Most of us speak of rivers, but geoscientists tend to call everything a stream. A stream is any body of running water that occupies a channel: it can even be underground in a cave or underneath a glacier. A river is a large surface stream, but other than that there isn't a clear boundary between a stream and a river. 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 intermittentoccurring 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 stream networks is stream order. 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. The largest river on Earth, the Amazon, is a twelfth-order stream. Together, the first- through third-order tributaries making up the source of a river are known as its headwaters. 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.