rivers. Lakes and lagoons are numerous, and a high water table makes digging wells easy. Three systems of rivers drain the land: rivers flow westward to the Río Uruguay, eastward to the Atlantic or tidal lagoons bordering the ocean, and south to the Río de la Plata. The Río Uruguay, which forms the border with Argentina, is flanked by low banks, and disastrous floods sometimes inundate large areas. The longest and most important of the rivers draining westward is the Río Negro, which crosses the entire country from northeast to west before emptying into the Río Uruguay. A dam on the Río Negro at Paso de los Toros has created a reservoir—the Embalse del Río Negro—that is the largest artificial lake in South America. The Río Negro's principal tributary and the country's second most important river is the Yí River. The rivers flowing east to the Atlantic are generally shallower and have more variable flow than the other rivers. Many empty into lagoons in the coastal plain. The largest coastal lagoon, Laguna Merín, forms part of the border with Brazil. Six smaller lagoons, some freshwater and some brackish, line the coast farther south. Climate Located entirely within the temperate zone, Uruguay has a humid subtropical climate (Cfa according to the Köppen climate classification) that is fairly uniform nationwide. Seasonal variations are pronounced, but extremes in temperature are rare. As would be expected by its abundance of water, high humidity and fog are common. The absence of mountains, which act as weather barriers, makes all locations vulnerable to high winds and rapid changes in weather as fronts or storms sweep across the country. Weather is sometimes humid. Seasons are fairly well defined, and in most of Uruguay spring is usually damp, cool, and windy; summers are warm; autumns are mild; and winters are chilly and somewhat uncomfortably damp. Northwestern Uruguay, however, is farther from large bodies of water and therefore has warmer summers and milder and drier winters than the rest of the country. Average highs and lows in summer (January) in Montevideo are , respectively, with an absolute maximum of ; comparable numbers for Artigas in the northwest are , with the highest temperature ever recorded . Winter (July) average highs and lows in Montevideo are , respectively, although the high humidity makes the temperatures feel colder; the lowest temperature registered in Montevideo is . Averages in July of a