Landsat image of Nioghalvfjerdsbræ in 2017 with supraglacial stream indicated by arrows and calving front yellow dots. 

I first observed the impressive discharge and length of the supraglacial streams working on Jakobshavn Glacier more than 30 years ago.  The longest persistent supraglacial stream in Greenland at present that I am aware of is on Nioghalvfjerdsbræ (79 Glacier).  The longest streams will occur on glaciers with low slopes in the ablation zone and limited crevassing, suggesting an extended region of compressive flow. The compressive flow and low slope also would help suppress moulin formation. Rignot et al (1997) first noted that flux from glaciers in this region was 3.5x the iceberg production due primarily to ice loss from basal melt, Nioghalvfjerdsbræ has a 20 km wide ice shelf, and the bed remains below sea level over a distance of 150 km upstream from the grounding line, and 200 km inland of the calving front, as indicated Bamber (2013). The basal melt rate in this reach is comparatively low  less than 10 m per year maximum and ~5 m per year average seen in figure 2 from Wilson et al (2017) and.  The velocity declines from 1000 m/year near the start of the stream to less than a 100 m/year near the calving front.

Landsat image of Nioghalvfjerdsbræ in 2011 with supraglacial stream indicated by arrows. 

The supraglacial stream indicated in the 2011 and 2017 images along most of its length has the same path.  The streamflow directly to the ice front in 2011 with a length of 73 km without measuring secondary wiggles.  In 2017 the stream  turns south to reach southern margin with a length of 71 km.  Below are three images of the stream in 2014 Google Earth images.  The width of the stream remains relatively constant over most of the length ranging from 20-35 m.  You can also see how the stream drains numerous small supraglacial lakes.  This reduces the size of the lakes and on Petermann Glacier, was observed to reduce the destabilizing nature of the lakes on the floating tongue (Macdonald et al, 2018).  The smaller lakes are less likely to develop moulins or force fracture propagation. This stream survival depends on not encountering a moulin or crevasse.  In West Greenland the melt rates are higher and the surface slopes greater where the detailed observations of supraglacial streams and moulins have led to model development for stream discharge see the fantastically detailed work of Banwell et al (2012) and  (Smith et al., 2017)

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Lower reach of supraglacial stream

Mid reach of supraglacial stream

Upper reach of supraglacial stream