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.
This river has a lot of trout (describing only for this river).
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.