Monthly sedimentation in some reservoirs of hydroelectric stations in Finland

Abstract

Using a sediment sampler anchored to the bottom, measurements were made at different points in 14 reservoirs in the water courses of the Kemijoki, the Oulujoki, the Kokemäenjoki, the Pielisjoki and the Höytiäinen channel in 1974. The quantity and the quality of sedi­ments and the factors influencing sedimentation e.g. precipitation, discharge and qualities of both water and the deposits of surrounding areas were studied.

Suspended load was clearly highest (60‑120 mg/1) in the Paimion­joki which flows through an area with argillaceous deposits, but the rate of sedimentation was rather small (monthly mean about 300 mg/ cm²), as a result of the very fine‑grained suspended material. Sedi­mentation was highest in the Kokemäenjoki on average (monthly mean about 1650 Mg/cm²), although the average suspended load was not more than 8‑20 mg/1. The high rate of sedimentation can be explained by the qualities of the drainage area: many areas of argillaceous deposits, the rather high percentage of arable land, widespread indus­try and high population density. An especially high rate of sedimen­tation (2400 mg/cm²) and a high density of sediments was recorded in the Oulujoki in August when a considerable amount of silt was depos­ited in the area of sandy silt during the exceptionally high flood. The smallest sedimentation (about 30 mg/cm² per month) was recorded in the reservoirs situated just after a large lake and also in the drainage areas on till with plenty of forests, peat and lakes. Sedimentation in till drainage areas seems to be concentrated strongly to the month of spring flood, but in the areas composed of argillaceous deposits the autumn rains also have a great influence. The movement of sediments was stopped most effectively in the reservoirs of the Kokemäenjoki and the Pielisjoki. The reservoirs of the Paimionjoki reduced sediment transportation very little.

The percentages of organic material in the sediments were clearly highest in the forest ‑ peat land areas and on till, and lowest in areas with argillaceous deposits. The absolute amount of organic material was largest in the Kokemäenjoki. More organic material was deposited in upper than in lower reservoirs.