Hydrology and thermal regime of an ice‐contact proglacial lake: Implications for stream temperature and lake evaporation

Abstract

The formation of lakes as glaciers retreat can potentially influence stream temperature and enhance catchment water losses via evaporation. This study quantified the summer water and energy budgets and thermal regime of an ice‐contact proglacial lake below Bridge Glacier, British Columbia, to link ongoing glacier retreat with downstream effects. The ice‐proximal and distal portions of the lake behave differently due to a mid‐lake recessional moraine, against which icebergs are pinned by the dominant katabatic wind. Iceberg cover of the ice‐proximal basin ranged from 16% to 75%. Temperature profiles in the ice‐proximal basin were dominantly near‐isothermal at 1 to 1.4°C, with the exception of diurnal warming of the top 1 m of water and the presence of a colder layer near the lake bed at some sites, presumably caused by subglacial discharge of turbid water. Subaqueous iceberg melt limited warming by consuming heat from the water column. The distal basin exhibited varying degrees of thermal stratification, with turbidity inferred to have a greater influence on water density than temperature. Most of the warming between lake inflow and outflow occurred in the distal basin, where net radiation was the dominant surface energy input and net lateral advection the dominant energy sink. As a result of the suppressed water temperature, humidity gradients over the lake favoured condensation, not evaporation. Estimates of iceberg melt volume for 2013 exceeded glacier discharge into the lake. Therefore, when the glacier eventually becomes land‐terminating, icebergs are expected to disappear within a year, resulting in higher water temperatures both in the lake and downstream. Higher lake temperatures could also cause an eventual shift from condensation to evaporation, which would tend to reduce catchment water yield over and above reductions associated with loss of glacier area.

Lake surface and downstream river temperature response to the retreat of a lake‐terminating glacier

“…However, lake surface and downstream river temperatures did begin warming in association with a decline in iceberg density following its peak in around 2012, even though the glacier remained lake terminating. As explained by Bird et al (2022), a decrease in iceberg density in the proximal basin would promote warming by resulting in a greater water surface area over which energy inputs to the lake water column could occur, as well as by reducing the transfer of heat from the water column to the icebergs to drive subaqueous melting. In addition, Bird et al (2022) observed that a 'wall' of icebergs was typically pinned against the submerged moraine by the dominant katabatic winds, and that water flowing through this wall from the proximal to the distal basin was cooled to about 0.9 C. In more recent years, this wall has been less continuous or even absent.…”

“…A potential contributing factor to lake surface warming is inflow from the tributary streams, particularly South Creek, which has been observed to reach temperatures of about 14 C, well above the F I G U R E 1 0 Time series of near-surface water temperature recorded by Bird et al (2022) at profile E (light blue line) and Landsatderived lake surface temperature at the outlet point (black circles), which is less than 100 m from profile E.…”

“…Bridge Lake is expected to grow only another few hundred meters in length given the surface slope and current thickness of the glacier tongue. Indeed, field observations in summer 2021 indicated that the northern edge was within about 400 m of becoming indicate that South Creek can reach almost 14 C in summer (Bird et al, 2022;Dufficy, 2019).…”

“…Warming of ice-contact lakes can be further reduced by subaqueous melting of icebergs, which consumes heat from the water column (Bird et al, 2022;Landl et al, 2003). Bird et al (2022) proposed a hypothetical conceptual model describing how downstream river temperature would respond to glacier retreat and formation of an ice-contact proglacial lake. The model invokes an initial phase of warming as the glacier terminus retreats above the point of interest, with a reduction of the rate of warming during development of an ice-contact lake with icebergs.…”

“…However, warming of proglacial lakes can be moderated by high inflows, which reduce the mean residence time of lake water (and thus the opportunity for warming) (Bird et al, 2022;Richards et al, 2012). Warming of ice-contact lakes can be further reduced by subaqueous melting of icebergs, which consumes heat from the water column (Bird et al, 2022;Landl et al, 2003). Bird et al (2022) proposed a hypothetical conceptual model describing how downstream river temperature would respond to glacier retreat and formation of an ice-contact proglacial lake.…”