Thjorsa is 230 km long and flows to the SW. It follows the polygons and the same direction as that of the East Volcanic Zone. But it can be analyzed further:
The start of the main part of the river is at the edge of polygon 1, it flows just about equadistance from the two relevant division lines (also marking the western borger of the East Volcanic Zone) until it reaches the border line between polygon 2 and 3, where it bends along the division line. That is the point where the division between highland and lowland creates conditions for Iceland’s first large scale hydro-powerplant, Búrfell Powerplant. Then the river flows down to the 64th parallel, and flows along it, as the 64th parallel is also the axis of the South Iceland Seismiz Zone. At the very center of polygon 3, the river turns back to SW orientation, flowing down to the estuary, and the coast there coincides with the edge of the polygon.