Gluggafoss waterfall

Gluggafoss is one of those waterfalls that becomes more interesting the closer you look. Rather than falling cleanly over a single cliff, the Merkjá River has worked its way through softer volcanic rock, cutting tunnels, grooves and openings before dropping over harder basalt below.

These openings give the waterfall its name. Gluggar means windows in Icelandic, and at suitable water levels several of them can be seen in the rock face behind and around the falling water. Gluggafoss is also known as Merkjárfoss, after the river Merkjá.

What makes the waterfall particularly unusual is that the windows visible today are not simply ancient geological features. Their appearance changed dramatically after the 1947 eruption of Hekla, and the river has spent decades reshaping them.

The waterfall with windows

Gluggafoss drops about 50 metres in total through several distinct levels rather than one uninterrupted plunge. The geology of the cliff helps explain its unusual form.

The upper part consists largely of palagonite or tuff, material formed when volcanic eruptions interact with water or ice. This rock is considerably easier for running water to erode than the harder basalt forming lower sections of the falls.

Over time, the Merkjá River exploited weaknesses in the softer rock and began cutting into it. Instead of eroding the cliff evenly, the water developed channels and tunnels through the rock. Sections eventually opened through the face of the waterfall, producing the characteristic windows.

The harder basalt below erodes more slowly, creating ledges and steps that interrupt the descent of the water. The result is a waterfall whose structure is controlled as much by differences between the rock layers as by the volume of the river itself.

What Hekla did to Gluggafoss

The appearance of Gluggafoss changed dramatically in 1947. That year Hekla erupted in one of its largest eruptions of modern times. Ash fell across a wide area of South Iceland, including the drainage system feeding the Merkjá River.

Around 20 centimetres of volcanic ash were deposited in the area. Rain and river water then began carrying the loose ash downstream towards Gluggafoss.

Before the eruption, much of the upper waterfall reportedly travelled through a near-vertical tunnel inside the cliff. The water could be seen through several openings in the rock, including three windows arranged one above another.

The enormous quantity of fresh ash entering the river partly filled this tunnel system and blocked many of the openings. Gluggafoss effectively changed appearance.

The river then began the slow process of removing the material again. Year after year, flowing water carried ash out of the tunnels and continued eroding the surrounding rock. It took decades before the openings became clearly visible again, with accounts suggesting that almost half a century passed before the waterfall began to resemble its earlier form.

The story makes Gluggafoss an unusually clear example of how two different natural processes can interact. Hekla altered the waterfall almost instantly. Merkjá then spent decades undoing part of that change.

A waterfall that continues to change

The process has not stopped. Merkjá is not a glacial river. Its flow depends largely on rainfall, groundwater and melting snow, and its water is generally much clearer than the sediment-heavy rivers flowing from Iceland’s larger glaciers.

Changes in water level therefore have an immediate effect on how Gluggafoss appears. During higher flow, water can cover or obscure some of the openings in the upper cliff. The waterfall can look fuller and more powerful, but the structure behind it becomes harder to see.

At lower flow, more of the tunnels, grooves and windows become visible. Neither condition is necessarily better. Higher water emphasises the waterfall itself. Lower water reveals more clearly how the waterfall was made.

The river also continues widening channels and removing material from the softer rock, so the form of Gluggafoss should not be considered permanent. The same erosion that created the windows is still changing them.

Visiting Gluggafoss

Gluggafoss lies in the rural landscape of Fljótshlíð, east of Hvolsvöllur and away from the busiest line of South Coast attractions.

The lower part of the waterfall is the straightforward place from which to experience it and gives a good view of the different tiers.

It is possible to approach higher parts of the falls, but reaching the upper level is considerably more difficult. The terrain becomes steeper and rougher, and this should not be treated as an easy extension of the lower viewpoint.

Access also crosses private farmland. Visitors should therefore remain considerate of the land, follow existing routes and avoid disturbing livestock or damaging cultivated ground.

There is little reason to force a climb simply to say that the top has been reached. Much of what makes Gluggafoss distinctive — the relationship between the rock, the windows and the falling water — is best understood from below.

Gluggafoss ultimately demonstrates how small structural irregularities in volcanic rock can shape entire waterfall systems. The falls are not defined by height alone, but by interaction between fracture, flow, and gradual widening of openings.

Its tiered design makes the geological process visible, providing a clear example of how Icelandic waterfalls evolve where rivers intersect layered basalt.

• • Gluggafoss is also known as Merkjárfoss, after the Merkjá River.
  • Gluggar means “windows” in Icelandic and refers to openings cut through the upper rock face by the river.
  • The upper cliff is largely palagonite or tuff, while harder basalt forms lower ledges.
  • Ash from the 1947 eruption of Hekla partly filled the tunnel system through which the upper waterfall flowed.
  • The river gradually removed the ash again over the following decades.
  • Water level strongly affects whether the windows can be seen clearly.