Sri Lanka's rivers rise in the Central Highlands and flow in a radial pattern toward the sea. Most of these rivers are short. There are sixteen principal rivers longer than 100 kilometers in length, with twelve of them carrying about 75 percent of the mean river discharge in the entire country. The longest rivers are the Mahaweli Ganga (335 kilometers) and the Aruvi Aru (170 kilometers). In the highlands, river courses are frequently broken by discontinuities in the terrain, and where they encounter escarpments, numerous waterfalls and rapids have eroded a passage. Once they reach the plain, the rivers slow down and the waters meander across flood plains and deltas. The upper reaches of the rivers are wild and usually unnavigable, and the lower reaches are prone to seasonal flooding. Human intervention has altered the flows of some rivers in order to create hydroelectric, irrigation, and transportation projects. In the north, east, and southeast, the rivers feed numerous artificial lakes or reservoirs (tanks) that store water during the dry season. During the 1970s and 1980s, large-scale projects dammed the Mahaweli Ganga and neighboring streams to create large lakes along their courses. Several hundred kilometers of canals, most of which were built by the Dutch in the eighteenth century, link inland waterways in the southwestern part of Sri Lanka.