When was the first great migration in Chinese history? Why is this happening?

At the end of the Western Jin Dynasty, the water conservancy in the Central Plains was in disrepair, and droughts, locusts and epidemics continued year after year. In order to get food, people in the Central Plains began to move south. The destruction of the Eight Kings Rebellion and the impact of various ethnic groups in the west and north caused a greater migration tide. People from the Yellow River basin moved to the Yangtze River basin, and people from the Yangtze River basin moved further south.