Lakes and reservoirs are found throughout North America. The combination of extensive glaciations and widespread dam building has resulted in a continent that is rich in lakes of all sizes, from the smallest features tens of meters across to the Great Lakes. The number of lakes in the Canadian Shield is very large. The 5 Great Lakes are joined by several other very large lakes that account for ten of the 25 largest lakes in the world. The cordilleran system of North America includes many lakes, formed by glaciation and/or tectonics. In the more arid portions of the region many lakes have no outlets to the sea. The Arctic lowlands are home to tens and perhaps hundreds of thousands of shallow lakes. When Europeans occupied the continent in the 17th century they quickly began to build small dams that created ponds supplying power to mills for various purposes. In the early 20th century, the scale of reservoir construction increased markedly. In addition to the thousands of large reservoirs that have been built in North America, millions of small impoundments dot the landscape, especially in the United States. The cumulative impacts of the placement of millions of artificial water bodies on the landscape of North America are both profound and pervasive. There are no major watersheds in the United States without reservoirs.