Hydrological code
Encyclopedia
A hydrological code or hydrologic unit code is a sequence of numbers or letters that identify a hydrological feature like a river
, river reach
, lake
, or area like a drainage basin
(also called watershed or catchment).
One system, developed by Strahler, known as the Strahler stream order
, ranks streams based on a hierarchy of tributaries. Each segment of a stream or river within a river network is treated as a node in a tree, with the next segment downstream as its parent. When two first-order streams come together, they form a second-order stream. When two second-order streams come together, they form a third-order stream, and so on.
Another example is the system of assigning IDs to watersheds devised by Otto Pfafstetter, known as the Pfafstetter Coding System or the Pfafstetter System. Drainage areas are delineated in a hierarchical fashion, with "level 1" watersheds at continental scales, subdivided into smaller level 2 watersheds, which are divided into level 3 watersheds, and so on. Each watershed is assigned a unique number, called a Pfafsetter Code, based on its location within the overall drainage system.
receive a higher number. The seas are limited using the so-called definitions made by the International Hydrographic Organization
in 1953. The coasts of these seas are defined clockwise from north west to south east from the strait where the sea connects to the ocean or the other seas.
Subsequently every watershed along this coast is assigned a number using the Pfafstetter System. This implies that the four largest watersheds are selected and receive numbers 2,4,6, or 8. The watersheds in between the large systems receive numbers 3, 5, and 7. Numbers 1 and 9 are used for the small watersheds on the edges of the strait. The smaller systems can subsequently be numbered recursively or kept together for grouping purpose.
created a hierarchical system of hydrologic units originally called regions, sub-regions, accounting units, and cataloging units. Each unit was assigned a unique Hydrologic Unit Code (HUC). As first implemented the system had 21 regions, 221 subregions, 378 accounting units, and 2,264 cataloging units. Over time the system was changed and expanded. As of 2010 there are six levels in the hierarchy, represented by hydrologic unit codes from 2 to 12 digits long, called regions, subregions, basins, subbasins, watersheds, and subwatersheds. The table below describes the system's hydrologic unit levels and their characteristics, along with example names and codes.
The original delineation of units, down to subbasins (cataloging units), was done using 1:250,000 scale
maps and data. The newer delineation work on watersheds and subwatersheds was done using 1:24,000 scale maps and data. As a result, the subbasin boundaries were changed and adjusted in order to conform to the higher resolution watersheds within them. Changes to subbasin boundaries resulted in changes in area sizes. Therefore, older data using "cataloging units" may differ from newer, higher resolution data using "subbasins".
The regions (1st level hydrologic units) are geographic areas that contain either the drainage area
of a major river, such as the Missouri region, or the combined drainage areas of a series of rivers, such as the Texas–Gulf region. Each subregion includes the area drained by a river system, a reach
of a river and its tributaries in that reach, a closed basin or basins, or a group of streams forming a coastal drainage area. Regions receive a two-digit code. The following levels are designated by the addition of another two digits. The hierarchy was designed and the units subdivided so that almost all the subbasins (formerly called cataloging units) are larger than 700 square miles (1,813 km²). Larger closed basins were subdivided until their subunits were less than 700 square miles. The 10-digit watersheds were delineated to be between 40,000 and 250,000 acres in size, and the 12-digit subwatersheds between 10,000 and 40,000 acres. In addition to the hydrologic unit codes, each hydrologic unit was assigned a name corresponding to the unit's principal hydrologic feature or to a cultural or political feature within the unit.
The boundaries of the hydrologic units usually correspond to drainage basins with some exceptions; for example, subregion 1711, called "Puget Sound", includes all U.S. drainage into not only Puget Sound but also the Strait of Georgia
, Strait of Juan de Fuca
, and the Fraser River
. Also, region and subregion boundaries end at the U.S. international boundary.
In general, hydrologic units were delineated such that all surface drainage within each unit converges at a single outlet point—a type of hydrologic unit called a "classic hydrological unit". It was not always possible to delineated units in this way while adhering to the size and subdivision standards of the system. There are several "non-classic" types of drainage areas, each requiring special criteria for delineation and subdivision.
"Remnant areas" occur along coasts where individual streams are too small for the given subdivision type. Such remnants were combined into a single unit if they were adjacent and could be combined. These "composite" units are called "frontal units". They are non-classic because they have more than one outlet.
For example, the coastal area along Puget Sound
between Seattle and Mukilteo
, is delineated at the finest "subwatershed" level as "Shell Creek-Frontal Puget Sound", HUC 171100190203. This hydrologic unit includes numerous small streams that drain directly to Puget Sound, including Pipers Creek
and Boeing Creek
. As a consequence of the smallest "subwatershed" being non-classic, every higher level unit containing it are also non-classic "frontal" units—"Lunds Gulch-Frontal Puget Sound" (HUC 17110019), "Puget Sound" (HUC 171100 and 1711), and "Pacific Northwest Region" (HUC 17).
"Noncontributing areas" are drainage areas within a hydrologic unit that do not drain to the unit's outlet. They can be caused by such things as potholes and kettles, closed basins, playas
, and cirque
s. If a noncontributing area is large enough, it was designated as a hydrologic unit of its own. The largest such example is the Great Basin
, designated a hydrologic unit the Region level. When a noncontributing area was not large enough to be designated a hydrologic unit, it was merged into the surrounding or bordering larger hydrologic unit. Special decisions were required for "semiconfined basins" that contribute flow to another area in wet years but become noncontributing in dry years—Goose Lake
, for example. The USGS instructed the people doing the delineation work to take extra care in the case of semiconfined basins and to seek assistance from others, but to ultimately make their own decision on whether the semiconfined basin should be designated a noncontributing area or not. Another special case occurs when noncontributing areas very small and dispersed, or scattered throughout a drainage area. These were considered part of the encompassing hydrologic unit. In short, noncontributing areas cannot be subject to strict criteria for delineating, and methods vary from state to state, landform type to type, and special cases. The effect of noncontributing areas on specific hydrologic units is explained in metadata
as best it can.
The Goose Lake example illustrates how USGS hydrologic units do not always conform strictly to drainage basins. Despite being part of the Upper Sacramento River basin (or accounting unit), HUC 180200, and the Sacramento River subregion, HUC 1802, the Goose Lake subbasin (or cataloging unit), HUC 18020001, was defined as a closed basin during the watershed and subwatershed delineation process. Therefore, the area of the Sacramento River subregion and the Upper Sacramento River basin, as published by the USGS (27600 sq mi (71,483.7 km²) and 7650 sq mi (19,813.4 km²) respectively), are too large by at least the size of the Goose Lake subbasin/cataloging unit, 1080 sq mi (2,797.2 km²).
Other non-classic drainage issues that have an effect on hydrologic unit delineation and subdivision include reservoir
s, diverted waters ranging from small irrigation ditch
es to interbasin transfer
s, islands, and coastal areas with large tidal range
s. The 5th and 6th level hydrologic units, called "watersheds" and "subwatersheds", were assigned one of seven attribute codes to indicate drainage type: standard (classic, one outlet), closed basin (no outlet), frontal (multiple outlets), water (predominately water with adjacent land areas), island (one or more islands and adjacent water), and unclassified (an area that cannot be defined or does not fit one of the other types).
River
A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...
, river reach
Reach (geography)
A reach in geography can mean several things. Most generally, a reach is any length of a stream between any two points. The points may be selected for any reason, such as gauging stations, river miles, natural features, and topography. They may be arbitrary or vague.A reach may also be an expanse,...
, lake
Lake
A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...
, or area like a drainage basin
Drainage basin
A drainage basin is an extent or an area of land where surface water from rain and melting snow or ice converges to a single point, usually the exit of the basin, where the waters join another waterbody, such as a river, lake, reservoir, estuary, wetland, sea, or ocean...
(also called watershed or catchment).
One system, developed by Strahler, known as the Strahler stream order
Strahler number
In mathematics, the Strahler number or Horton–Strahler number of a mathematical tree is a numerical measure of its branching complexity....
, ranks streams based on a hierarchy of tributaries. Each segment of a stream or river within a river network is treated as a node in a tree, with the next segment downstream as its parent. When two first-order streams come together, they form a second-order stream. When two second-order streams come together, they form a third-order stream, and so on.
Another example is the system of assigning IDs to watersheds devised by Otto Pfafstetter, known as the Pfafstetter Coding System or the Pfafstetter System. Drainage areas are delineated in a hierarchical fashion, with "level 1" watersheds at continental scales, subdivided into smaller level 2 watersheds, which are divided into level 3 watersheds, and so on. Each watershed is assigned a unique number, called a Pfafsetter Code, based on its location within the overall drainage system.
Europe
A comprehensive coding system is in use in Europe. This system codes from the ocean to the so-called primary catchment. The system determines a set of oceans of endorheic systems identified by a letter. The systems are subdivided by seas. The seas are numbered 1 to 9. Seas lying far from the ocean, for example the Black SeaBlack Sea
The Black Sea is bounded by Europe, Anatolia and the Caucasus and is ultimately connected to the Atlantic Ocean via the Mediterranean and the Aegean seas and various straits. The Bosphorus strait connects it to the Sea of Marmara, and the strait of the Dardanelles connects that sea to the Aegean...
receive a higher number. The seas are limited using the so-called definitions made by the International Hydrographic Organization
International Hydrographic Organization
The International Hydrographic Organization is the inter-governmental organisation representing the hydrographic community. It enjoys observer status at the UN and is the recognised competent authority on hydrographic surveying and nautical charting...
in 1953. The coasts of these seas are defined clockwise from north west to south east from the strait where the sea connects to the ocean or the other seas.
Subsequently every watershed along this coast is assigned a number using the Pfafstetter System. This implies that the four largest watersheds are selected and receive numbers 2,4,6, or 8. The watersheds in between the large systems receive numbers 3, 5, and 7. Numbers 1 and 9 are used for the small watersheds on the edges of the strait. The smaller systems can subsequently be numbered recursively or kept together for grouping purpose.
United States
The United States Geological SurveyUnited States Geological Survey
The United States Geological Survey is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, and the natural hazards that threaten it. The organization has four major science disciplines, concerning biology,...
created a hierarchical system of hydrologic units originally called regions, sub-regions, accounting units, and cataloging units. Each unit was assigned a unique Hydrologic Unit Code (HUC). As first implemented the system had 21 regions, 221 subregions, 378 accounting units, and 2,264 cataloging units. Over time the system was changed and expanded. As of 2010 there are six levels in the hierarchy, represented by hydrologic unit codes from 2 to 12 digits long, called regions, subregions, basins, subbasins, watersheds, and subwatersheds. The table below describes the system's hydrologic unit levels and their characteristics, along with example names and codes.
| Name | Level | Digits | Average size (square miles) |
Number of HUs (approximate) |
Example name | Example code (HUC) |
|---|---|---|---|---|---|---|
| Region | 1 | 2 | 177,560 | 21 | Pacific Northwest | 17 |
| Subregion | 2 | 4 | 16,800 | 222 | Lower Snake | 1706 |
| Basin | 3 | 6 | 10,596 | 370 | Lower Snake | 170601 |
| Subbasin | 4 | 8 | 700 | 2,200 | Imnaha River | 17060102 |
| Watershed | 5 | 10 | 227 (40,000–250,000 acres) |
22,000 | Upper Imnaha River | 1706010201 |
| Subwatershed | 6 | 12 | 40 (10,000–40,000 acres) |
160,000 | South Fork Imnaha River | 170601020101 |
The original delineation of units, down to subbasins (cataloging units), was done using 1:250,000 scale
Scale (map)
The scale of a map is defined as the ratio of a distance on the map to the corresponding distance on the ground.If the region of the map is small enough for the curvature of the Earth to be neglected, then the scale may be taken as a constant ratio over the whole map....
maps and data. The newer delineation work on watersheds and subwatersheds was done using 1:24,000 scale maps and data. As a result, the subbasin boundaries were changed and adjusted in order to conform to the higher resolution watersheds within them. Changes to subbasin boundaries resulted in changes in area sizes. Therefore, older data using "cataloging units" may differ from newer, higher resolution data using "subbasins".
The regions (1st level hydrologic units) are geographic areas that contain either the drainage area
Drainage basin
A drainage basin is an extent or an area of land where surface water from rain and melting snow or ice converges to a single point, usually the exit of the basin, where the waters join another waterbody, such as a river, lake, reservoir, estuary, wetland, sea, or ocean...
of a major river, such as the Missouri region, or the combined drainage areas of a series of rivers, such as the Texas–Gulf region. Each subregion includes the area drained by a river system, a reach
Reach (geography)
A reach in geography can mean several things. Most generally, a reach is any length of a stream between any two points. The points may be selected for any reason, such as gauging stations, river miles, natural features, and topography. They may be arbitrary or vague.A reach may also be an expanse,...
of a river and its tributaries in that reach, a closed basin or basins, or a group of streams forming a coastal drainage area. Regions receive a two-digit code. The following levels are designated by the addition of another two digits. The hierarchy was designed and the units subdivided so that almost all the subbasins (formerly called cataloging units) are larger than 700 square miles (1,813 km²). Larger closed basins were subdivided until their subunits were less than 700 square miles. The 10-digit watersheds were delineated to be between 40,000 and 250,000 acres in size, and the 12-digit subwatersheds between 10,000 and 40,000 acres. In addition to the hydrologic unit codes, each hydrologic unit was assigned a name corresponding to the unit's principal hydrologic feature or to a cultural or political feature within the unit.
The boundaries of the hydrologic units usually correspond to drainage basins with some exceptions; for example, subregion 1711, called "Puget Sound", includes all U.S. drainage into not only Puget Sound but also the Strait of Georgia
Strait of Georgia
The Strait of Georgia or the Georgia Strait is a strait between Vancouver Island and the mainland coast of British Columbia, Canada. It is approximately long and varies in width from...
, Strait of Juan de Fuca
Strait of Juan de Fuca
The Strait of Juan de Fuca is a large body of water about long that is the Salish Sea outlet to the Pacific Ocean...
, and the Fraser River
Fraser River
The Fraser River is the longest river within British Columbia, Canada, rising at Fraser Pass near Mount Robson in the Rocky Mountains and flowing for , into the Strait of Georgia at the city of Vancouver. It is the tenth longest river in Canada...
. Also, region and subregion boundaries end at the U.S. international boundary.
In general, hydrologic units were delineated such that all surface drainage within each unit converges at a single outlet point—a type of hydrologic unit called a "classic hydrological unit". It was not always possible to delineated units in this way while adhering to the size and subdivision standards of the system. There are several "non-classic" types of drainage areas, each requiring special criteria for delineation and subdivision.
"Remnant areas" occur along coasts where individual streams are too small for the given subdivision type. Such remnants were combined into a single unit if they were adjacent and could be combined. These "composite" units are called "frontal units". They are non-classic because they have more than one outlet.
For example, the coastal area along Puget Sound
Puget Sound
Puget Sound is a sound in the U.S. state of Washington. It is a complex estuarine system of interconnected marine waterways and basins, with one major and one minor connection to the Strait of Juan de Fuca and the Pacific Ocean — Admiralty Inlet being the major connection and...
between Seattle and Mukilteo
Mukilteo, Washington
Mukilteo , which means "good camping ground", is a city in Snohomish County, Washington, United States. The population was 20,254 at the 2010 census. It is on the shore of the Puget Sound, and is the site of a Washington State Ferries terminal linking it to Clinton, on Whidbey Island.Mukilteo is...
, is delineated at the finest "subwatershed" level as "Shell Creek-Frontal Puget Sound", HUC 171100190203. This hydrologic unit includes numerous small streams that drain directly to Puget Sound, including Pipers Creek
Pipers Creek
Pipers Creek is a urban stream in the Broadview and Blue Ridge neighborhoods of Seattle, Washington, whose entire length is within the boundaries of Carkeek Park. Its tributaries are Venema Creek and Mohlendorph Creek. It empties into Puget Sound. The creek was renamed "Piper's" by early White...
and Boeing Creek
Boeing Creek
Boeing Creek is a stream in the U.S. state of Washington, located in the city of Shoreline, just north of Seattle. It is about long and empties into Puget Sound. The creek is heavily modified along its course, and in many places has been diverted into culverts. The watershed of Boeing Creek is...
. As a consequence of the smallest "subwatershed" being non-classic, every higher level unit containing it are also non-classic "frontal" units—"Lunds Gulch-Frontal Puget Sound" (HUC 17110019), "Puget Sound" (HUC 171100 and 1711), and "Pacific Northwest Region" (HUC 17).
"Noncontributing areas" are drainage areas within a hydrologic unit that do not drain to the unit's outlet. They can be caused by such things as potholes and kettles, closed basins, playas
Dry lake
Dry lakes are ephemeral lakebeds, or a remnant of an endorheic lake. Such flats consist of fine-grained sediments infused with alkali salts. Dry lakes are also referred to as alkali flats, sabkhas, playas or mud flats...
, and cirque
Cirque
Cirque may refer to:* Cirque, a geological formation* Makhtesh, an erosional landform found in the Negev desert of Israel and Sinai of Egypt*Cirque , an album by Biosphere* Cirque Corporation, a company that makes touchpads...
s. If a noncontributing area is large enough, it was designated as a hydrologic unit of its own. The largest such example is the Great Basin
Great Basin
The Great Basin is the largest area of contiguous endorheic watersheds in North America and is noted for its arid conditions and Basin and Range topography that varies from the North American low point at Badwater Basin to the highest point of the contiguous United States, less than away at the...
, designated a hydrologic unit the Region level. When a noncontributing area was not large enough to be designated a hydrologic unit, it was merged into the surrounding or bordering larger hydrologic unit. Special decisions were required for "semiconfined basins" that contribute flow to another area in wet years but become noncontributing in dry years—Goose Lake
Goose Lake (Oregon-California)
Goose Lake is a large alkaline glacial lake located in the Goose Lake Valley on the Oregon-California border. The north end of the lake is in Lake County, Oregon and the south end is in Modoc County, California. The mountains at the north end of the lake are part of the Fremont National Forest...
, for example. The USGS instructed the people doing the delineation work to take extra care in the case of semiconfined basins and to seek assistance from others, but to ultimately make their own decision on whether the semiconfined basin should be designated a noncontributing area or not. Another special case occurs when noncontributing areas very small and dispersed, or scattered throughout a drainage area. These were considered part of the encompassing hydrologic unit. In short, noncontributing areas cannot be subject to strict criteria for delineating, and methods vary from state to state, landform type to type, and special cases. The effect of noncontributing areas on specific hydrologic units is explained in metadata
Metadata
The term metadata is an ambiguous term which is used for two fundamentally different concepts . Although the expression "data about data" is often used, it does not apply to both in the same way. Structural metadata, the design and specification of data structures, cannot be about data, because at...
as best it can.
The Goose Lake example illustrates how USGS hydrologic units do not always conform strictly to drainage basins. Despite being part of the Upper Sacramento River basin (or accounting unit), HUC 180200, and the Sacramento River subregion, HUC 1802, the Goose Lake subbasin (or cataloging unit), HUC 18020001, was defined as a closed basin during the watershed and subwatershed delineation process. Therefore, the area of the Sacramento River subregion and the Upper Sacramento River basin, as published by the USGS (27600 sq mi (71,483.7 km²) and 7650 sq mi (19,813.4 km²) respectively), are too large by at least the size of the Goose Lake subbasin/cataloging unit, 1080 sq mi (2,797.2 km²).
Other non-classic drainage issues that have an effect on hydrologic unit delineation and subdivision include reservoir
Reservoir
A reservoir , artificial lake or dam is used to store water.Reservoirs may be created in river valleys by the construction of a dam or may be built by excavation in the ground or by conventional construction techniques such as brickwork or cast concrete.The term reservoir may also be used to...
s, diverted waters ranging from small irrigation ditch
Ditch
A ditch is usually defined as a small to moderate depression created to channel water.In Anglo-Saxon, the word dïc already existed and was pronounced 'deek' in northern England and 'deetch' in the south. The origins of the word lie in digging a trench and forming the upcast soil into a bank...
es to interbasin transfer
Interbasin transfer
Interbasin transfer or transbasin diversion are terms used to describe man-made conveyance schemes which move water from one river basin where it is available, to another basin where water is less available or could be utilized better for human development...
s, islands, and coastal areas with large tidal range
Tidal range
The tidal range is the vertical difference between the high tide and the succeeding low tide. Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of the Earth...
s. The 5th and 6th level hydrologic units, called "watersheds" and "subwatersheds", were assigned one of seven attribute codes to indicate drainage type: standard (classic, one outlet), closed basin (no outlet), frontal (multiple outlets), water (predominately water with adjacent land areas), island (one or more islands and adjacent water), and unclassified (an area that cannot be defined or does not fit one of the other types).