Why are there so many fish by the river when there is a flood in the countryside?

I remember that it usually rains once or twice in summer and is flooded. I remember a few years ago when there was a flood in the summer, a lot of fish ran out of the pond in the village above us, both in the fields and in the ditches of your. Many people just use fishing nets to catch fish in the river ditch, but the water is too big and washed away carelessly, but there are really many fish, mostly crucian carp? And mullet, the water receded in the next few days. Basically, I fished it online in about three days. I remember I fished about 50 kilograms online. I sent some to my cousins and relatives and some to my village. They were all looking at my net, so I gave some. This fish pond has not been repaired much. The pond used to be muddy, unlike now, I'm going to get water from the shore. now