The Environmental Impacts of Large Dams Land and water are ecologically linked in a natural system called a watershed. From the smallest droplet to the mightiest river, water works to shape the land, taking with it sediment and dissolved materials that drain to watercourses and, in most cases, eventually to the sea. So is the river a product of the land it inhabits--the type of rock and soil, the shape of the land, and the amount of vegetation are some of the factors that determine the river's shape, size and flow. When these ties between the land and the river are broken by a large dam, the consequences are felt throughout the water