Image caption The volume of water at the terminus of Ngozumpa is about a third of a cubic km and growing

Ngozumpa Glacier in Nepal snakes away from the sixth highest mountain in the world, Cho Oyo.

It's not the greatest glacier to look at - far from it. It's smothered in a layer of rocky debris that's fallen from the surrounding cliffs, giving it a very grey, dirty appearance.

But Ngozumpa is generating a lot of scientific interest at the moment.

The Nepalese Himalayas have been warming significantly more than the global mean temperature in recent decades.

Glaciers in much of the region are showing signs of shrinking, thinning, and retreating; and this is producing a lot of melt water.

On Ngozumpa, some of this water is seen to pool on the surface and then drain away via a series of streams and caverns to the snout of the glacier.

There, some 25km from the mountain, an enormous lake is growing behind a mound of dumped rock fragments.

This lake, called Spillway, has the potential to be about 6km long, 1km wide and 100m deep.

The concern is that this great mass of water could eventually breach the debris dam and hurtle down the valley, sweeping away the Sherpa villages in its path. The threat is not immediate, but it's a situation that needs monitoring, say scientists.

One of the researchers at work on Ngozumpa is Ulyana Horodyskyj, from the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado in Boulder, US.

She is setting up remote cameras to monitor the surface, or supraglacial, pools of water that dot the length of Ngozumpa. Some are small; some are big - the size of several football fields.

Already, she has been able to establish just how dynamic these water features can be as they drain and fill in rapid time.

The volumes involved can be prodigious. In one event, her cameras spied a supraglacial lake losing more than 100,000 cubic metres of water in just two days. Within five days, the lake had recovered half the volume, fed by waters from higher up the glacier.

"Say I came the week before and the week after a lake drained - it would seem like nothing had happened because the lake level would appear to be the same," Ms Horodyskyj told BBC News.

"But my timelapse photography tells me that something has happened - 40 Olympic-size swimming pools just got sent down the glacier."

The CIRES researcher wants to understand the part these supraglacial lakes play in the erosion of Ngozumpa.

Image caption Horodyskyj is placing cameras on the cliffs to monitor the water features on the glacier below

Debris-covered glaciers don't melt in the same way as clean glaciers. The rock covering, depending on its depth, will insulate the ice from solar radiation. But remove it - as happens in these fluctuating lakes - and the rate of melting will increase.

"The enhanced melting comes from the bare ice walls in the lakes," she explains.

"The melt rate below the debris covering is about 2cm per day, but on these walls it's 4cm per day. As the lake drains, it exposes the walls which can then calve."

Ms Horodyskyj's assumption is that many of the lakes on Ngozumpa's surface are directly connected; and as one of them drains, it's likely that another lake at lower elevation is filling. However, the routes taken by the plumbing system are not always obvious.

This is being investigated by Doug Benn from the University Centre in Svalbard (UNIS), Norway.

He's been climbing through the vast channels cut by flowing water inside Ngozumpa. Some of these "ice pipes" open up into spectacular caverns.

"It's widely recognised that the glaciers in this region are melting down as a result of global warming, but what hasn't been realised is that they're also being eaten away from the inside as well," he says.

"These glaciers are becoming like Swiss cheeses, so everything is happening more rapidly than is apparent by just looking at the surface."

Dr Benn visits the conduits after the melt season, after the water has stopped flowing. It would be too dangerous to get inside them at the height of summer.

It would seem the channels control where some surface pools and lakes form. It is as if the conduits are the templates.

"They're lines of weakness. As the glacier melts down, the roofs of the tunnels fall in and bare ice is exposed," explained Dr Benn. "The rock debris on the surface would normally slow down melting, but the existence of these weaknesses inside Ngozumpa opens it up and makes it melt far faster than would otherwise be the case."

Media playback is unsupported on your device Media caption Timelapse of a Ngozumpa supraglacial lake filling and draining (Courtesy of CIRES)

One of his students, Sarah Thompson, is concentrating her study on the end story - the snout of the glacier. This is where the water sent down Ngozumpa is gathering, in the rapidly growing Spillway Lake.

It is bounded by the moraine - an enormous pile of granite fragments dropped by the glacier over millennia.

At this point the glacier is stagnant; it is not moving. Again, the exposed ice walls that line Spillway Lake calve into the water, raising its level.

"We've got quite a short time period - the past 10 years - but it's an exponential growth in area," Ms Thompson says of Spillway's size. "And when we look at other similar lakes in the region, Spillway is on the same sort of trajectory to their development."

The Swansea University researcher added: "The expansion is way beyond what you would expect from the rates of ice melting, ablation and even calving.

"We need to understand at an early stage the processing rates so that we can predict ahead of time what is likely to happen and, if needs be, go in and mitigate all of this before it becomes such a significant hazard.

"In my work, we've been trying to identify where there might be weak points in the moraine dam, and we believe we've identified a few areas where in future you might want to take action."

Spillway is not expected to burst out anytime soon. It could be two decades or more before a 6km-long body of water is built up. But the difficulty of working in the region and bringing heavy equipment into the area means a long-term strategy for managing the lake's evolution is essential.

Image caption The conduits are investigated after the melt season, after all the water has gone

Jonathan.Amos-INTERNET@bbc.co.uk