Stream Terminology and Definitions

River delta patterns, Columbia River, Western Washington and Western Oregon, USA
River patterns of the Columbia River, Western Washington and Western Oregon, and its tributaries. Sunset Avenue Productions / Getty Images

A stream is any body of running water that occupies a channel. It is normally above ground, eroding the land that it flows over and depositing sediment as it travels. A stream can, however, be located underground or even underneath a glacier. 

While most of us speak of rivers, geoscientists tend to call everything a stream. The boundary between the two can get a little blurry, but in general, a river is a large surface stream.

It is made up of many smaller rivers or streams.

Streams smaller than rivers, roughly in order of size, may be called branches or forks, creeks, brooks, runnels and rivulets. The very smallest kind of stream, just a trickle, is a rill.

Streams may be permanent or intermittent—occurring only part of the time. So you could say that the most important part of a stream is its channel or streambed, the natural passage or depression in the ground that holds the water. The channel is always there even if no water is running in it. The deepest part of the channel, the route taken by the last (or first) bit of water, is called the thalweg (TALL-vegg, from the German for "valley way"). The sides of the channel, along the edges of the stream, are its banks. A stream channel has a right bank and a left bank: you tell which is which by looking downstream.

Stream channels have four different channel patterns, the shapes they show when viewed from above or on a map.

The curviness of a channel is measured by its sinuosity, which is the ratio between the length of the thalweg and the distance downstream along the stream valley. Straight channels are linear or nearly so, with a sinuosity of nearly 1. Sinuous channels curve back and forth. Meandering channels curve very strongly, with a sinuosity of 1.5 or more (although sources differ on the exact number).

Braided channels split and rejoin, like the braids in hair or a rope.

The top end of a stream, where its flow begins, is its source. The bottom end is its mouth. In between, the stream flows through its main course or trunk. Streams gain their water through runoff, the combined input of water from the surface and subsurface.

Most streams are tributaries, meaning that they drain into other streams. An important concept in hydrology is stream order. A stream's order is determined by the number of tributaries that flow into it. First-order streams have no tributaries. Two first-order streams combine to make a second-order stream; two second-order streams combine to make a third-order stream, and so on. 

For context, the Amazon River is a 12th order stream, the Nile an 11th, the Mississippi a tenth and the Ohio an eighth. 

Together, the first through third-order tributaries making up the source of a river are known as its headwaters. These make up approximately 80% of all of the streams on Earth. Many large rivers divide as they near their mouths; those streams are distributaries.

A river that meets the sea or a large lake may form a delta at its mouth: a triangle-shaped area of sediment with distributaries flowing across it.

The area of water around a river mouth where seawater mixes with freshwater is called an estuary.

The land around a stream is a valley. Valleys come in all sizes and have a variety of names, just like streams. The smallest streams, rills, run in tiny channels also called rills. Rivulets and runnels run in gullies. Brooks and creeks run in washes or ravines or arroyos or gulches as well as small valleys with other names.

Rivers (large streams) have proper valleys, which may range from canyons to enormous flat lands like the Mississippi River Valley. The larger, deeper valleys are usually v-shaped. The depth and steepness of a river valley depends on the size, slope and speed of the river as well as composition of the bedrock. 

Edited by Brooks Mitchell