1000 m) scale (e.g., Morris and Wingham, 2011; Brown et al., 2012). Intermediate (1–100 m)-scale perme- ability relevant for supraglacial stream channel formation and spacing (Marston, 1983) is not well constrained. Fig. 1. Landsat 7 image (taken 15 August 2001) of the Juneau Ice- ◦ ◦ Here we develop novel methodsFig. 1. toLandsat infer field-scale 7 image (taken per- Augustfield. 15, The 2001) Llewellyn of the Juneau Glacier Icefield. study site The is Llewellynlocated at 59 glacier◦4036 study.8300 N, site is located at 59 4’36.83”N, 134 meability in supraglacial systems,7’24.54”W, and present indicated measurements by the solid red134 circle.◦7024.5400 W, indicated by the solid red circle. of a supraglacial drainage basin high in the ablation zone of the Llewellyn Glacier on the Juneau Icefield, British Columbia, Canada, over four days in August 2010 aimed at characterizing both surface and subsurface components of than rain. Average yearly rainfall in Atlin, British Columbia, the supraglacial network. These measurements reveal how Canada (60 km East and 450 m lower than the Llewellyn near-surface permeability is expressed in the distribution and site) is 0.192 m yr−1 (http://www.theweathernetwork.com). time variation of melt transport. We measure time-varying Our study site is a drainage basin several kilometers down discharge, water temperature and geometrical properties of glacier from the equilibrium line near a medial moraine a supraglacial drainage basin including channel aspect ratio, (Fig. 2a), exhibiting supraglacial channels that draw water channel spacing and the height of the subsurface water table from a layer of weathered, partially melted surface ice (e.g., away from channels. We then develop two methods to infer Müller and Keeler, 1969). In the time frame of our study (4– near-surface permeability, on a length scale of 1–10 m that 7 August 2010), we observed the transient snow line (TSL) at is relevant to channel development at the Llewellyn Glacier elevations ∼ 75–100 m higher than our study site, implying site. One method is based on modeling the elevation of the ∼ 3 weeks of bare ice exposure prior to our study if TSL mi- subsurface water table that extends perpendicular to chan- gration rates measured at other glaciers on the Juneau Icefield nels, while the other uses a phase shift observed between (∼ 4–5 m day−1 in August 2010, Pelto, 2011; Pelto et al., stream water temperature and discharge to infer a subsurface 2013) are also assumed for our site. Fractures and crevasses transport lag. Both methods result in similar estimates for in this ice-marginal area are rare, with spacing much larger permeability of near-surface ice. We end by illustrating how than typical stream spacing. these methods could be extended via remote sensing obser- Although some inheritance of channels may occur, many vations to large-scale supraglacial drainage networks such as supraglacial channels form anew each year when melt pro- those on the western Greenland Ice Sheet. duction exceeds subsurface transport capacity and ensuing surface flow locally enhances thermal erosion (Fig. 2b). Be- cause of rapid glacier surface melting (several cm day−1 – 2 Site summary and methods our study period coincided with higher than average temper- atures as recorded in Atlin), we observed that channels in this The Llewellyn Glacier drains the Juneau Icefield, on the east- environment form and become abandoned on day- to several- ern side of the North American Continental Divide (Fig. 1). day timescales (Fig. 2c), creating a hummocky glacier sur- Because of the local rain shadow and relatively high eleva- face topography in the upper parts of the drainage basin. This tion (1440 m), the primary input to the supraglacial hydro- topography reflects a competition between localized erosion logic network at this site is locally produced meltwater rather by streams and large-scale surface lowering. The Llewellyn The Cryosphere, 8, 537–546, 2014 www.the-cryosphere.net/8/537/2014/ L. Karlstrom et al.: Near-surface permeability 539 Fig. 2. (a) Marginal region of the Llewellyn Glacier that contains the study drainage basin (dashed line), photo taken from medial moraine. Black arrows indicate starting locations of survey transects, while red arrow is the stream in which the DTS temperature sensor was deployed. Boxed letters provide approximate locations of photographs in subsequent panels. (b) Inception of a supraglacial channel high in the upper study drainage basin, with porous icy substrate surrounding. (c) Recently abandoned channel, note difference in color between recently wetted channel ice and the glacier surface. (d) Example of a highly regular meandering reach. Black scale bars in panels (b–d) are approximately 50 cm. study site thus is well suited for a short duration and spa- Surveying of streams within the basin was accomplished tially focused study: supraglacial transport is widespread, the with a combination of measuring tapes/sticks and a laser channel networks evolve rapidly, and both surface and sub- range finder (TruPulse 200B) with ∼ 1 cm accuracy.