The basin of subarctic Thingvallavatn (64-degrees 10'N, 21-degrees 10'W) was originally formed by tectonic subsidence and glacial erosion. Since the glacial recession some 10,000 yr ago, the basin has been strongly modified by volcanic activity. The catchment area is characterized by lava flows, of which four entered the lake, and one dammed it. One eruption occurred within the lake. Fault movements have resulted in horizontal and vertical displacements. The lake, area 83 km2, is at 100.5 m a.s.l. It has a mean depth of 34 m, and a maximum of 114 m. The catchment area is 1000 km2, of which ca. 50% is postglacial lava. Interglacial lavas and subglacially formed mountains rising to 800-1100 m, glaciers and glacial deposits account for another 50%. The mean discharge of the last 50 yr is 100 m3 S-1. The precipitation, estimated at 2000-2500 mm yr-1, is mostly percolated; hence the lake is ca. 90% spring-fed. The main spring areas in the N show a constant temperature of 2.8-3.5-degrees-C. Warmer ground water (7.5-10.7-degrees-C) enters the lake in the SW owing to the nearby Hengill geothermal area. A discrepancy between potential water balance and discharge is explained by an underestimate of the amount of precipitation in the mountainous areas, by evapotranspiration and underground flow from the Langjokull glacier bed from outside the topographic catchment area. The water level has been regulated since 1960, and the pattern of water level fluctuations before and after are discussed. Irradiance (PAR) measurements since 1982 reveal strong diel fluctuations and a reflection of 49% from the lake surface. Values from PAR measurements and global radiation estimates show fairly consistent results. The euphotic layer varies between 20 m in spring to 40 m in summer. Temperature measurements cover 30 yr. The lake is dimictic and the temperature commonly reaches 10-11-degrees-C in August with a gradient of 2-3-degrees-C over the thermocline. On an average, the ice-cover tasted 95 d during the period 1974 to 1990.
