Monthly sedimentation in the reservoirs and channel of the Paimionjoki river, Finland

Abstract

A method is described by means of which the monthly rate of sedi­mentation in reservoirs and in a 'normal' river channel can be measured. The investigated area is the water course of the Paimionjoki river in the clay region of south‑western Finland. The observation period was from April to the end of October in 1973.

The rate of sedimentation was observed in the reservoirs of three hydroelectric stations at depths of 5.8‑11.0 m and three other points upstream in shallow water. Monthly samples from surface water were taken from eight points. Suspended load in the surface water was 60‑160 mg/1 in spring, 20‑60 mg/1 in summer and 40‑80 mg/1 in autumn. Conductivity was greatest in spring (650‑900 * 10⁶) and smallest in summer (500‑600 * 10⁶).

The average monthly dry weight of the sediments at the observation points, from the head to the mouth of the river, was: (1) 47, (2) 114, (3) 101, (4) 310, (5) 294 and (6) 272 Mg/CM2 (points 4‑6 = reservoirs). The corresponding thicknesses of the uncompressed sediments were 3.4, 7.4, 4.7, 10.3, 10.2 and 9.3 mm. The monthly maximum at point 4 in July (875 mg/cm² and 22 mm). The rate of sedimentation was highest in July, mean 389 mg/cm², and smallest in October, 54 mg/cm². Intensity of rain (monthly precipitation/number of rainy days) and moisture of the preceding month have great influence on the quantity of moving sediments and sedimentation.