why did jim believe it would be good to camp in the high up cavern

I think the cave was big enough for him to stand straight up in.

The river was also rising because of the storm. The cave offered both height and protection.

JIM predicted that there was a huge storm and flooding coming their way. Moving to the cave high up in the cavern would keep their things safe and dry...... it would also provide them shelter when the flood moved in. Jim was right....... the flood waters rose for 12 days, and the cave provided them the shelter he knew it would afford.