Environmental properties and material transport of two rivulets in Lammi, southern Finland

Abstract

The research is centred upon two streams, Koiransuolenoja and Löyttynoja, which flow into Lake Pääjärvi in southern Finland. The drainage basins of these streams, of area 6.75 km² and 8.02 km² respectively, are located at alti­tudes of 103–180 m a.s.l. The main streams flow largely across arable land. Detailed maps of the geomorphology, topography, gradients, surficial de­posits, land use and vegetation of the basins are constructed in order to ex­plain the environmental factors affecting sediment load in the streams, which was assessed from a total of 576 water samples taken over a space of two years.

The first year studied, 1981, was exceptionally wet, with a total precipi­tation of 850 min compared with a mean for 1931–60 of 580 min. Precipitation in 1982 was only 59 % of that recorded in the previous year, i.e. 502 mm. Peak discharge in Koiransuolenoja was approx. 100 l s‑¹ and minimum discharge approx. 0.3 1 s‑¹, the corresponding values for Löyttynoja being over 60 1 s‑¹ and below 4 1 s‑¹. The spring floods are greatly affected by air temperatures, with peak discharge rates achieved in the evenings and at night and minimum levels in the afternoons. The streams thus provide typical examples of temperature‑controlled snowmelt run‑off.

Annual mean concentrations of suspended solids at the sampling sites on Koiransuolenoja were in the range 22–76 mgl^-l, and those for the Löyttynoja sites 6–39 mgl^-1, the corresponding figures for dissolved substances being 128–166 mgl^‑l for Koiransuolenoja and 65–92 mgl^‑l for Löyttynoja. Nitro­gen content in Koiransuolenoja was 1–4 mgl^‑l and phosphorus content 25­–150 ugl^‑l, with figures of 0.5–3 mgl^‑l and 15–40 ugl^‑l respectively for Löyt­tynoja. The pH of the water varied in the range 6.0–7.3. The peak phosphorus and nitrogen values were associated with the release of material into the environment from agricultural sources.

Total transport in Koiransuolenoja amounted to 82 tkm^‑2 in 1981 and 40 tkm^‑2 in 1982, of which dissolved substances accounted for 60 tkm^‑2 and 31 tkM^‑2 respectively. The corresponding figures for Löyttynoja were 57 tkm^‑2 in 1981 and 38 tkm^-2 in 1982, of which 44 tkm^‑2 and 32 tkm^‑2 respec­tively were in dissolved form. Transport was heavily concentrated in the short peak flood season. At the present rate of erosion the land surface in the basins concerned would be lowered by an average of approx. 5 cm in 1000 years, although this erosion is not evenly distributed, but is most pronounced in areas of marine‑littoral and fluvial relief containing fine‑grained material, areas which are usually subject to intensive agriculture.