Do you know if there are some rules to how fast the water should be? I know in siberia there are some rivers with suuuper slow speed, you almost can't see that

Do you know if there are some rules to how fast the water should be? I know in siberia there are some rivers with suuuper slow speed, you almost can't see that it moves. I know that there are myriads of mosquitos too. Would be interesting to know if there's a connection to water speed and the amount of mosquitos breeding.