## 2 Materials and Methods
### Study Area  
The Nakdong river water system, the target of this study, is composed of eight multifunctional veins, which are the main water structures of the main stream. Changes have been made to the environment of the watershed with the installation of multifunctional veins as part of a river project in 2012. Therefore, watershed management measures according to environmental changes are urgently needed. As the water depth of the Nakdong river has increased and the flow speed has decreased after the Daehacheon Project, its water has stagnated, and consequently, changes in water quality and environment have been observed. An algae warning is issued every year due to heat waves and droughts in summer, and on 22 August 2018, the number of harmful cyanobacterial cells per 1 mL of water in Hapcheon-Changnyeong Weir was recorded at 1,264,052, which is the worst case of algal bloom in history [19]. In the main stream of the Nakdong river, eight multifunctional veins were installed as part of a river project: Sangju, Nakdan, Gumi, Chilgok, Gangjeong-Goryeong, Dalseong, Hapcheon-Changnyeong, and Changnyeong-Haman weirs, which are located from upstream to downstream. Figure 1 shows the location of the multifunctional weir installed in the Nakdong river. Among the eight weir structures, Nakdan, Gangjeong-Goryeong, and Hapcheon-Changnyeong weirs were selected as the target areas. Table 2 shows the multifunction weir specifications. After the construction of the weirs, the hydraulic residence time increased, the characteristics of the stagnant water area were observed, and problems with algae due to heat waves and drought continued to occur.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Multifunctional Weir**} & \multicolumn{2}{c}{**Input Variable**} & \multicolumn{2}{c}{**Data Set**} & \multirow{2}{*}{
\begin{tabular}{c} **Data** \\ **Period** \\ \end{tabular} } \\ \cline{2-2} \cline{5-6}  & **Water Quality** & **Hydraulic** & **Meteorological** & & **Training** & **Validation** & **Data** \\  & **Data** & **Data** & **Data** & & & **Period** \\ \hline Nakdan & & & & & & \\ (ND) & & & & & & \\ \hline Gangjung–Goryeong (GG) & pH, EC, DO, BOD, COD, TP, and Chl-a & Out Flow & SPI & & & & \\ (GG) & & & HRT & Temperature & & & \\ \hline Hapcheon–Changnyeong (HC) & & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Machine learning data sets per multifunctional weir point.  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Multifunctional Weir**} & \multicolumn{2}{c}{**Total Length**} & \multirow{2}{*}{**Type**} \\  & **(m)** & & \\ \hline Nakdan (ND) & 285 & \begin{tabular}{c} 1–3 movable weir (second roller gate) and \\ fixed weir (gravity-type concrete) \\ \end{tabular} \\ \hline Gangjung–Goryeong (GG) & 953 & \
\begin{tabular}{c} 1–2 movable weir (rising sector gate) and \\ fixed weir (gravity-type concrete) \\ \end{tabular} \\ \hline Hapcheon–Changnyeong (HC) & 398 &
\begin{tabular}{c} 1–3 movable weir (truss lift type), \\ flap movable weir (flap gate), and \\ fixed weir (gravity-type concrete) \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table}
Table 2: Design specifications of the multifunctional weirs’ structure.  
The hydraulic and hydrological characteristics of the multifunctional weirs located in the Nakdong river were examined through a visualization analysis. Table 3 summarizes the water level, reservoir capacity, HRT, inflow, discharge, and number of cyanobacterial cells for each multifunctional weir. The data were used when the algae population in each weir was 2000 cells/mL or more. Compared to other weirs, the water storage capacity was small, and the HRT was short.