Stífla is an area of the Fljót district in Skagafjörður, Iceland, in the interior of valley. The name originally referred to the group of hills that cuts directly across the valley and was called either or , but the name later came to refer to the area between the hills. It was previously a flat, grassy area and beautiful valley, and there were a good many farms. The river ran through the area, but past its name changes to . 
Around 1940, it was decided to create a hydroelectric power station in Siglufjörður that harnessed the river, and production began in 1942. A dam was built in the ravine in and the Skeiðsfoss power station officially started running in 1945. There was a lake among the hills, called , that became significantly larger once the power plant started running, and as a result, a lot of land was largely submerged, and some land was abandoned. The lake is now 3.9 km2.