Sedimentation in an Intermountain Lake, Lake Okotanpe, Hokkaido.

I. Sedimentary Processes Derived from the Grain Size Distribution of Surficial Sediments.

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In Lake Okotanpe, Hokkaido, surficial sediments were picked up at 59 points from the lake bottom, lake shore, and river bed and -bank. All the samples were analyzed for grain size and ignition loss. From the analytical results, the following conclusions can be drawn :
1. The lake bottom sediments, as a whole, consist of more than 80% silt and clay in weight. Excessive sedimentation in the lake could take place during the torrential floods of influent rivers, carrying a large washload of much silt and clay.
2. The lake sedimentary processes are represented by transportation and deposition of the fine suspended grains due to river induced turbidity currents and sediment dispersion.
3. The turbidity currents have a hydrodynamic feature of being controlled considerably by the topography of the lake basin. Therefore, the 'threshold' and the 'constricted' wall characteristic of the basin shape of Lake Okotanpe, can induce the convergence and subsequent lateral spreading of turbidity currents. Consequently, they cause sediments to have a more fine-textured, poorly sorted and almost log-normal grain size distribution.
4. The flowing condition of sediment dispersion is controlled by convective currents induced in the inflow-outflow system of the lake. Its sedimentary region is limited to the southmost area of the lake.
The flowing processes of suspension, controlled by turbidity currents and sediment dispersion, are consistent with the flowing conditions of river induced currents observed in October 1983.
Ignition loss for the lake bottom sediments, as a whole, ranges from 5 to 20%. The organisms almost all originate in the terrestrial plants. In the northeast recess, however, the ignition loss reaches the maximum of 30%, where an artificial effect is added.