Story of Seasons: Materials when Swimming

At the beginning of Story of Seasons there are some materials hard to find such as iron, glass stone, brick and gems. Some of these items can be found in one of the rivers that you can swim in. The river that generally produces a lot of these items is north of the Grain and Water Paddy plots owned by the Guild. Here is a video showing the river, though I did not find any materials this time around.