Melt ponds and meandering streams on an ice island--cool!

Peterman Ice Island, Image from NASA
Astronaut photograph taken on August 29, 2011from the International Space Station

More than a year ago, a chunk of ice five times the size of Manhattan broke off of Greenland's Petermann Glacier (reference for this post is here.) It is smaller now (4 x 3.5 kilometers), having splintered several times in its journey of a few thousand kilometers on the ocean. This piece is referred to as Petermann Ice Island A, fragment 2, and it is currently off the northeast coast of Newfoundland. During August it became stuck for 11 days on a shoal or shallow sea floor. It broke free on August 18, but within a week had split into two large pieces.

The image is NASA's Earth Observatory image of the day today (Sept. 16), and the accompanying description discusses how it is behaving in some ways as if it was still a glacier instead of an iceberg. Specifically, during these warm summer months ice on top melts and water forms streams and ponds as it moves downhill toward the edges of the ice.  Sometimes the water hits a crevass and drains out the bottom of the ice instead of making it all the way to the edge.

The features that caught my eye in this photo above are the incredible meandering streams.More than a year ago, a chunk of ice five times the size of Manhattan broke off of Greenland's Petermann Glacier (reference for this post is here.) It is smaller now (4 x 3.5 kilometers), having splintered several times in its journey of a few thousand kilometers on the ocean. This piece is referred to as Petermann Ice Island A, fragment 2, and it is currently off the northeast coast of Newfoundland. During August it became stuck for 11 days on a shoal or shallow sea floor. It broke free on August 18, but within a week had split into two large pieces.

Back in the 1970's these so-called supraglacial streams were a topic of considerable interest, but it appears that not much has been done recently, and possibly nothing on these streams on ice islands.  Leopold and Wolman observed that they were similar in form to alluvial meandering streams, and noted that since they don't carry sediment, the meanders have a hydrodynamic origin. According to Ferguson (ref below) they form in unfissured hollows that experience appreciable surface ablation and where meltwater from a "sufficiently large" drainage area is concentrated.  It is not clear if they originate on the surface bare ice or not.  Ablation reaches its peak in late spring and early summer when extensive winter snow cover remains. The streams seem to develop at the interface between saturated snowpack and underlying ice, which means that they may already be well established when they become obvious at the surface.  Old channels survive for many years because once cut, the only way that they can be obliterated is by ablation. They erode by frictional melting of the channels, but preferential melting along crystal boundaries may be important, and solar radiation penetrating through flowing water can melt the channel bed as well. Ferguson showed that stream widths are proportional to the square root of the (presumeably peak) discharge (discharge in a glacial environment depends strongly on the time of day.

Parker (1975) studied these and concluded that the instability that triggers the meanders only occurs in supercritical flow, and that the meander pattern does not migrate downstream. He found that the meander wavelength is determined by channel width, depth, and Froude number.

Ferguson, R.I., Sinuosity of supraglacial streams, Bulletin of the Geological Society of America, v. 84, 251-256, 1973.

Parker, G., Meandering of mupraglacial melt streams, Water Resources Research, 11(4), 551-552, 1975.