Bathymetrical mapping and sedimentrogical analyses of the bottom surface sediments were carried out in Lake Koyama-ike, a coastal lagoon facing the Japan Sea, southwest Honshu. The bottom topography of the lake is almost flat but for the gentle inclination to northeast, exceptionally with a small funnel-shaped depression located at the north end of the lake which has the deepest record of 6.5m, Artificial bottom features such as deredged surfaces and a slipped-out mound from the reclaimed land are observed in the northeast half of the lake. The distribution of grain size and sand content of the surface sediments sampled from 30 stations show that the sandy material from dunes covering the northern area scarcely affects the sedimentation of the lake. The main source of sediment supply is River Nagara which has the largest catchment area among the inflowing rivers. The distribution of C/N ratio, however, suggests that the amount of terrigenous materials supplied from the river is very little compared with the case of Lake Shinji, a typical estuarine lagoon in Japan. Although the amount of input materials from land area is small, the content of total organic carbon (TOC) of the sediments shows rather high value than in the case of Lake Shinji. Therefore, it is considered that the larger amount of primary production or the smaller ratio of sedimentation compared with the case of Lake Shinji characterizes the bottom condition of Lake Koyama-ike.
Key words: bottom topography, C/N ratio, grain-size analysis, ignission loss, Lake Koyamaike, total organic carbon