Abstract
Despite an interest in the hydraulic functioning of supraglacial and englacial
channels over the last 4 decades, the processes and forms of such
ice-bounded streams have remained poorly documented. Recent glaciological
research has demonstrated the potential significance of so-called
"cut-and-closure" streams, where englacial or subglacial flow paths are
created from the long-term incision of supraglacial channels. These flow paths
are reported to exhibit step-pool morphology, comprising knickpoints and/or
knickzones, exaggerated in dimensions in comparison to supraglacial channels.
However, little is known of the development of such channels' morphology.
Here, we examine the spatial organisation of step pools and the upstream
migration of steps, many of which form knickzones, with repeated surveys over
a 10-year period in an englacial conduit in cold-based Austre
Brøggerbreen, Svalbard. The observations show upstream step recession to
be the dominant process for channel evolution. This is paralleled by an
increase in average step height and conduit gradient over time.
Characteristic channel-reach types and step-riser forms are consistently
observed in each of the morphological surveys reported. We suggest that the
formation of steps has a hydrodynamic origin, where step-pool geometry is
more efficient for energy dissipation than meanders. The englacial channel
system is one in rapid transition towards a quasi-equilibrium form within a
decadal timescale. The evolution and recession of knickzones reported here
result in the formation of a 37 m deep moulin shaft, suggesting that over
time an incising supraglacial channel may evolve towards an englacial channel
form exhibiting a stable end-point characterised by a singular vertical
descent, which potentially can reach the glacier bed. This challenges the
prevailing notions that crevasses or hydrofractures are needed to form deep
moulins. Our observations highlight the need to further examine the
adjustment processes in cut-and-closure channels to better understand their
coupling to supraglacial meltwater sources and potential significance in
cold-based glacier hydrology and ice dynamics.
Citation
ID:
140960
Ref Key:
vatne2016hydrologymorphological