Waterlines: How Water Shapes Our World

Takeaway: A moulin can act less like a drainpipe and more like a hidden water tank, softening daily pressure pulses under Greenland’s ice.

Greenland’s meltwater does not just run across the surface and disappear. In summer, blue rivers plunge into holes in the ice and help decide how much water pressure builds at the bed of the ice sheet, where ice can slide faster toward the ocean. That matters for coastal communities, sea-level planning, and anyone trying to understand how a warming climate changes frozen landscapes from the inside out.

In this episode of Waterlines, we explore a study that takes listeners into Greenland’s moulins: steep meltwater shafts that connect the surface of the ice sheet to its hidden drainage system below. Researchers measured water levels inside moulins, modeled how daily melt pulses move through the system, and directly explored two moulins with very different shapes. Their surprising finding: some moulins are far larger than many models have assumed, with storage spaces big enough to dampen daily pressure swings under the ice.

We talk through what a moulin is, why subglacial water pressure matters for ice sliding, how a big hole can act like a water tank rather than a simple drainpipe, and why scientists still need many more observations before predicting how future melt will affect Greenland’s motion. We also discuss the field reality of studying these places: pressure sensors lowered into dark shafts, rope-access exploration, uncertainty in models, and the practical stakes for sea-level rise.

Full paper citation: Covington, M. D., Gulley, J. D., Trunz, C., Mejia, J., & Gadd, W. (2020). Moulin volumes regulate subglacial water pressure on the Greenland Ice Sheet. Geophysical Research Letters, 47, e2020GL088901. https://doi.org/10.1029/2020GL088901

Disclosure: This episode uses AI-generated voices for the host conversation.

Takeaway: In Pâkitsoq, Greenland, the cracks mostly stayed put for ten years, but their changing size may shift where summer meltwater reaches the ice sheet’s hidden floor.

Greenland’s ice sheet is not just melting at the edges; every summer, water also runs across its surface, finds openings, and may plunge toward the bed far below. That hidden plumbing matters because water at the base can change how fast ice slides toward the ocean. In this episode, we visit Pâkitsoq in central-western Greenland, where researchers used satellite radar and a new deep-learning tool called MimiNet to map crevasse fields year by year from 2015 to 2024. The surprising result: many crevasse fields stayed in the same places for a decade, while their total area grew and shrank from year to year with no clear long-term trend. We talk through what radar “sees,” why crevasses are different from moulins, how AI can help map ice-sheet hazards, and what the uncertainty means for understanding future sea-level rise.

Citation: Khan N, Poinar K, Briner J, Nowicki S, Mejia JZ, Avellaneda A and Verné M (2026) Steady crevasse locations in Pâkitsoq, Greenland, over one decade: Results from MimiNet, a new deep-learning model for crevasse detection. Journal of Glaciology 72, e69, 1–19. https://doi.org/10.1017/jog.2026.10185

Disclosure: This Waterlines episode package is written for production using AI-generated voices.

Takeaway: A sudden flood under Greenland’s ice can drain hidden water pockets at the bed, making the ice grip harder and slow down even while meltwater keeps flowing.

Greenland’s summer melt does more than make rivers on the ice surface; it can change how the whole ice sheet grips the ground below. That matters because the speed of Greenland’s ice helps shape future sea level, coastal planning, and the way climate change shows up far from the Arctic. In this episode, we follow scientists to a moulin—a deep vertical drain in the ice—and listen as a hidden floodwave from draining surface lakes rushes under the ice sheet. The surprise: the flood did not simply carve bigger plumbing and speed things along. It appears to have drained hidden pockets of pressurized water at the bed, helping the ice stick more firmly and slow down to winter-like speeds. We unpack what moulins are, how researchers measure water pressure and ice motion in a dangerous, shifting landscape, and why Greenland may not behave like the smaller mountain glaciers scientists often use as examples. Citation: Mejia, J. Z., Gulley, J. D., Trunz, C., Covington, M. D., Bartholomaus, T. C., Xie, S., & Dixon, T. H. (2021). Isolated cavities dominate Greenland Ice Sheet dynamic response to lake drainage. Geophysical Research Letters, 48, e2021GL094762. https://doi.org/10.1029/2021GL094762. Disclosure: This Waterlines episode package is written for AI-generated host voices; the voices listeners hear are AI-generated.

Takeaway: On some limestone islands, caves that look cut off from the world can still pulse with the ocean through tiny connected holes in the rock.

Coastal islands are not solid blocks of rock; many are more like old coral sand turned into Swiss cheese, with hidden pathways that can move seawater, freshwater, pollution, and even pressure from the ocean far inland. In this episode, we visit San Salvador Island in the Bahamas, where researchers used the rhythm of the tides to ask a deceptively simple question: are isolated coastal caves really isolated? The answer changes how we picture island aquifers, cave formation, and the underground plumbing that communities and ecosystems depend on.

The study focuses on flank margin caves, rounded cave chambers that form inside young limestone near ancient freshwater lenses. Scientists long thought these caves mostly stored water but did not connect efficiently to larger flow networks until burial, cracking, or collapse. Breithaupt and colleagues tested that idea by measuring how ocean tides showed up in wells, boreholes, blue holes, and cave pools across rocks of different ages. Their results suggest older limestone can develop region-wide networks of connected solution pores, called touching vugs, and that some caves can plug into those networks before burial.

Full paper citation: Breithaupt, C. I., Gulley, J. D., Moore, P. J., Fullmer, S. M., Kerans, C., & Mejia, J. Z. (2021). Flank margin caves can connect to regionally extensive touching vug networks before burial: Implications for cave formation and fluid flow. Earth Surface Processes and Landforms, 46, 1458–1481. https://doi.org/10.1002/esp.5114

Disclosure: This Waterlines episode package is written for public science listening and uses AI-generated host voices.

Takeaway: On Greenland’s ice sheet, a winter snow plug can make a summer meltwater river abandon one hole in the ice and carve a route to another.

Greenland’s summer meltwater is not just water running downhill and out to sea. Some of it dives through deep holes in the ice, reaches the bed, and can help decide how the ice sheet slides. This episode follows a field team watching blue surface streams, snow-filled channels, and sudden reroutes that show Greenland’s drainage plumbing can reorganize from one year to the next.

We unpack how three neighboring surface catchments on the western Greenland Ice Sheet merged over the 2017–2019 melt seasons. The surprising culprit was ordinary winter snow: it filled old meltwater canyons, blocked previous routes, and helped steer water over low divides into new channels and different moulins—the vertical shafts that carry water down through the ice. That matters because many models treat these drainage basins and moulin locations as fairly fixed, while this study shows that some can switch, merge, and change the timing and location of water reaching the ice-sheet bed.

Citation: Mejia, J., Gulley, J., Trunz, C., Breithaupt, C., and Covington, M.: Greenland supraglacial catchment consolidation by streams breaching drainage divides, The Cryosphere, 19, 6445–6460, 2025, https://doi.org/10.5194/tc-19-6445-2025.

Disclosure: This Waterlines episode uses AI-generated voices for the host conversation.

Takeaway: Firn is the old snow that works like a frozen sponge, and as it fills with ice, more meltwater can escape to the sea and weaken ice shelves.

Ice sheets do not just melt from the top and send water straight to the ocean. Much of that water first meets firn: older snow packed with tiny air spaces, like a cold sponge spread across Greenland and Antarctica. This episode matters because that sponge helps decide how fast ice sheets add to sea level, how vulnerable Antarctic ice shelves are to cracking apart, and how scientists read climate history from air bubbles trapped in ice.

We unpack a major review of what researchers know about firn: how snow turns into ice, how meltwater percolates, refreezes, forms ice layers or hidden aquifers, and why warming can make this natural buffer less reliable. Greenland already retains about half of its surface meltwater in firn, but warming has reduced average firn pore space there by about 5% since 1980. In Antarctica, firn on ice shelves can keep meltwater from ponding in cracks; when that storage space disappears, water can help force fractures open.

Along the way, we talk through the field science behind the findings: snow pits, firn cores, radar, satellites, weather stations, and models that try to follow water through a changing, layered material. The story is practical, uncertain in important places, and deeply human: to understand future sea level, scientists have to understand what happens inside old snow.

Citation: The Firn Symposium team. “Firn on ice sheets.” Nature Reviews Earth & Environment 5, 79–99 (2024). https://doi.org/10.1038/s43017-023-00507-9

Disclosure: This Waterlines episode uses AI-generated voices for the host conversation.

Takeaway: Even in Greenland’s dark winter, heat at the glacier bed can keep hidden water pathways pressurized and ready beneath the ice.

Greenland’s fast outlet glaciers help decide how much ice reaches the ocean, but some of their most important plumbing is buried under hundreds to thousands of meters of ice. This episode goes beneath Helheim Glacier in east Greenland, where researchers used a stripped-down computer model to ask a simple, surprising question: even in winter, when no surface meltwater is pouring down, is there still an active water system under the ice?

We unpack how water pressure at a glacier bed can help ice slide, why “effective pressure” is like the grip between a tire and a road, and why it is so hard to measure what is happening under a giant glacier. The paper’s model suggests that heat made at the bed—especially frictional heat from fast ice sliding over rock and sediment—can make enough meltwater to keep winter drainage pathways alive. It also predicts that the glacier bed is a patchwork: some places transmit water easily, while others are poorly connected and can hold high pressure.

We also talk about uncertainty: how much frictional heat is realistic, why bed topography matters, and why models are not crystal balls but carefully tested maps of what might be happening in places scientists cannot directly see.

Citation: Sommers A, Meyer C, Morlighem M, Rajaram H, Poinar K, Chu W, Mejia J (2023). Subglacial hydrology modeling predicts high winter water pressure and spatially variable transmissivity at Helheim Glacier, Greenland. Journal of Glaciology 69(278), 1556–1568. https://doi.org/10.1017/jog.2023.39

Disclosure: This Waterlines episode package is designed for production with AI-generated voices.

Takeaway: A Greenland lake can vanish through a crack not just because ice breaks, but because the ice slowly gives way enough to keep the drain open.

When a lake sitting on top of the Greenland Ice Sheet suddenly drains, it can send millions of tons of meltwater rushing to the glacier bed, briefly lifting the ice and changing how it slides. This matters far beyond one icy basin: these fast drainage events are one way surface warming can reach deep into an ice sheet, linking summer melt, glacier motion, and future sea-level rise.

In this episode, we unpack a new modeling study that asks what really keeps these water-filled cracks moving. The surprising answer is not mainly melting inside the crack. It is the way ice behaves a little like a solid and a little like a very slow fluid. Over minutes to hours, that “give” can keep a fracture open long enough for lake water to keep pouring downward and then spread along the bed.

We explain hydraulic fracture without assuming a science background, compare brittle-cracking intuition with the slow sag and creep of real glacier ice, and look at why older elastic-only models struggled to match observed lake-drainage behavior. We also discuss what the model leaves out, including its simplified 2D geometry and limited treatment of subglacial drainage, and why those caveats matter for turning detailed fracture physics into ice-sheet-scale forecasts.

Citation: Hageman, T., Mejía, J., Duddu, R., and Martínez-Pañeda, E. (2024). Ice viscosity governs hydraulic fracture that causes rapid drainage of supraglacial lakes. The Cryosphere, 18, 3991–4009. https://doi.org/10.5194/tc-18-3991-2024

Disclosure: This Waterlines episode package is designed for production using AI-generated voices.

Takeaway: A Greenland moulin is not just a drainpipe into the ice; its water level can reveal a whole hidden plumbing network working far below the surface.

Greenland’s summer meltwater does not simply run off the ice sheet like rain down a sidewalk. Some of it pours into deep blue shafts called moulins, reaches the bed hundreds of meters below, and helps set the water pressure that can speed up or slow down the ice itself. In this episode, we follow one small moulin in western Greenland and ask a deceptively simple question: why didn’t its water level bounce up and down as much as the researchers’ first models said it should? The answer points to a hidden network of channels beneath the ice, where water from many places may be smoothing out the daily pulse of melt. That matters because these unseen drainage systems help shape how Greenland responds to warmer summers and how much ice eventually reaches the ocean. Featured paper: Trunz, C., Poinar, K., Andrews, L. C., Covington, M. D., Mejia, J., Gulley, J., and Siegel, V.: Observed and modeled moulin heads in the Pâkitsoq region of Greenland suggest subglacial channel network effects, The Cryosphere, 17, 5075–5094, 2023, https://doi.org/10.5194/tc-17-5075-2023. This Waterlines episode uses AI-generated voices for the host conversation.

Takeaway: In a flat flood-prone basin, knowing how much water is locked in snow can depend on whether you look from the ground, a plane, a model, or a microwave satellite.

Spring flooding is not just a river problem; it can begin weeks earlier as quiet snow sitting on fields, roads, and roofs. In the Red River of the North basin, where the land is very flat and the river flows north into colder conditions, knowing how much water is stored in snow can shape flood warnings, emergency planning, and public trust. This episode follows scientists comparing satellite microwave estimates of snow water with the models used in forecasting, and asks a practical question: when ground measurements are sparse, can space help communities see flood risk more clearly?

Paper featured: Ronny Schroeder, Jennifer M. Jacobs, Eunsang Cho, Carrie M. Olheiser, Michael M. DeWeese, Brian A. Connelly, Michael H. Cosh, Xinhua Jia, Carrie M. Vuyovich, and Samuel E. Tuttle, “Comparison of Satellite Passive Microwave With Modeled Snow Water Equivalent Estimates in the Red River of the North Basin,” IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, vol. 12, no. 9, pp. 3233–3246, September 2019. DOI: 10.1109/JSTARS.2019.2926058.

Disclosure: This Waterlines episode package is written from the source paper and is intended for public science communication. The episode uses AI-generated voices.

Full citation: Schroeder, R., Jacobs, J. M., Cho, E., Olheiser, C. M., DeWeese, M. M., Connelly, B. A., Cosh, M. H., Jia, X., Vuyovich, C. M., & Tuttle, S. E. (2019). Comparison of Satellite Passive Microwave With Modeled Snow Water Equivalent Estimates in the Red River of the North Basin. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 12(9), 3233–3246. https://doi.org/10.1109/JSTARS.2019.2926058