Debora Ranck, age 12, of Van Nuys, Calif., for her question:

How wide is the mouth of the Amazon?

A river is one of the world's most wondrous creations, and the Amazon of South America is one of the world's greatest. Its source in the western and east is fed icy waters from mountain lakes. It runs down the eastern slopes and then ambles eastward across the plains. Huge tributaries join it from the north and the south. It drains almost half of South America and carries more water than any river in the world. The Upper Amazon has a series of major river systems in Peru and Ecuador, some of which flow into the Marañón and others directly into the Amazon proper. Among others, these include the following rivers: Morona, Pastaza, Nucuray, Urituyacu, Chambira, Tigre, Nanay, Napo, Huallaga, and Ucayali. The headstreams of the Marañón—which for many years had been seen as the origin of the Amazon—flow from high above central Peru's Lake Lauricocha, from the glaciers in what is known as the Nevado de Yarupa. Rushing through waterfalls and gorges in an area of the high jungle called the pongos, the Marañón River flows about 1,600 kilometres (1,000 mi) from west-central to northeast Peru before it combines with the Ucayali River, just below the provincial town of Nauta, to form the Amazon River.

After its 3,900 mile course eastward across the continent, the mighty Amazon rendezvous with the Atlantic. Here it is joined by the 40 mile wide and the two rivers share the same outlet. The river mouth is a silty delta, including a flat island the size of Denmark. The total width of the delta is 207 miles, and the fresh waters of the vast river system push 300 miles out into the salty ocean.