地球上大約97%的水都在海洋中，而隻有大約2.5%是淡水。其中大部分被束縛在冰蓋中或位於地下。事實上，地球上隻有約0.3%的淡水存在於河流和湖泊中，這是我們日常生活中大多數水的來源。

當太陽加熱海洋中的水時，它蒸發成水蒸氣，上升並凝結成雲，然後以雨或者雪的降水方式落下。冰可以在冰帽和冰川中儲存數千年，其中包含瞭地球上大約70%的淡水。

雨水從陸地上流下，進入河流，流向海洋或進入大部分淡水湖泊。湖泊的類型有許多種，比如 "牛角湖"。當流水的力量逐漸使得蜿蜒的曲線擴張，直到它與主河道斷開時，就會形成牛角湖。位於美國和加拿大邊境的蘇必利爾湖通常被認為是面積最大的淡水湖，占地82400平方公裡（31820平方英裡）。

About 97 percent of all water on Earth is in the salty oceans, while only about 2.5 percent is freshwater. Most of that is tied up in the ice caps or lies underground. In fact, only about 0.3 percent of Earth's freshwater is in rivers and lakes, the sources of most water we use in everyday life.

When the Sun heats water in the oceans, it evaporates as water vapour that rises and condenses into clouds, before falling down as precipitation including rain and snow. Ice can be stored for thousands of years in the ice caps and glaciers, which contain about 70 percent of Earth's freshwater.

Rainwater runs off land into rivers that flow to the oceans or into largely freshwater lakes. There are many types of lakes, including 'oxbow lakes' that form when the force of flowing water gradually exaggerates a meandering curve until it cuts off from the main river channel. Lake Superior on the US-Canada border is often regarded as the largest freshwater lake by area, covering 82,400 square km(31,820 square miles).