What is at the Root of Ruth Glacier?
Recently, I came across film footage showing tons of water igniting out of a spring close to the Ruth Glacier. The filmmakers were stunted by this mysterious spring’s origin in Denali National Park, Alaska. This mystery was resolved when viewed from space and demonstrates what can be done with daily satellite imagery.
The footage seems to show what is the foreland of a glacier. However, the film is taken overlooking the glacier! It sounds strange, but trees are starting to grow on the glacier. This migration of trees can be seen when old satellite images from 40+ years ago are compared to a recent satellite scene.
There are several things to see on this flickering image. Firstly, the vegetation is starting to cover the rocks of the glacier snout. If you look higher up the glacier, the different rock types make it possible to track patterns. By looking at these patterns, you can see that these features are displaced. Thus the glacier is moving, slowly but steadily. If you look even higher up, you can see the ice, but what is also clear is the decrease of area of clean ice. The ice melts, but leaves behind the rocks that were in the ice. These rocks absorb more sunlight, thus enhancing the melting. However, when the cover is sufficiently thick, the rock cover functions as an isolation blanket, as is the case here on the snout of Ruth Glacier.
Trees on glaciers are not a rare phenomena, as multiple glaciers in Alaska are becoming covered by trees, such as Malaspina Glacier. Thus, the spring is not in the forest, but at the end of the glacier. The next question is: Where does this water come from?
An answer may be found just north of the spring. There are two lakes situated on top of the glacier. Because water is heavier than ice, the water is able to fracture the ice and find its way to the bottom of the glacier and out. This seems to be the case here.
We can find out more about this mysterious spring. For example, by looking at the Planet imagery, it is possible to see the lake extent change over time. In two days (between the 5th and the 7th of August, around the time the film was shot) the lake reduces in size. How much water this reduction is can be roughly estimated by looking at the surface topography. I took a closer look at the high resolution topography from the ArcticDEM. A transect of one of the lakes can be drawn:
From this profile we can estimate the mean slope, which is around 18 degrees, or a ratio .31:1. This means that every meter of outward or inward of lake extent is roughly an increase of 31 centimeters in lake level rise or fall, respectively. So the Planet data can be used as a gauge to estimate the amount of water that drained.
A profile is drawn and here I simply look at the intensity change. Then, the drops can be found by fitting a parabolic function. This results in a change of 3.8 meters from the 5th to 7th of August, which would relate to a lake level drop of 1.1 meters! The area of the lake is roughly 64'000 square meters and the spring is just 10 meters lower from the lake. However, two days later the lake seem to have grown, growing 0.9 meter further in respect to the 5th of August. This is strange. So, where did this water come from?
Communicating vessels
The water cannot come from the neighborhood, as the catchment of the lake is small. Hence, these lakes need to be connected with another water source. This source is not at the surface. These lakes are connected to the hydrological system under the glacier. Thus, these lakes function as a gauge for the water pressure of this system. This we know, but now we are able to map this as well with the help of high-resolution imagery from Planet. The lake level variation can be very complicated, as it depends on surface melt high up on the glacier and the tunnel network underneath a glacier. This network underneath a glacier is more complicated than you think (as this short movie highlights). Now we are able to observe this from space!
What started with some amazement over a video brought me, after some steps, to a clear understanding of what is happening at a far and remote glacier in Alaska! It is amazing that nowadays these kind of processes can be measured! Such analysis are helpful tools for people like me that study glaciers. Especially this effect of hydrological connectivity, as this relates to glacier sliding, which is still a less resolved issue.
Further reading:
Miles and others. 2017. Pond dynamics and supraglacial-englacial connectivity on debris-covered Lirung Glacier, Nepal. Frontiers of earth science
