On its main forks, the Coquille is also an alluvial river valley, meaning it was once a drowned river valley—a lake, or perhaps a much wider river, with a sandy bottom. The soil in the valley is very fertile. However, the rise and fall of the water level, and flooding in winter, prevent ordinary types of farming. A lush grass grows in the valley lowlands which can survive either drought or inundation. Tide-gates along the river can bring in water when needed. On this sweeping stand of grass, in summer many cattle find pasture.