Cyanobacterial bloom exhibits huge annual variation in the regulated river system.

Self-Organizing Map was used to analyze patterns between cyanobacterial bloom and environmental factors for the Nakdong River.

Annual cyanobacterial blooms are separated by rainfall and river flow in a river experiencing eutrophication.

Seasonal cyanobacterial blooms were distinguished and separated based on heat source, rainfall and nutrient variations.

Cyanobacterial bloom reduction during drought periods can be suppressed by regulated flow and nutrient.