Overall water quality in Han, Nakdong, Geum, and Yeongsan rivers has improved in May compared to that of April, 2002.                                            
                                         
BOD level in the main stream of Han river has improved (range 0.3-2.3 mg/l). BOD level of Paldang water reservoir in upper stream of Han river increased from 1.4mg/l to 1.7mg/l due to the higher water temperature than April.

BOD level in main and lower stream of Nakdong river improved (0.8-3.4mg/l), keeping the second best water quality level (level II from 5 levels). Large quantity of water inflow from increased precipitation contributed to water quality improvement by enhancing the self-purification capacity and decreasing the concentration of algae. In contrast, BOD level of upper stream of Nakdong river increased from 0.9 to 1.2mg/l.

Daecheong lake in upper stream of Geum river maintains the level I water quality with BOD 1.0mg/l. The main stream in all Geum river watershed has also improved(BOD 0.3-0.4mg/l).

Water quality in all areas on Yeongsan river improved (BOD 0.3-3.5mg/l) because water inflow from dams has increased. In addition, Juam lake in upper stream of Seoumjin river maintains the level I with BOD 0.8mg/l.