Headwater in the context of "Columbia River"

In hydrology, a main stem or mainstem (also known as a trunk) is "the primary downstream segment of a river, as contrasted to its tributaries". The mainstem extends all the way from one specific headwater to the outlet of the river, although there are multiple ways to determine which headwater (or first-order tributary) is the source of the mainstem. Water enters the mainstem from the river's drainage basin, the land area through which the mainstem and its tributaries flow. A drainage basin may also be referred to as a watershed or catchment.

Hydrological classification systems assign numbers to tributaries and mainstems within a drainage basin. In the Strahler number, a modification of a system devised by Robert E. Horton in 1945, channels with no tributaries are called "first-order" streams. When two first-order streams meet, they are said to form a second-order stream; when two second-order streams meet, they form a third-order stream, and so on. In the Horton system, the entire mainstem of a drainage basin was assigned the highest number in that basin. However, in the Strahler system, adopted in 1957, only that part of the mainstem below the tributary of the next highest rank gets the highest number.

The Sava is a river in Central and Southeast Europe, a right-bank and the longest tributary of the Danube. From its source in Slovenia it flows through Croatia and along its border with Bosnia and Herzegovina, and finally reaches Serbia, feeding into the Danube in its capital, Belgrade.

The Sava is 990 kilometres (615 miles) long, including the 45-kilometre (28 mi) Sava Dolinka headwater rising in Zelenci, Slovenia. It is the largest tributary of the Danube by volume of water, and the second-largest after the Tisza in terms of catchment area (97713km²) and length. It drains a significant portion of the Dinaric Alps region, through the major tributaries of Drina, Bosna, Kupa, Una, Vrbas, Lonja, Kolubara, Bosut and Krka. The Sava is one of the longest rivers in Europe and among the longest tributaries of another river.

The Mekong or Mekong River (UK: /miːˈkɒŋ/ mee-KONG, US: /ˌmeɪˈkɔːŋ/ may-KAWNG) is a transboundary river in East Asia and Southeast Asia. It is the world's twelfth-longest river and the third-longest in Asia with an estimated length of 4,909 km (3,050 mi) and a drainage area of 795,000 km (307,000 sq mi), discharging 475 km (114 cu mi) of water annually. From its headwaters in the Tibetan Plateau, the river runs through Southwest China (where it is officially called the Lancang River), Myanmar, Laos, Thailand, Cambodia, and southern Vietnam. The extreme seasonal variations in flow and the presence of rapids and waterfalls in the Mekong make navigation difficult, though the river remains a major trade route between Tibet and Southeast Asia. The construction of hydroelectric dams along the Mekong in the 2000s through the 2020s has caused serious problems for the river's ecosystem, including the exacerbation of drought.

The Werra (German pronunciation: [ˈvɛʁa] ), a river in central Germany, is the right-bank headwater of the Weser. "Weser" is a synonym in an old dialect of German. The Werra has its source near Eisfeld in southern Thuringia. After 293 kilometres (182 mi) the Werra joins the river Fulda in the town of Hann. Münden, forming the Weser. If the Werra is included as part of the Weser, the Weser is the longest river entirely within German territory at 744 kilometres (462 mi).

Its valley, the Werratal, has many tributaries and is a relative lull between the Rhön Mountains and the Thuringian Forest.

The Someșul Mic (Little Someș, Hungarian: Kis-Szamos) is a river in north-western Romania (Cluj County). At its confluence with the Someșul Mare in Mica, the Someș is formed. Its total length is 178 km (111 mi), and its drainage basin area is 3,773 km (1,457 sq mi). It is formed at the confluence of two headwaters, Someșul Cald ("Warm Someș") and Someșul Rece ("Cold Someș"), that originate in the Apuseni Mountains. From the confluence, in Gilău, the Someșul Mic flows east and north through Cluj-Napoca, Apahida and Gherla, until it meets the Someșul Mare in Dej.

Lake Itasca (/aɪˈtæskə/ eye-TASS-kə) is a small glacial lake, approximately 4.7 square kilometres (470 hectares; 1,200 acres) in area. It is located in Itasca State Park, in south-eastern Clearwater County, in the Headwaters area of north-central Minnesota, and is notable for being the headwater of the Mississippi River. It has an average depth of 20 to 35 feet (5 to 10 m) and is 1,475 feet (450 m) above sea level.

The Ojibwe name for the lake is Omashkoozo-zaaga'igan (Elk Lake). The first European explorers in the area were French Canadian and they named the lake Lac Labiche (Doe Lake, often mistranslated as Elk Lake). The name was later changed by Henry Schoolcraft to "Itasca", coined from a combination of the Latin phrase veritas caput ("true head [of the Mississippi]"). It is one of several examples of pseudo-Native American place names created by Schoolcraft.

Trout (pl.: trout) is a generic common name for numerous species of carnivorous freshwater fishes belonging to the genera Oncorhynchus, Salmo, and Salvelinus, all of which are members of the subfamily Salmoninae in the family Salmonidae. The word trout is also used for some similar-shaped but non-salmonid fish, such as the spotted seatrout/speckled trout (Cynoscion nebulosus, which is actually a croaker).

Trout are closely related to salmon and have similar migratory life cycles. Most trout are strictly potamodromous, spending their entire lives exclusively in freshwater lakes, rivers and wetlands and migrating upstream to spawn in the shallow gravel beds of smaller headwater creeks. The hatched fry and juvenile trout, known as alevin and parr, will stay upstream growing for years before migrating down to larger waterbodies as maturing adults. There are some anadromous species of trout, such as the steelhead (a coastal subspecies of rainbow trout) and sea trout (the sea-run subspecies of brown trout), that can spend up to three years of their adult lives at sea before returning to freshwater streams for spawning, in the same fashion as a salmon run. Brook trout and three other extant species of North American trout, despite the names, are actually char (or charr), which are salmonids also closely related to trout and salmon.

River ecosystems are flowing waters that drain the landscape, and include the biotic (living) interactions amongst plants, animals and micro-organisms, as well as abiotic (nonliving) physical and chemical interactions of its many parts. River ecosystems are part of larger watershed networks or catchments, where smaller headwater streams drain into mid-size streams, which progressively drain into larger river networks. The major zones in river ecosystems are determined by the river bed's gradient or by the velocity of the current. Faster moving turbulent water typically contains greater concentrations of dissolved oxygen, which supports greater biodiversity than the slow-moving water of pools. These distinctions form the basis for the division of rivers into upland and lowland rivers.

The food base of streams within riparian forests is mostly derived from the trees, but wider streams and those that lack a canopy derive the majority of their food base from algae. Anadromous fish are also an important source of nutrients. Environmental threats to rivers include loss of water, dams, chemical pollution and introduced species. A dam produces negative effects that continue down the watershed. The most important negative effects are the reduction of spring flooding, which damages wetlands, and the retention of sediment, which leads to the loss of deltaic wetlands.