A river from source to mouth

The Danube starts as the spring of a small stream in Germany’s Black Forest and ends in the Black Sea.

The source

The place where a river begins is its source, or headwaters. National Geographic Education points out that a river usually gathers water from many small streams, and the one that starts farthest from the river’s end counts as the source. Sources come in several kinds. Some rivers start from melting glaciers: the Ganges begins at the Gangotri Glacier in the Himalayas of northern India. Some start from springs, where groundwater reaches the surface: the spring of the Breg River in Germany’s Black Forest is counted as the source of the Danube. And some flow out of lakes: the Mississippi starts as a stream leaving Lake Itasca in Minnesota.

Tributaries and confluences

A tributary is a stream that flows into a larger one, and the place where two streams meet is a confluence. Every confluence adds water, so a river usually grows wider and carries more water downstream. The Nile has two main tributaries: the White Nile, traced from Lake Victoria, and the Blue Nile, which drains Lake Tana in the Ethiopian highlands. In the scene, the blue lines get thicker each time a stream joins another, because the model draws each stretch by how much land drains through it.

Steep at the top, gentle at the bottom

Draw a profile along a river from its source to its mouth and you usually get a curve that is steep at the top and flattens toward the sea, like the one above. Near the source, small streams cut down into the slopes and carry rock and gravel. Farther down, the river has more water but less slope, and it spends more of its energy moving sideways across its valley. In the scene, the readout gives the starting height, the length of the drop’s path and how far it fell. Divide the fall by the length and you have the average slope of that path.

Meanders and oxbow lakes

On flat, low-lying plains rivers often swing in wide bends called meanders. On the outside of each bend the water flows faster and wears away the bank. On the inside it slows and drops silt and sand. Over time the bends grow, and two neighbouring bends can creep so close that the river cuts across the narrow neck between them. The abandoned loop is left beside the river as a curved lake, an oxbow lake, named after the U-shaped collar of an ox yoke. In Australia it is called a billabong. National Geographic notes that engineers have cut off hundreds of Mississippi meanders, so the river is shorter now than it was in the nineteenth century.

The mouth and the delta

Where a river slows down at its mouth, it drops the sediment it has been carrying. Over centuries the deposits can build a flat, low plain called a delta. The name comes from the Greek capital letter delta, Δ, because many deltas spread out in a triangle or fan. Deltas and floodplains are among the most fertile farmland on Earth; the Nile and its delta helped give rise to ancient Egypt. Even under the sea the Nile keeps building: an underwater fan of its sediment now covers about 70,000 square kilometres.