Simplify the following text for a Grade 5 native English-speaking person.
Original: It is 128 km long and has a drainage area of 3990 kmÂ . With the Mogami River and the Kuma River , it is regarded as one of the three most rapid flows of Japan .
Simplified: It is 128 k long and has a watershed of 3990 k. With the Mogami River and the Kuma River , it is regarded as one of the three most rapid flows of Japan .