The Lake Biwa is a deep (up to 100m) tectonic basin with rapid accumulation of muddy sediments, and show an asymmetrical profile with stacked submerged terraces on the east and steep slope on the west.
The results of acoustic survey off the Echi River and the facies analyses of sediment cores show that these terraces are composed of sand-rich deposits formed under delta, barrier, and alluvial plain environments, which are similar to those in the present time and characterized by wave-dominated edimentary structures. They are successively built up 40-m-thick coastal sedimentary prisms, where at least nine sedimentary sequences are identified. Each sequence has dominant regressive facies represented by prograding delta-front and shoreface deposits followed by lagoon or alluvial-plain deposits behind them, but fewer transgressive facies.
In addition to the stacked terraces, barrier-growth, lagoon-filling, overwash, and coastal onlap structures around the sequence boundaries can prove the water-level rise during the last 30, 000yr. Thin sand sheets or reworked sediments on the ravinement surfaces, washover deposits, and well-preserved surface topographic features are suggestive of rapid shoreface retreat or in situ drowning. The average amplitude and period of lake-level rise are estimated as 3.5m and 3, 500yr, respectively.
According to the previous works (TAKAYA and NISHIDA, 1964; YOSHIOKA, 1987; KAWABE, 1989), it is expected that the tectonic block including the drainage channel of the lake water (present Seta River) is uplifted, and that the lake basin is tilting westward to the active faults along the western basin boundary. Therefore, such a periodic rise can be sttributed to tectonically-induced relative subsidence compared to the level of the drainage channel, rather than a climatic control.