Difference between NはNが多い and NounにNが多い


この川にマスが多い。This river is full of trout.

新潟は雪が多い。 There’s much snow in Niigata

What’s the difference between に and は here ?

I learned only Noun1はNoun2がAdjい/な structure




The difference might be in the context. N1はN2が多い sounds like an explanation
only for N1, while N1にN2が多い sounds like an explanation compared to several
others.

> この川はマスが多い。  
>  This river has a lot of trout (describing only for this river).
>
> この川にマスが多い。  
>  _This_ river has a lot of trout (compared to some other rivers).

But は is sometimes used to imply a comparison with others. It may depend on
the context.

