KOINOBORI in the water

There is a custom in Japan that " carp-shaped streamer (KOINOBORI) ".
We can see the first ten days of May.
It usually swim in the sky.
But...these are swimming in the stream!!!
Why ? I thought.Then I asked an uncle.
And he tought me that we can't damage KUMAMOTO CASTLE because it is cultual heritage.
It is good for me to hear the reazon(>_<)!