ABSTRACT
Summary This chapter describes the physico-chemical and ecological features of the Yongdam reservoir. Flow rate, meteorological conditions and water quality were monitored over three years, 2005-2007 at ten sampling stations in the reservoir as well as five tributaries. The research site Yongdam reservoir is located in the upper region of the Geum river basin flowing through the mid-west of the Korean Peninsula that belongs to the temperate zone. The Yongdam reservoir has particular features such as a dendritic narrow, deep and long shape, thermal stratification, turbid density flows, and intake facilities for surface water. The retention time of the reservoir amounts to 318 days. About 70% of the annual rainfall is concentrated during the period of the summer monsoon from June to September; a main reason for the reservoir construction has been flood control and water storage for the dry season. In this study, physico-chemical and ecological parameters monitored at three depths in the water column identify double thermoclines of the reservoir water body and turbid density flows due to density differences between inflowing water and the receiving water body. Analyses of nutrients and eutrophication status show that the Yongdam reservoir is at the threshold of a limnological evolution, viz. through aging processes; thus highly increased nutrient and phytoplankton contents were found in 2005, i.e. only 3 years after filling the reservoir. The COD and phosphorus loadings etc. have been decreasing over the three years 2005-2007. The levels of pollutants in the upper reservoir section are generally higher than for the upstream sections. The middle layer, due to density flows during thermal stratification, shows an opposite results compared to the surface layer. This means that the water body of the Yongdam reservoir has been responding to the watershed with a complex mechanism. According to the Algal Growth Potential Test (AGPT), the Yongdam reservoir is a phosphate-limited water body; there may be inhibitors that hinder the algal growth in case of sufficient bio-available limiting nutrients existing in the water column. The lacustrine zone in the lower reach gives a better condition for algal growth because of a smaller turbulent length scale than that in the riverine and transition zones. The AGPT also identifies a mixing pattern of nutrients and the reservoir zones through their longitudinal and vertical AGP distributions. The surface water discharge of the Yongdam reservoir induces a transfer of pollutants from the middle layer to the surface layer. This nutrient transportation mechanism reduces nutrient accumulation at the bottom of the reservoir, but it also offers an opportunity for continuous algal production during thermal stratification of the surface water.
