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Ice at its largest
Ice at its largestToday

Schematic retreat: only the maximum outline comes from a published source.

Retezat Mountains, Southern Carpathians, Romania

Retezat under the ice, about 20,600 years ago, and its lakes today

Watch the cirque of Bucura Lake fill with ice, drawn inside the outline published by the researchers who dated its moraines. Scroll to see it melt away, then pick a lake.

Drag the mountain to turn it

About 20,600 years ago

The ice at its largest

About 20,600 years ago, in the last glacial maximum, glaciers filled the valleys of the Retezat Mountains. Researchers dated the moraines they left, the ridges of rock and gravel pushed up at the ice edge, and mapped how far the ice went. The gold line is the outline they published: 78 km², from the ridge crest down to about 1,030 m on the terrain model. The date, 20.6 thousand years (+0.8, −1.3), is the one measured for the maximum in the southern valleys.

The ground is a satellite photograph of August 2024 over the real relief. Ice, outline and lakes are what change.

Bucura cirque

Roughly the height of the Palace of the Parliament

Over Bucura Lake, which lies at 2,032 m, the ice is estimated at 53 to 126 m thick, about 103 m with the middle assumption. The Palace of the Parliament in Bucharest is 84 m high.

Nobody measured this ice. The numbers come from the slope of the ice surface and from how stiff ice is when it flows, which is known only within a factor of three; the range is that uncertainty. The research group’s own conference poster mentions up to about 330 m at the thickest point, well above this page’s maximum of 148 m for the whole outline at the middle assumption.

The melt

The ice goes, the lakes appear

Scroll, or move the slider under the mountain. The low ice thins and leaves first, the high ice last. Only the maximum outline is drawn here from a published source; the retreat is a sketch of the order in which a glacier shrinks, not a map of dated stages.

As each lake emerges it lights up. The gold line fades once the ice is going.

Today

44 mapped water bodies inside the outline

OpenStreetMap maps 82 water bodies in this part of the mountains, 44 of them inside the old ice. The cirques that hold them appear to have formed over several glacial phases; the group that dated the last glaciation found its ice eroded them little, and older rock surfaces survive in them.

The 28 lakes on the cards are the ones between 0.4 and 40 ha in the high cirques. The biggest are Bucura, Zănoaga Mare and Negru. Each freezes in winter and opens in spring or summer on its own calendar.

When does the ice leave your lake?

Choose a lake, or tap one on the mountain. The card shows what the satellite saw in each spring since 2018.

Lake by lake: when the satellite saw the ice go

Twenty-eight lakes between 0.4 and 9.3 ha, every spring from 2018 to 2026. “Springs with a pair” counts the years in which a clear pass saw the lake frozen and a later clear pass saw it open. Dates are medians over those years, and the true ice-out of any single spring lies between the two dates of that spring.

LakeAltitudeAreaSprings with a pairLast seen frozen (median)First seen open (median)Ice then (est.)
Lacul Bucura2,032 m9.3 ha521 May11 July53–126 m
Lacul Zănoaga Mare1,996 m6.8 ha715 May16 June42–118 m
Negru2,006 m4.8 ha615 May24 June–
Galeș1,966 m3.9 ha55 May7 July45–92 m
Zănoaga Mică1,966 m3.8 ha228 April31 May–
Slăvelu1,932 m3.7 ha87 May18 June37–88 m
Ana1,975 m3.6 ha55 May16 June57–148 m
Gemenele1,892 m2.7 ha614 May11 July–
Tăul Custura Mare2,191 m2.4 ha121 June26 June–
Peleaga2,125 m2.3 ha416 May19 June30–83 m
Lacul Știrbu2,082 m1.4 ha127 May21 July–
Lia1,903 m1.2 ha610 May1 July53–140 m
Peleguța2,096 m1.0 ha418 May19 June26–94 m
Judele2,132 m0.9 ha526 May22 June47–90 m
Tăurile Radeșului1,922 m0.9 ha111 May17 July–
Viorica2,062 m0.9 ha526 May11 July40–121 m
Tău Răsucit2,095 m0.8 ha219 May19 July35–85 m
Lacul Adânc2,158 m0.8 ha0–––
Stânișoara2,028 m0.8 ha216 May7 June50–122 m
Unnamed lake1,825 m0.8 ha0–––
Lacul Mare din Valea Rea2,188 m0.7 ha323 May17 July42–101 m
Tăul Custura Mică2,175 m0.7 ha0–––
Florica2,081 m0.5 ha0––47–106 m
Ștevia2,055 m0.5 ha518 June6 July–
Tău Urât2,087 m0.5 ha321 May20 June35–126 m
Tăul Porții2,208 m0.5 ha426 May17 July36–62 m
Unnamed lake1,878 m0.4 ha0––44–126 m
Caprelor2,129 m0.4 ha0–––

A dash in the last column marks a lake that lies outside the outline used here, which draws no ice over it. A dash in a date column means no spring with a clear frozen-then-open pair.

Why the dates are intervals

Sentinel-2 can see Retezat every few days, though the interval varies. Mountain cloud and cirque shadow leave a lake usable on about one pass in five (19.7%, from 2018 to 2026; from 12% in 2019 to 29% in 2021). The plan was to give a day for the ice-out of at least ten lakes in seven or more springs, within three days. It did not hold: of 252 lake-springs, 90 have a clear frozen pass followed by a clear open pass, none of those pairs are less than three days apart, and only two lakes have seven or more.

So each spring on the card is the interval between the last pass that saw the lake frozen and the first that saw it open. The ice left somewhere inside it, and for a typical pair that is a month. A lake counts as frozen when at least half of its pixels are bright (green reflectance above 0.15), and as open when almost none are and the near-infrared reflectance is below 0.02; dark ice, which looks like water in visible light, would otherwise be counted as open. 2017 is left out: the archive has only six scenes, from 11 June.

Two kinds of ice

The ice that filled the valleys 20,600 years ago was glacier ice, moving slowly downhill, estimated at up to roughly 100–180 m at its thickest. The ice the satellite watches is the layer that forms on a lake each winter and melts each spring. The glaciers are long gone from Retezat; the lakes freeze on their own.

How far to trust the ice drawn here

The outline is published and dated: 78.4 km² of ice, with the southern maximum at 20.6 thousand years (+0.8, −1.3). Two older outlines for the same mountains differ a lot: one covers 65.6 km² of the Pietrele valley, another 324 km² across the whole massif. The thickness is an estimate: ice yield stress of 50, 100 or 150 kPa, the surface slope smoothed over about 300 m, and a margin that thins like a plastic glacier. It gives at most 102, 148 or 182 m inside the outline; the research group’s own conference poster mentions about 330 m, well above these figures. The terrain is a surface model, so the lakes are drawn level at their shore height, and their depth is not in the thickness. Ground colours and snow tones are an illustration.

Sources and method

Sources

  1. Ruszkiczay-Rüdiger, Z., Kern, Z., Urdea, P., Madarász, B., Braucher, R. (2021). Limited glacial erosion during the last glaciation in mid-latitude cirques (Retezat Mts, Southern Carpathians, Romania). Geomorphology 384: 107719. Maximum ice extent, 10Be exposure ages (southern valleys 20.6 +0.8/−1.3 ka). link
  2. Lima, A. C. et al. GLACIMONTIS: global compilation of empirically reconstructed mountain palaeoglaciers (Zenodo, CC BY 4.0). The outline drawn here is its digitisation of Ruszkiczay-Rüdiger et al. (2021): 78.4 km², three polygons. The compilation also holds Urdea et al. (2011, 324 km²) and Reuther et al. (2007, 65.6 km²) for the same mountains. link
  3. Copernicus DEM GLO-30 (ESA, 30 m), the relief: a surface model, so forest canopy is included and the lakes are drawn level. Contains modified Copernicus data. link
  4. Copernicus Sentinel-2 L2A, scene S2A_34TFR_20240813 (true colour) as the ground photograph; 413 scenes from 15 April to 1 August of 2018–2026 for the ice series (Earth Search COG archive). Contains modified Copernicus Sentinel data. link
  5. OpenStreetMap contributors (ODbL): outlines and names of the water bodies, peaks and altitudes of the labels; 82 water bodies in the scene, 44 of them inside the outline (three reservoirs excluded). link
  6. Madarász, B. et al. (EGU 2016) and Ruszkiczay-Rüdiger, Z. et al. (EGU 2017, EGU 2024), conference abstracts: glaciers reached about 1,040 m in the north and 1,130 m in the south; the 2024 poster lists a maximum thickness of about 330 m, not used here. link
  7. Palace of the Parliament, Bucharest: 84 m high (Wikipedia; Skyscraper Center). Some sources give 86 m. link

mariuscomper.uk · Relief and photograph: Copernicus (ESA) · Outline: Ruszkiczay-Rüdiger et al. 2021 via GLACIMONTIS · Water and peaks: OpenStreetMap