I’m not a linguist nor am I Icelandic. However, when visiting I did notice that the language has this “Eth” (Ð ð) letter and [the “Thorn” Þ þ letter.](https://en.wikipedia.org/wiki/Thorn_(letter)).

My simple, English-based understanding is that they represent two variations of the English “th” sound. Eth is voiced, softer, and similar to the “th” in “the” or “father” and Thorn is unvoiced, harsher, and similar to the “th” sound in its name “thorn” or “thank”. It’s subtle and I never remember learning about the differences in my schooling.

I’ve also noticed there is a small “movement”, here and a few other sites online, to try and bring one or both of these back, replacing “th” with one or both these characters in English posts and comments.

Edit: Pronouncing the actual name of the waterfall, the Wikipedia page has a playable pronunciation that sounds to me like “go the foss” which matches an English pronunciation of just replacing Eth with TH, but that’s just an approximation.