## 1 Introduction  
Changes have occurred in the river environment of the Nakdong river due to the construction of multifunctional weirs as part of the Four Major Rivers Project. This river currently exhibits characteristics that are similar to those of a stagnant water area in which the river depth increases and the flow velocity decreases. Therefore, the river exhibits characteristics of a lake, and consequently, the occurrence of algae is becoming more frequent [1]. A large-scale occurrence of blue-green algae causes problems, such as oxygen depletion and increased organic matter in water bodies through proliferation and death. In addition, toxic substances secreted by blue-green algae are harmful to the aquatic ecosystem as well as humans [2]. Moreover, the blue-green algae problem in the Nakdong river, which is used as a source of water, affects the water purification supply system by secreting geosmin and 2-MIB that are responsible for the unpleasant tastes and earthy odors in tap water. The occurrence and flourishing of algal blooms degrade the quality of drinking-water supply reservoirs, which ultimately causes public health problems and increases the treatment costs for water utilities [3]. Various factors are associated with the occurrence of blue-green algae. Therefore, identifying the causal relationship between these causative factors is necessary. Most factors that affect the water environment, such as water quality, hydraulics, and weather, are included, and the pattern of algal occurrence and changes in the influencing factors must be analyzed. Based on previous studies on cyanobacteria after the construction of weirs in the main section of the Nakdong river in Korea, [PERSON] (2018) investigated the succession pattern of cyanobacteria in a section of a Nakdong river weir after its installation and identified the morphological characteristics of the genus Dolichosperm in the natural population [4]. [PERSON] (2013) investigated the phytoplankton community structure in the middle and lower reaches of the Nakdong river from January to December 2012 and calculated species diversity and dominance indices. Diatoms accounted for 50% and green algae accounted for 28% of the composition ratio of the species appearing in the summer, indicating a high dependence on diatoms and green algae; in summer, _Microcytsis aeruginosa_ was the predominant species [5]. In addition, [PERSON] (2018) investigated major environmental factors affecting water quality at weekly intervals in eight weir pools of the Nakdong river from January 2013 to July 2017 and compared and analyzed the dominant currents and correlations. The main causative algae of harmful green algae and red tides were the blue-green algae _Microcytsis_ and the diatom _Stephanddiscus_ populations, respectively [6]. [PERSON] (2017) used chlorophyll a (Chl-a) and algae population data, along with water quality and hydrological and meteorological factors in the Mulgeum area of the lower Nakdong river, to analyze the pattern of algal occurrence and changes in influencing factors after the construction of a weir and examined the correlation. After the weir's construction, the number of cyanobacteria in the Mulgeum area increased significantly, whereas the number of diatoms decreased [7]. [PERSON] (2017) compared and analyzed water quality parameters at four points in the middle and lower reaches of the Nakdong river after a weir's installation and estimated the correlation between water quality parameters and the number of cyanobacteria through factor analysis. Water temperature and pH, which are important water quality factors for the growth of blue-green algae, were found to be significantly correlated. In particular, the phosphorus elution of the sediment occurred with an increase in pH, which was judged as a condition in which eutrophication and nutrient salts could be continuously maintained [8].