Sedimentation and water quality in the flood basin of the river Kyrönjoki in Finland

Abstract

The river Kyrönjoki on the west coast of Finland has a catchment area of 4920 km2 and a mean discharge of 43 m3/s, which can increase to 500 m3/s during the spring flooding season, when the water level in the river can be more than 6 m higher than during dry periods. The floodwaters frequently spread over the surrounding flat farm land and also cause considerable damage to property in years of serious flooding.

The Ilmajoki flood basin studied here is practically 30 kin in length, 1–6 km wide and has a maximum water depth on the fields of almost 3 m. Sedimentation in this flood lake was measured using a total of 150 samplers of three types placed at given points on the flood plain before the rise in water level. The measurements were carried out in spring 1983 and 1984. The dry weight of the sediment collected ranged from a few grammes to over 6000 g per m2 (60,000 kg/ha). Sedimentation rates of 20,000–50,000 kg/ha were common in water of over 2 m in depth. Maximum water depth served to explain 83 % of the variance in sedimentation rate.

The sediments deposited during severe flooding contained very little organic matter, usually about 6–15 %, but those deposited by floods of aver­age proportions contained very much more, a median value of 34 %. The inorganic material deposited was of a grain size of 0.006‑0.02 min. Suspended solids were of the order of 100 mg/l at the height of the flood, but the water of the fields soon cleared after that, to give values of 10–20 mg/1. The only exceptions to this were caused by clouding of the water at the edge of the food lake due to waves whipped up by the wind. The water contained 138 mg/1 dissolved substances during the severe flooding of 1984, and 158 mg/1 during the ordinary flooding of 1983. The mean pH of the water was 5.0 in the former case and 5.6 in the latter. Large amounts of nutrients were carried in the water, the total nitrogen content being 11–14 mg/1 during normal flooding and 5–10 mg/1 even at high flood levels. Other values varied generally in the following ranges: phosphorus 0.3–0.5 mg/1, calcium 5–10 mg/1, potassium 3–7 mg/l, magnesium 2–8 mg/1, sulphates 25–70 mg/1, chlorides 5–15 mg/1, manganese 0.05–0.6 mg/l, iron 0.3–1.0 mg/1 and zinc 0.01–0.04 mg/1.