Lake Suigetsu ('Suigetsu-ko') is a tectonic lake, situated close to the Sea of Japan coast at Wakasa Bay, Honshu Island, central Japan (35˚ 35' N, 135˚ 53' E). The lake is approximately 34 m deep, covers a surface area of approximately 4.3 km2, and has a diameter of approximately 2 km in both north-south and east-west directions. At present, Suigetsu is situated 0 m above sea level (a.s.l.); however, this relative elevation would have been higher in the past because sea level was much lower during the Last Glacial period (with significant volumes of water locked up in extensive ice sheets).
Suigetsu forms part of a five lake system, the 'Mikata-goko' ('Mikata Five Lakes'), with Suigetsu representing the largest of these lakes. The only significant water supply into Suigetsu comes via the neighbouring Lake Mikata from the Hasu River. Since Mikata and Suigetsu are connected by a very shallow (approximately 4 m deep) and narrow (approximately 45 m wide) channel, only fine sediment can be carried across into Suigetsu, with any larger material preferentially deposited in Lake Mikata. This hydrological linkage of the two lakes protects Suigetsu from any high energy hydrological events (such as floods), which therefore cannot disturb the basal sediments deposited at the bottom of Lake Suigetsu. Additionally, Suigetsu is naturally protected from winds by the surrounding ring of Palaeozoic hills (with a maximum elevation of approximately 400 m), providing further protection to the deposited benthic sediment.
The Mikata-goko are situated in the mid-latitude western Pacific, with a regional climate that is typically characterised by both summer and winter monsoons. In winter, the predominant wind direction comes from the north-west, bringing cool air (originating in Siberia), which has picked up moisture over the relatively warmer Sea of Japan, providing heavy winter precipitation to Japan. Conversely, the predominant wind direction in the summer comes from the south-east, bringing warm, moist air originating from over the Pacific Ocean, and again producing heavy summer precipitation. The position of Suigetsu in relation to the important geoclimatic boundary of the Asian monsoon front makes it ideally situated to provide important palaeoclimatic information about any changes in this boundary back through time.
The marked seasonality at Lake Suigetsu generates differential deposition of material through the seasons, which results in the layers within the sediment profile that contribute to the Suigetsu varves. The hydrology of the lake is such that anoxic (deoxygenated) benthic water conditions exist, preventing bioturbation by basal-dwelling organisms. In combination with the physical protection afforded to the lake, these anoxic conditions enable the laminations initially formed to be perfectly preserved in the sedimentary record.