This study was carried out to elucidate the development of unprecedented water-bloom caused by a single species of colonial green algae Eudorina elegans in the upstream area of the Seungchon weir located in the Yeongsan River from late April to May 2013. The Yeongsan River is typically regulated system and the waterbody is seriously enriched by both external and internal sources of nutrients. Seasonal algal outbreaks were highly probable due to various potential factors, such as the excessive nutrients contained in treated wastewater, slow current, high irradiation and temperature, in diatom (winter), green algae (spring) and bluegreen algae (summer). Spring green-tide was attributed to E. elegans with level up to 1,000 mg m-3 (>50×104 cells mL-1). The bloom was exploded in the initial period of the algal development and after then gradually diminished with transporting to the downstream by the intermittent rainfall, resulting in rapid expansion of the distribution range. Although the pulsed-flows by the weir manipulation was applied to control algal bloom, they were not the countermeasures to solve the underlying problem, but rather there still was a remaining problem related to the impact of pulsed-flows on the downstream. The green-tide of E. elegans in this particular region of the Yeongsan River revealed the blooming characteristics of a colonial motile microalga, and fate of vanishing away by the succeeding episodic events of mesoscale rainfall. We believe that the results of the present study contribute to limno-ecological understanding of the green-tide caused by blue-green algae in the four major rivers, Korea.