Fossá is an extremely beautiful river in the upper part of Þjórsárdalur in Gnjúpverjahreppur and has its source in Fossárdrög. It merges with Þjórsá somewhat below Þjóðveldisbæinn of Stöng. The river is fished from both banks. Nice deep pools and beautiful runs alternate. Some salmon migrate into the river in July, but the best time is nearer the autumn. In recent years, many good salmon have been caught in the river during the summer. Arctic char, sea trout and resident brown trout are also caught in the salmon area. The river is fished from morning to evening.