Rivers and streams dot the island; many of them are formed in the crevices between land created by new and old lava flows. Drainage radiates from the central tableland to the sea, and many rivers are steeply graded with rapids and falls. Torrential flows are common during storms and cyclones. Marshes and ponds lie in the tableland and on the coastal plain, but the country has only two natural lakes, both crater lakes. The largest of several manmade reservoirs is the Mare aux Vacoas.