STREAMS - A stream can 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. 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.

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Streams

Stream Terminology and Definitions

by Andrew Alden  

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.

Characteristics of Streams

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.

Understanding Stream Order

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.

Land Around a Stream

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

Andrew Alden

·   Professional geologist, writer, photographer, and geological tour guide

·   Thirty-seven years of experience writing about geological subjects

·   Six years as a research guide with U.S. Geological Survey (USGS)

·   Member of Geological Society of America (GSA), and American Geophysical Union (AGU)

Experience

Andrew Alden is a former writer for ThoughtCo who contributed hundreds of articles for more than 17 years. Andrew works as a geologist, writer, editor, and photographer. He has written on geological subjects since 1981 and participates actively in his field. For example, Andrew spent six years as a research guide with the U.S. Geological Survey, leading excursions on both land land and at sea. And since 1992, he has hosted the earthquakes conference for the online discussion platform, The Well, which began as a dialogue between the writers and readers of the Whole Earth Review. 

In addition, Andrew is a longtime member of the member of the Geological Society of America — an international society that serves members in academia, government, and industry; and the American Geophysical Union — a community of earth and space scientists that advances the power of science to ensure a sustainable future.

Andrew lives in Oakland, California; and though he writes about the whole planet and beyond, Andrew finds his own city full of interest too and blogs about its geology. 

Education

Andrew Alden holds a bachelor's (B.A.) degree in Earth Science from the University of New Hampshire, College of Engineering and Physical Sciences, in Durham, N.H.

Awards and Publications

·   Andrew Alden on Earthquakes ​(The Well Group, Inc., 2011)

·   Assessment of River — Floodplain Aquifer Interactions (Environmental and Engineering Geoscience, 1997)

·   Andrew Alden on Hosting (The Well Group, Inc., 1995)

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