The Maule river (Mapudungun: rainy) is one of the most important rivers of Chile and is inextricably linked to this country's pre-Hispanic (Inca) times, the country's conquest, colonial period, wars of Independence, modern history, agriculture (wine, traditional crops), culture (literature, poetry, folklore), religion, economy and politics. The Maule River marked the southern limits of the Inca Empire. Many famous men and women in Chile's history have been born in the Region named after the river. The river has also lent its name to one of the viticultural regions of the country (also known as appellations), the Valley of Maule, a subregion of the Central Valley.

The river is 240 km long and its basin covers around 20,600 km². Thirty percent of the basin is located in the Andean range. It is born in Laguna del Maule in Talca Province, at an altitude of 2,200 mt in the vicinity of the border with Argentina. From that point the rivers flows westwardly and is soon joined on its left bank by one of its main tributaries, the Melado River, which is born in Laguna Dial, in Linares Province. Further on, and still in its upper course, the Maule is joined, this time on its right bank by rivers Colorado, Claro, Puelche and Cipreses.

A crater lake is a lake that forms in a volcanic crater or caldera, such as a maar; less commonly and with lower association to the term a lake may form in an impact crater caused by a meteorite, or in an artificial explosion caused by humans. Sometimes lakes which form inside calderas are called caldera lakes, but often this distinction is not made. Crater lakes covering active (fumarolic) volcanic vents are sometimes known as volcanic lakes, and the water within them is often acidic, saturated with volcanic gases, and cloudy with a strong greenish color. Lakes located in dormant or extinct volcanoes tend to have fresh water, and the water clarity in such lakes can be exceptional due to the lack of inflowing streams and sediment.

Lava lakes, a rare phenomenon where a volcanic crater can sustainably contain a significant volume of molten lava, are discussed in a separate article.

Crater lakes form as precipitation within the rim fills the created depression. The water level rises until an equilibrium is reached between the rate of incoming and outgoing water. Sources of water loss singly or together, may include evaporation, subsurface seepage, and in places, surface leakage or overflow when the lake level reaches the lowest point on its rim. At such a saddle location, the upper portion of the lake is contained only by its adjacent natural volcanic dam; continued leakage through or surface outflow across the dam can erode its included material, thus lowering lake level until a new equilibrium of water flow, erosion and rock resistance is established. If the volcanic dam portion erodes rapidly or fails catastrophically, the occurrence produces a breakout or outburst flood. With changes in environmental conditions over time, the occurrence of such floods is common to all natural dam types.