Water currents and spreading of river load in Lake Pyhäselkä, Saimaa, Finland

During the summers of 1989–1990, the pattern of water flow and the water quality in Lake Pyhaselka, Finland, were studied using field observations and a 3-dimensional model. The lake receives its major point-source loading, including pulp mill effluents, via the Pielisjoki River in the northeastern part of the lake, where the water is slightly eutrophic. The flow pattern was measured with three types of recording current meters, two of which were conventional mechanical meters and the third a new acoustic current profiler. The profiler was used in direct reading mode on a research vessel and also in self-contained mode on a float installed in the deep basin of the lake. The water-quality part of the model simulates oxygen, BOD, total phosphorus, AOX, and phytoplankton biomass. The three-dimensional model applied here simulates accurately the flow pattern of the water in Lake Pyhaselka. The results of this study made it possible to detect different flow patterns in the lake. The main load is clearly affected by the wind-induced pattern of flow.