If the precipitation in the central plains of China is lower than in previous years, the water level of rivers in the region will drop and the flow rate will slow down  This is good for the growth of water plants in the river, and the total amount of water plants in the river usually increases
However, after an extreme drought in the region last year, the amount of water grass in one of the region's rivers did not increase, but remained at a very low level, even though the flow rate was very slow
The slower the flow of a river, the less its water temperature changes, which is conducive to the growth and reproduction of water plants.