Þingvallavatn is in a vast, oblong graben or dell which stretches from the southwest to the northeast, from Hengill to Langjökull. The nutrient-rich groundwater is the foundation for the diverse biota in the lake. Sogið, the greatest non-glacial Icelandic river, runs out of the lake. It is so broad in some places that it actually forms the lakes Úlfljótsvatn and Álftavatn. Þingvallavatn is usually covered with ice during the latter part of winter but an air hole remains open most of the time along the northern shore of the lake where spring water of a steady temperature flows continuously all year round. The same is true of Sogið; it never ices up completely, even in extremely cold weather. For this reason, birds seek this area in winter. The vegetation around Þingvallavatn is mostly dwarf-shrub heaths and birch thickets. Along Sogið the riverbanks are grassy all the way down to the water. Birdlife is diverse. The great northern diver, loom, graylag, barrow's goldeneye, harlequin duck, goosander and other duck species are among the water birds found there. In addition, there are many heath land birds, such as wrens and ptarmigans. Among the bird-watching sites in this region are the national park and the banks along the northern part of the lake, as well as Hestvík and Hagavík at its southern end. The main sites beneath Þingvallavatn are Úlfljótsvatn and a dam at the Ljósafoss and Írafoss electric power plants. Harlequin ducks dwell at Sogið, particularly in spring, and various duck species can be seen in all seasons down by the bridge where the highway crosses the river. Common goldeneyes are often seen on Sogið during the winter. They are annual winter guests but they have never been known to nest in Iceland.