We actually couldn't see where all the water was going until we climbed up to this high vantage point. Right after I took this picture, both me and my camera got drenched from spray blown by the strong wind.

"Gullfoss is one of the most popular tourist attractions in the country. The wide Hvítá rushes southward. About a kilometre above the falls it turns sharply to the right and flows down into a wide curved three-step "staircase" and then abruptly plunges in two stages (11 m and 21 m) into a crevice 32 m (105 ft) deep. The crevice, about 20 m (60 ft) wide, and 2.5 km in length, extends perpendicular to the flow of the river. The average amount of water running over this waterfall is 140 m³/s in the summertime and 80 m³/s in the wintertime. The highest flood measured was 2000 m³/s.

"As one first approaches the falls, the crevice is obscured from view, so that it appears that a mighty river simply vanishes into the earth." (Wikipedia)

A full view of the double falls: