Temporal variations in water quality and fluvial erosion in a small drainage basin in southern Finland

A study is made of temporal and spatial variations in material concentrations and transport in connection with spring and summer quick flow periods and episodes in a small drainage basin, Koiransuolenoja, of size 6.75 km². More long-term variations detectable in fluvial erosion and the development of the valleys of the streams are also examined. A total of 382 water samples for analysis were collected in 1984 and 1985, mostly using an automatic sampling device, in order to investigate variations within the course of individual flood events. The highest concentrations of suspended solids were consistently recorded during the rising stage in the quick flow period, or 3—5 hours before the peak discharge rate in the case of diurnal variations. Seasonal peak concentration values were very many times greater than the pre-flood levels, around 900 mg 1-1 compared with 10 mg 1-1 while diurnal variations, which were similarly greatest at the beginning of the flood season, entailed maxima which were as much as 10 times the minimum recorded on the same day. The concentration of dissolved substances increased at the beginning of the high water period, but decreased once more at the time of peak discharge, with very little diurnal variation. Nitrogen and phosphorus, especially the latter, which was dependent on total suspended solids, showed marked diurnal variations and differences between the stages of the quick flow season. Mean concentrations were best represented by samples taken daily between midday and 4.00 p.m. or 8.00 p.m. and midnight, while the highest concentration levels reached during summer flood situations were over in a few hours. Samples taken once a day or less frequently can easily lead to underestimation of the extent of erosion. Up to 1 t km-2 of suspended solids can be removed from the basin in a day as a result of heavy summer rains, and up to 4 t km-2 at the height of the spring flood, while total transport in the course of the spring high water season can amount to as much as 40 t km-2 , accounting for the majority of the year's total. Examination of the development of the valley of the main stream allows a phase of initial rapid erosion to be distinguished on emergence from the sea, followed by phases of slowly progressing erosion and sedimentation of the lower reaches, and finally a period of accelerated erosion brought about by human agency