The stream order hierarchy appeared in 1952, proposed in Arthur Newell Strahler's article "Hypsometric Analysis of Erosional Topology" in the Geological Society of America Bulletin. It organizes streams by size into twelve orders. Those orders can be expressed generically as Strahler's numbers, and are applicable in any field dealing with dendritic shapes. Examples include respiratory systems, trees, and social networks.
Two first order streams combine to make a second order stream; two second order streams make a third order stream, and so on. Streams of different orders do not combine into a larger order; the smaller stream merely terminates.
Strahler's numbers expressed on a network of streams might look something like this:
---1---\ \ ___3_/
\ \ / /
---1------2--\ 1 / /
---1-----2---/ / \
---1---/ 2-----/ 2
Some eighty percent of streams are third order and lower, originating high in the landscape and roiling over steep terrain. Streams that are seventh order and higher are classified as rivers. As of March 2015, the Amazon River is the world's only twelveth order stream.
A river's middle is known as a channel, while the more placid outside is known as the floodplain. This can grow to significant size on flatter land, or during much precipitation. Rivers nearly always end at another river, or at the ocean, in a brackish flaring-out known as a mouth. They are otherwise freshwater. The area of ground that drains to a specific river is that river's watershed. The Mississippi River's watershed, for example, includes the eastern half of the Rocky Mountains.
Rivers embody the disparate sources of first-order streams: runoff, upwellings of groundwater, springs, snow. Their speed and size directly influence the types and numbers of life-forms at their edges. At their mouths, humans living in ostensibly different worlds found enough mineral nutrient in the silt to support the building of armies. Underground rivers are usually surrounded by limestone.
Rivers follow a path determined by gravity and the robustness of substrate. While it is conceivable that the Coriolis force would influence which bank of a river erodes more quickly, it does not. A river's overall shape is largely a function of whether the river is alluvial or bedrock.
Alluvial rivers build from a deepening cut in soft earth and are more likely to meander; alluvium refers to sediment. Bedrock rivers, meanwhile, follow the usually-straighter paths of fractured stone. Of course, rivers can alternate between alluvial and bedrock, and most bedrock sections of river have alluvium on their beds. Being more variable, the shapes of alluvial rivers occupy more classifications: anastomose, braided, meandering, straight, or wandering. In the beginning of the twentieth century William Morris Davis proposed the "cycle of erosion" system of classifying rivers by age, using mistaken and largely anecdotal criteria.
A river's hyporheic zone is its surrounding substrate, through which water also flows. It is biologically productive enough that wells drilled near rivers often yield invertebrates. Less consolidated substrate allows for a larger hyporheic zone.
The aforementioned meander--or bend--is usually the result of a widening channel. The decreased flow-rate allows for the accumulation of silt at the center; as outside banks erode and inside banks accumulate, the meander grows more pronounced, eventually becoming a loop. Silt quickly cuts off the loop, making it a temporary body known as an oxbow lake. Large, old floodplains often bear fluorishing meander scars.
River ecosystems are lotic, from the Latin lotus, for "washed." The main characteristic of lotic systems is the unidirectional flow of water; other lotic ecosystems include other streams. Lentic ecosystems, meanwhile, coalesce around still water. The oceans are marine ecosystems.
Lotic ecosystems experience much variability, both within a single system and between different systems. They are often understood in terms of energy inputs and the functions of collected life forms, which tend to be broadly similar between locations. Most streams are exposed to light, receive and distribute sediment and biotic material, have temperature(s), and experience chemistry. Likewise, the animals in streams can be near-uniformly categorized as microbes, collectors, shredders, grazers, or predators. These categories are a function of the material acted upon by whichever being, and their balance changes with that of the available materials in different waterways. As in all aquatic ecosystems, algae are the most important primary producers.
The terms Riparian corridor or riparian area refer to the area along a stream's banks occupied by specialized plants and animals. The Colorado River, for example, is bordered by (among other things) willows, which require damp roots and hang curtains of fine leaf into the current. The riparian corridor is the most direct contributor of biotic material and the prime indicator of a river's health.
Robin Vannote, first director of the Stroud Water Research Center, introduced the River Continuum Concept in the early 1970s. Simply, it is the assertion that producer and consumer communities develop in harmony with available materials, within physical conditions. Such knowledge is useful in the restoration of destroyed ecosystems. Smaller or headwater streams, for example, experience much influence from the riparian corridor and tend to be dominated by shredders and collectors, which depend on coarse organic matter (like leaves). Mid-sized rivers, being more exposed to light, tend to host more algae-consuming grazers and piscivores; larger rivers, being host to light, silt, and much-processed organic material, are dominated by grazers and collectors. Predators live everywhere.
Chamisa Freeshell. "How a River Flows" and "Basics of Stream Ecology."
Nezette Rydell. "Re: What determines the direction of river flow? Elevation, Topography,Gravity??"
Wikipedia. "River" and "River Ecosystem."
About.com. "Stream Order."
Oxbow River and Stream Inc. "Riparian: River Continuum Concept."
Wheeling Creek Watershed. "River as a Continuum."
State of Washington Department of Ecology. "River and Stream Monitering Water Quality Index."