The Thjórsá River's ever changing channels meander through a valley of volcanic detritus. It's source is the Hofsjökull glacier, in the very heart of Iceland, and it flows 143 miles to the Atlantic, making it Iceland's longest river. The once grassy Thjorsa River Valley was devastated by the massive eruption of Mt. Hekla in 1104, which covered half of Iceland in volcanic debris