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56-507: The Tahltan River's watershed covers 1,851 km (715 sq mi), and its mean annual discharge is 21.5 m/s (760 cu ft/s). The mouth of the Tahltan River is located about 15 km (9.3 mi) northeast of Telegraph Creek , British Columbia, about 200 km (120 mi) east of Juneau, Alaska , and about 375 km (233 mi) southeast of Whitehorse , Yukon. The Tahltan River's watershed's land cover

112-426: A "raw" DEM, or analysts can download hydrologically-conditioned DEMs such as the near-global HydroSHEDS, MERIT-Hydro, or EDNA for the continental United States. The usual steps for hydrologic conditioning of a DEM are: Additionally, some methods allow for "fencing ridgelines" and burning in flow pathways through lakes. Some methods also enforce a small slope onto flat areas so that flow will continue to move toward

168-426: A DEM. The first method, introduced by Australian geographers O'Callaghan and Mark in 1984, is referred to as D8. Water flows from a pixel to one of 8 possible directions to a neighboring cell (including diagonally), based on the direction of steepest slope. There are disadvantages to this method as water flow is limited to 8 directions, separated by 45°, which may result in unrealistic flow patterns. Also, because all of

224-454: A drainage basin, and there are different ways to interpret that data. In the unlikely event that the gauges are many and evenly distributed over an area of uniform precipitation, using the arithmetic mean method will give good results. In the Thiessen polygon method, the drainage basin is divided into polygons with the rain gauge in the middle of each polygon assumed to be representative for

280-540: A drainage boundary is referred to as watershed delineation . Finding the area and extent of a drainage basin is an important step in many areas of science and engineering. Most of the water that discharges from the basin outlet originated as precipitation falling on the basin. A portion of the water that enters the groundwater system beneath the drainage basin may flow towards the outlet of another drainage basin because groundwater flow directions do not always match those of their overlying drainage network. Measurement of

336-403: A fundamental geographic unit in hydrology , the science concerned with the movement, distribution, and management of water on Earth. Delineating watersheds may be considered an application of hydrography , the branch of applied sciences which deals with the measurement and description of the physical features of oceans, seas, coastal areas, lakes and rivers. It is also related to geomorphometry ,

392-495: A number of vector datasets representing watersheds as polygons that can be displayed and analyzed with GIS or other software. In these datasets, the entire land surface is divided into "subwatersheds" or "unit catchments." Individual unit watersheds can be combined or merged to find larger watersheds. The unit catchments have linked hydrological code data or similar metadata to create a flow network , so flow pathways and connections can be determined via network analysis. This list

448-426: A point where surface water is lost underground . Drainage basins are similar but not identical to hydrologic units , which are drainage areas delineated so as to nest into a multi-level hierarchical drainage system . Hydrologic units are defined to allow multiple inlets, outlets, or sinks. In a strict sense, all drainage basins are hydrologic units but not all hydrologic units are drainage basins. About 48.71% of

504-428: A succession of elevated features, such as ridges and hills . A basin may consist of smaller basins that merge at river confluences , forming a hierarchical pattern . Other terms for a drainage basin are catchment area , catchment basin , drainage area , river basin , water basin , and impluvium . In North America, they are commonly called a watershed , though in other English-speaking places, "watershed"

560-446: A topographic map, for example to identify ridges, valleys, and the direction of steepest slope. Even in the computer era, manual watershed delineation is still a useful skill, in order to check whether watersheds generated with software are correct. Instructions for manual watershed delineation can be found in some textbooks in geography or environmental management, in government pamphlets, or in online video tutorials. According to

616-493: A transparent overlay. The watershed area can then be estimated using a planimeter , by overlaying graph paper and counting grid cells, or the result can be digitized for use with mapping software. The same process can be done on a computer, sketching the watershed boundary (with a mouse or stylus) over a digital copy of a topographic map. This is referred to as "heads up digitizing" or "on-screen digitizing." For "manual" watershed delination, one must know how to read and interpret

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672-475: A watershed, also referred to as a catchment, drainage basin , or river basin. It is an important step in many areas of environmental science, engineering, and management, for example to study flooding, aquatic habitat, or water pollution. The activity of watershed delineation is typically performed by geographers, scientists, and engineers. Historically, watershed delineation was done by hand on paper topographic maps , sometimes supplemented with field research. In

728-405: Is a tradeoff, as a finer grid with more pixels increases computing time. Nevertheless, even high-resolution data may not adequately capture flow pathways in complex environments like cities and suburbs, where flow is directed by curbs, culverts, and storm drains. Finally, some errors can result from the algorithm or the choice of parameters. Because errors are common, some authorities insist that

784-708: Is classified as 35.0% conifer forest , 29.6% shrubland , 14.0% barren , 9.1% herbaceous , 8.5% mixed forest , and small amounts of other cover. The Tahltan River is named for the Tahltan people and is in their traditional territory. The Tahltan River originates in high glaciated peaks on the east edge of the Boundary Ranges and the west edge of the Tahltan Highland. It first flows south and southeast then east and northeast, gathering many unnamed tributary streams. After passing north of Tahltan Lake it

840-621: Is joined by Johnny Tashoots Creeks, which flows from Tahltan Lake. The Tahltan River is then joined by Harper Reed Creek and Lovell Creek, both flowing from the north, then Tutesheta Creek, flowing from the south. After flowing north briefly the Tahltan River is joined by its main tributary, the Little Tahltan River near the Indian reserve of Tahltan Forks 5, of the Tahltan First Nation . Turning east and southeast,

896-434: Is not always correct. Some errors stem from incorrectly placing the watershed outlet on the digital river network, or "snapping the pour point." Another class of errors stems from inaccuracies in the digital terrain data, or where its resolution is too coarse to capture flow pathways. In general, DEMs with higher spatial resolution can more realistically describe topography of the land surface and flow direction. However, there

952-426: Is possible to use DEMs in different formats for watershed delineation, such as a Triangular Irregular Network (TIN), or Hexagonal tiling however most contemporary algorithms make use of a regular rectangular grid. In the 1980s and 1990s, digital elevation models were often obtained by scanning and digitizing the contours on paper topographic maps, which were then converted to a TIN or a gridded DEM. More recently,

1008-487: Is referred to as " watershed management ". In Brazil , the National Policy of Water Resources, regulated by Act n° 9.433 of 1997, establishes the drainage basin as the territorial division of Brazilian water management. When a river basin crosses at least one political border, either a border within a nation or an international boundary, it is identified as a transboundary river . Management of such basins becomes

1064-614: Is the Dead Sea . Drainage basins have been historically important for determining territorial boundaries, particularly in regions where trade by water has been important. For example, the English crown gave the Hudson's Bay Company a monopoly on the fur trade in the entire Hudson Bay basin, an area called Rupert's Land . Bioregional political organization today includes agreements of states (e.g., international treaties and, within

1120-404: Is used only in its original sense, that of the drainage divide line. A drainage basin's boundaries are determined by watershed delineation , a common task in environmental engineering and science. In a closed drainage basin, or endorheic basin , rather than flowing to the ocean, water converges toward the interior of the basin, known as a sink , which may be a permanent lake, a dry lake , or

1176-589: The 1980s, automated methods were developed for watershed delineation with computers and electronic data, and these are now in widespread use. Computerized methods for watershed delineation use digital elevation models (DEMs), datasets that represent the height of the Earth's land surface. Computerized watershed delineation may be done using specialized hydrologic modeling software such as WMS , geographic information system software like ArcGIS or QGIS , or with programming languages like Python or R . Watersheds are

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1232-713: The African Great Lakes , the interiors of Australia and the Arabian Peninsula , and parts in Mexico and the Andes . Some of these, such as the Great Basin, are not single drainage basins but collections of separate, adjacent closed basins. In endorheic bodies of water where evaporation is the primary means of water loss, the water is typically more saline than the oceans. An extreme example of this

1288-828: The Continental Divide , northern Alaska and parts of North Dakota , South Dakota , Minnesota , and Montana in the United States, the north shore of the Scandinavian peninsula in Europe, central and northern Russia, and parts of Kazakhstan and Mongolia in Asia , which totals to about 17% of the world's land. Just over 13% of the land in the world drains to the Pacific Ocean . Its basin includes much of China, eastern and southeastern Russia, Japan,

1344-714: The Korean Peninsula , most of Indochina, Indonesia and Malaysia, the Philippines, all of the Pacific Islands , the northeast coast of Australia , and Canada and the United States west of the Continental Divide (including most of Alaska), as well as western Central America and South America west of the Andes. The Indian Ocean 's drainage basin also comprises about 13% of Earth's land. It drains

1400-403: The groundwater . A drainage basin is an area of land where all flowing surface water converges to a single point, such as a river mouth, or flows into another body of water, such as a lake or ocean. Watershed delineation For watershed segmentation , a technique for image processing, see Watershed (image processing) . Watershed delineation is the process of identifying the boundary of

1456-433: The 1980s, and are now in widespread use in the science and engineering communities. Researchers have even used computer methods to delineate watersheds on Mars. Automated watershed delineation methods use digital data of the earth's elevation, a Digital Elevation Model , or DEM. Typically, algorithms use the method of "steepest slope" to calculate the flow direction from a grid cell (or pixel) to one of its neighbors. It

1512-456: The DEM is obtained by aerial or satellite remote sensing , using stereophotogrammetry , lidar , or radar . To use a rectangular grid DEM for watershed delineation, it must first be processed or "conditioned" in order to return realistic results. The result is sometimes referred to as a "hydro-enforced" DEM or a "HydroDEM." Most of the software packages listed below can perform these functions on

1568-566: The Little Tahltan confluence, sometimes acts as a barrier to fish migration. In 2014 a large rock slide partially filled in the Tahltan River about 1.1 km (0.68 mi) above the Stikine River confluence. This slide created a partial barrier to salmon migration. In 2018 large scale remediation efforts were completed, widening the river's channel and distributing rock debris downstream. This has improved fish migration although

1624-623: The Tahltan River is joined by numerous tributaries including Bear Creek , Beatty Creek , Newell Creek, Middle Creek , and Hartz Creek . In its last few kilometres the Tahltan River flows south to join the Stikine River in the Grand Canyon of the Stikine , at the locality of Tahltan and the Tahltan Indian Reserve Tahltan 1. The historic Yukon Telegraph Trail runs through the Tahltan River's watershed, following

1680-622: The Tutesheta Creek tributary and crossing the Tahltan River near the Little Tahltan River confluence. The Tahltan River is a major contributor to the Stikine River's salmon runs and the most significant contributor of Chinook salmon , sockeye salmon , and other salmonids like steelhead trout . Spawning habitat is found throughout the Tahltan's watershed. Decheeka Falls, located about 3 km (1.9 mi) upstream of

1736-677: The US Geological Survey, there are 5 steps to manual watershed delineation: General Rules: One disadvantage to manual watershed delineation is that it is subject to errors and the individual judgment of the analyst. The Illinois Environmental Protection Agency wrote, "bear in mind that delineating a watershed is an inexact science. Any two people, even if both are experts, will come up with slightly different boundaries." Especially for smaller watersheds and when accurate results are important, field reconnaissance may be needed to find features that are not shown on maps. "Going out into

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1792-623: The US, interstate compacts ) or other political entities in a particular drainage basin to manage the body or bodies of water into which it drains. Examples of such interstate compacts are the Great Lakes Commission and the Tahoe Regional Planning Agency . In hydrology , the drainage basin is a logical unit of focus for studying the movement of water within the hydrological cycle . The process of finding

1848-406: The basin, it can form tributaries that change the structure of the land. There are three different main types, which are affected by the rocks and ground underneath. Rock that is quick to erode forms dendritic patterns, and these are seen most often. The two other types of patterns that form are trellis patterns and rectangular patterns. Rain gauge data is used to measure total precipitation over

1904-430: The discharge of water from a basin may be made by a stream gauge located at the basin's outlet. Depending on the conditions of the drainage basin, as rainfall occurs some of it seeps directly into the ground. This water will either remain underground, slowly making its way downhill and eventually reaching the basin, or it will permeate deeper into the soil and consolidate into groundwater aquifers. As water flows through

1960-441: The drainage area is dependent on the soil type. Certain soil types such as sandy soils are very free-draining, and rainfall on sandy soil is likely to be absorbed by the ground. However, soils containing clay can be almost impermeable and therefore rainfall on clay soils will run off and contribute to flood volumes. After prolonged rainfall even free-draining soils can become saturated , meaning that any further rainfall will reach

2016-697: The drainage basin to the mouth, and may accumulate there, disturbing the natural mineral balance. This can cause eutrophication where plant growth is accelerated by the additional material. Because drainage basins are coherent entities in a hydrological sense, it has become common to manage water resources on the basis of individual basins. In the U.S. state of Minnesota , governmental entities that perform this function are called " watershed districts ". In New Zealand, they are called catchment boards. Comparable community groups based in Ontario, Canada, are called conservation authorities . In North America, this function

2072-805: The eastern coast of Africa, the coasts of the Red Sea and the Persian Gulf , the Indian subcontinent , Burma, and most parts of Australia . The five largest river basins (by area), from largest to smallest, are those of the Amazon (7 million km ), the Congo (4 million km ), the Nile (3.4 million km ), the Mississippi (3.22 million km ), and the Río de la Plata (3.17 million km ). The three rivers that drain

2128-416: The field allows you to identify human alterations, such as road ditches, storm sewers and culverts that could change the direction of waters flow and thus change the watershed boundaries." Using computer software to delineate watersheds can be much faster than manual methods. It may also be more consistent, as it removes analyst's subjectivity. Automatic methods of watershed delineation have been in use since

2184-958: The first watershed delineation software was written in FORTRAN, such as CATCH and DEDNM. Watershed delineation tools are a part of several Geographic Information System software packages such as ArcGIS , QGIS , and GRASS GIS . There are standalone programs for watershed delineation such as TauDEM. Watershed delineation tools are also incorporated into some hydrologic modeling software packages. Software developers have also published libraries or modules in several languages (see list below). Many of these packages are free and open source, which means they can be expanded or adapted by those willing and able to write or modify code. Finally, there are web applications for delineating watersheds. Some of these web apps have extra features for science and engineering like calculating flow statistics or watershed land cover types (e.g.: StreamStats, Model My Watershed). There are

2240-521: The flow is routed in one direction, the D8 method is unable to model situations where the flow diverges, such as on convex hillsides, in a river delta , or in branched or braided rivers . Alternative algorithms have been proposed and implemented to overcome this limitation, such as D∞. Nevertheless, the D8 algorithm remains in widespread use, and has been used to create important datasets such as HydroBasins and MERIT-Basins. Computerized watershed delineation

2296-583: The greatest portion of western Sub-Saharan Africa , as well as Western Sahara and part of Morocco . The two major mediterranean seas of the world also flow to the Atlantic. The Caribbean Sea and Gulf of Mexico basin includes most of the U.S. interior between the Appalachian and Rocky Mountains , a small part of the Canadian provinces of Alberta and Saskatchewan , eastern Central America ,

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2352-435: The ground and along rivers it can pick up nutrients , sediment , and pollutants . With the water, they are transported towards the outlet of the basin, and can affect the ecological processes along the way as well as in the receiving water body . Modern use of artificial fertilizers , containing nitrogen (as nitrates ), phosphorus , and potassium , has affected the mouths of drainage basins. The minerals are carried by

2408-781: The ground at its terminus, the area can go by several names, such playa, salt flat, dry lake , or alkali sink . The largest endorheic basins are in Central Asia , including the Caspian Sea , the Aral Sea , and numerous smaller lakes. Other endorheic regions include the Great Basin in the United States, much of the Sahara Desert , the drainage basin of the Okavango River ( Kalahari Basin ), highlands near

2464-639: The islands of the Caribbean and the Gulf, and a small part of northern South America. The Mediterranean Sea basin, with the Black Sea , includes much of North Africa , east-central Africa (through the Nile River ), Southern , Central, and Eastern Europe , Turkey , and the coastal areas of Israel , Lebanon , and Syria . The Arctic Ocean drains most of Western Canada and Northern Canada east of

2520-462: The most water, from most to least, are the Amazon, Ganges , and Congo rivers. Endorheic basin are inland basins that do not drain to an ocean. Endorheic basins cover around 18% of the Earth's land. Some endorheic basins drain to an Endorheic lake or Inland sea . Many of these lakes are ephemeral or vary dramatically in size depending on climate and inflow. If water evaporates or infiltrates into

2576-610: The outlet. The step of "burning in" stream channels involves artificially deepening the channel, by subtracting a large elevation value from pixels that represent the channel. This ensures that once flow has entered the channel, it will stay there rather than jumping out and flowing overland or into another channel. Some algorithms infer the location of channels automatically from the DEM. Better results are usually obtained by burning in mapped stream channels, or channels derived from satellite or aerial imagery. There are several different algorithms available for calculating flow direction from

2632-400: The quantitative science of analyzing land surfaces. Watershed delineation continues to be an active area of research, with scientists and programmers developing new algorithms and methods, and making use of increasingly high-resolution data from aerial or satellite remote sensing . The conventional method of finding a watershed boundary is to draw it by hand on a paper topographic map , or on

2688-418: The rainfall on the area of land included in its polygon. These polygons are made by drawing lines between gauges, then making perpendicular bisectors of those lines form the polygons. The isohyetal method involves contours of equal precipitation are drawn over the gauges on a map. Calculating the area between these curves and adding up the volume of water is time-consuming. Isochrone maps can be used to show

2744-631: The responsibility of the countries sharing it. Nile Basin Initiative , OMVS for Senegal River , Mekong River Commission are a few examples of arrangements involving management of shared river basins. Management of shared drainage basins is also seen as a way to build lasting peaceful relationships among countries. The catchment is the most significant factor determining the amount or likelihood of flooding . Catchment factors are: topography , shape, size, soil type, and land use (paved or roofed areas). Catchment topography and shape determine

2800-484: The results of automated delineation must be carefully checked. The US Geological Survey's standards for the US Watershed Boundary Dataset allow the use of software "to generate intermediate or “draft” boundary lines," which then must be verified by the analyst by overlaying them on a computer display over basemaps (scanned topographic maps, aerial photographs) to verify their accuracy. Some of

2856-461: The river rather than being absorbed by the ground. If the surface is impermeable the precipitation will create surface run-off which will lead to higher risk of flooding; if the ground is permeable, the precipitation will infiltrate the soil. Land use can contribute to the volume of water reaching the river, in a similar way to clay soils. For example, rainfall on roofs, pavements , and roads will be collected by rivers with almost no absorption into

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2912-401: The slide debris may still be a significant barrier under certain flow conditions. Drainage basin A drainage basin is an area of land in which all flowing surface water converges to a single point, such as a river mouth , or flows into another body of water , such as a lake or ocean . A basin is separated from adjacent basins by a perimeter, the drainage divide , made up of

2968-418: The speed with which the runoff reaches a river. A long thin catchment will take longer to drain than a circular catchment. Size will help determine the amount of water reaching the river, as the larger the catchment the greater the potential for flooding. It is also determined on the basis of length and width of the drainage basin. Soil type will help determine how much water reaches the river. The runoff from

3024-415: The time taken for rain to reach the river, while catchment size, soil type, and development determine the amount of water to reach the river. Generally, topography plays a big part in how fast runoff will reach a river. Rain that falls in steep mountainous areas will reach the primary river in the drainage basin faster than flat or lightly sloping areas (e.g., > 1% gradient). Shape will contribute to

3080-477: The time taken for runoff water within a drainage basin to reach a lake, reservoir or outlet, assuming constant and uniform effective rainfall. Drainage basins are the principal hydrologic unit considered in fluvial geomorphology . A drainage basin is the source for water and sediment that moves from higher elevation through the river system to lower elevations as they reshape the channel forms. Drainage basins are important in ecology . As water flows over

3136-673: The world's land drains to the Atlantic Ocean . In North America , surface water drains to the Atlantic via the Saint Lawrence River and Great Lakes basins, the Eastern Seaboard of the United States, the Canadian Maritimes , and most of Newfoundland and Labrador . Nearly all of South America east of the Andes also drains to the Atlantic, as does most of Western and Central Europe and

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