は marks the topic of the sentence.
Here, the topic is 川の流れ:
Then the sentence says something about that topic:
So the structure is:
A natural way to understand it is: As for the river’s current, it is fast.
This is a special feature of Japanese.
When は is used as the topic particle, it is pronounced wa, not ha.
So:
This only happens in certain grammatical uses. As a normal syllable in a word, は is usually pronounced ha.
In Japanese, sentences with i-adjectives often do not need a separate verb like is.
That means:
but in natural English, that becomes:
This is very normal in Japanese.
速い is an i-adjective.
That means it ends in -い and can directly describe something or act as the main predicate of a sentence.
Examples:
Because it is an i-adjective, you do not need です for it to be grammatical in plain style.
Yes, both are pronounced hayai, but they mean different kinds of fast/early.
Since a river current has to do with speed, 速い is the correct kanji here.
So:
The key point is:
Yes, 川の流れが速い is also possible, but the nuance changes.
Compare:
川の流れは速い。
As for the river’s current, it is fast.
This sounds like a general statement about it.
川の流れが速い。
The river’s current is fast. / It’s the river’s current that is fast.
This can sound more like new information or emphasis on what is fast.
Both are natural, but は is very common when stating a general fact.
Because this sentence is in plain style.
Both are correct.
Use:
The meaning stays basically the same.
Yes. 流れ can mean flow, current, or even a figurative course/trend depending on context.
Examples:
In this sentence, because of 川, it clearly means the physical current of the river.
Japanese does not use articles like English does.
So 川, 流れ, and 川の流れ do not automatically tell you a river, the river, a current, or the current. The exact meaning depends on context.
That is why the sentence could be translated in slightly different ways, such as:
In real use, context usually makes the intended meaning clear.
Yes, in natural English that can work, but it is not the most literal translation.
The Japanese sentence is talking about the river’s flow/current as a noun phrase:
So the most direct translation is:
But in smoother English, people might also say:
Those are natural English equivalents, even though the Japanese structure is noun-based.