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38-507: Water from a diversion weir in the Vaal River , near Warrenton , flows through a 1,176 km long network of canals. This system provides irrigation water to a total of 39,820ha scheduled land, industrial water to six towns and other industrial water users. This farmland is divided into individual blocks which each have their own letter, or letter group for identification. The blocks are divided into streets which have numbers that count up from

76-713: A monophyletic group. The order Salmoniformes first appeared during the Santonian and Campanian stages of the Late Cretaceous , and is most closely related to pike and mudminnows in the order Esociformes , to the extent that some authors have grouped the Esociformes within the Salmoniformes. Although it is assumed that salmon and pike diverged from one another during the Cretaceous, no definitive salmonids appear before

114-424: A change in the height of the river level. Weirs are also used to control the flow of water for outlets of lakes, ponds, and reservoirs. There are many weir designs, but commonly water flows freely over the top of the weir crest before cascading down to a lower level. There is no single definition as to what constitutes a weir. Weir can also refer to the skimmer found in most in-ground swimming pools, which controls

152-423: A closer examination of the salmonid genome, and has allowed for a more precise dating of the whole-genome duplication of the group, that places the latest possible date for the event at 88 million years ago. This more precise dating and examination of the salmonid whole-genome duplication event has allowed more speculation on the radiation of species within the group. Historically, the whole-genome duplication event

190-482: A negative effect on fish species that migrate as part of their breeding cycle (e.g., salmonids ), but it also can be useful as a method of preventing invasive species moving upstream. For example, weirs in the Great Lakes region have helped to prevent invasive sea lamprey from colonising farther upstream. Mill ponds are created by a weir that impounds water that then flows over the structure. The energy created by

228-415: A range of biota , including poor swimmers. Even though the water around weirs can often appear relatively calm, they can be extremely dangerous places to boat, swim, or wade, as the circulation patterns on the downstream side—typically called a hydraulic jump —can submerge a person indefinitely. This phenomenon is so well known to canoeists, kayakers, and others who spend time on rivers that they even have

266-416: A rueful name for weirs: "drowning machines". The Ohio DNR recommends that a victim should "tuck the chin down, draw the knees up to the chest with arms wrapped around them. Hopefully, conditions will be such that the current will push the victim along the bed of the river until swept beyond the boil line and released by the hydraulic." The Pennsylvania State Police also recommends to victims, "curl up, dive to

304-568: A single row of sharp teeth . Although the smallest salmonid species is just 13 cm (5.1 in) long for adults, most salmonids are much larger, with the largest reaching 2 m (6 ft 7 in). All salmonids are migratory fish that spawn in the shallow gravel beds of freshwater headstreams , spend the growing juvenile years in rivers , creeks , small lakes and wetlands , but migrate downstream upon maturity and spend most of their adult lives at much larger waterbodies. Many salmonid species are euryhaline and migrate to

342-449: A weir is broad-crested for much of its length, but has a section where the weir stops or is 'open' so that small boats and fish can traverse the structure. A notch weir is any weir where the physical barrier is significantly higher than the water level except for a specific notch (often V-shaped) cut into the panel. At times of normal flow all the water must pass through the notch, simplifying flow volume calculations, and at times of flood

380-405: Is a flat-crested structure, where the water passes over a crest that covers much or all of the channel width. This is one of the most common types of weir found worldwide. A compound weir is any weir that comprises several different designs into one structure. They are commonly seen in locations where a river has multiple users who may need to bypass the structure. A common design would be one where

418-473: Is a generic relationship and specific calculations are available for the many different types of weir. Flow measurement weirs must be well maintained if they are to remain accurate. The flow over a V-notch weir (in ft /s) is given by the Kindsvater–Shen equation: where As weirs are a physical barrier, they can impede the longitudinal movement of fish and other animals up and down a river. This can have

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456-651: Is an archaic salmonid, representing an important stage in salmonid evolution. Fossil scales of coregonines are known from the Late Eocene or Early Oligocene of California. A gap appears in the salmonine fossil record after E. driftwoodensis until about 7 million years ago ( mya ), in the Late Miocene , when trout-like fossils appear in Idaho , in the Clarkia Lake beds . Several of these species appear to be Oncorhynchus —

494-401: Is the fourth of its kind to happen in the evolutionary lineage of the salmonids, with two having occurred commonly to all bony vertebrates, and another specifically in the teleost fishes. Extant salmonids all show evidence of partial tetraploidy, as studies show the genome has undergone selection to regain a diploid state. Work done in the rainbow trout ( Onchorhynchus mykiss ) has shown that

532-575: The Eocene . The Salmonidae first appear in the fossil record in the Early Eocene with Eosalmo driftwoodensis , a stem- salmonine , which was first described from fossils found at Driftwood Creek , central British Columbia , and has been recovered from most sites in the Eocene Okanagan Highlands . This genus shares traits found in all three subfamily lineages. Hence, E. driftwoodensis

570-415: The biomass transfer provided by their mass migration from oceanic to inland waterbodies. Current salmonids comprise three main clades taxonomically treated as subfamilies : Coregoninae ( freshwater whitefishes ), Thymallinae ( graylings ), and Salmoninae ( trout , salmon , char , taimens and lenoks ). Generally, all three lineages are accepted to allocate a suite of derived traits indicating

608-683: The sea or brackish estuaries as soon as they approach adulthood, returning to the upper streams only to reproduce. Such sea-run life cycle is described as anadromous , and other freshwater salmonids that migrate purely between lakes and rivers are considered potamodromous . Salmonids are carnivorous predators of the middle food chain , feeding on smaller fish, crustaceans , aquatic insects and larvae , tadpoles and sometimes fish eggs (even those of their own kind), and in turn being preyed upon by larger predators. Many species of salmonids are thus considered keystone organisms important for both freshwater and terrestrial ecosystems due to

646-550: The subarctic and cool temperate waters of the Northern Hemisphere . The Atlantic salmon ( Salmo salar ), whose Latin name became that of its genus Salmo , is also the eponym of the family and order names. Salmonids have a relatively primitive appearance among teleost fish, with the pelvic fins being placed far back, and an adipose fin towards the rear of the back. They have slender bodies with rounded scales and forked tail fins , and their mouths contain

684-508: The Pliocene. Suggestions have gone back as far as the Early Miocene (about 20 mya). Based on the most current evidence, salmonids diverged from the rest of teleost fish no later than 88 million years ago, during the late Cretaceous . This divergence was marked by a whole-genome duplication event in the ancestral salmonid, where the diploid ancestor became tetraploid . This duplication

722-502: The bottom, and swim or crawl downstream". As the hydraulic jump entrains air, the buoyancy of the water between the dam and boil line will be reduced by upward of 30%, and if a victim is unable to float, escape at the base of the dam may be the only option for survival. There are many different types of weirs and they can vary from a simple stone structure that is barely noticeable, to elaborate and very large structures that require extensive management and maintenance. A broad-crested weir

760-501: The change in height of the water can then be used to power waterwheels and power sawmills, grinding wheels, and other equipment. Weirs are commonly used to control the flow rates of rivers during periods of high discharge. Sluice gates (or in some cases the height of the weir crest) can be altered to increase or decrease the volume of water flowing downstream. Weirs for this purpose are commonly found upstream of towns and villages and can either be automated or manually operated. By slowing

798-505: The crest of an overflow spillway on a large dam may therefore be referred to as a weir. Weirs can vary in size both horizontally and vertically, with the smallest being only a few centimetres in height whilst the largest may be many metres tall and hundreds of metres long. Some common weir purposes are outlined below. Weirs allow hydrologists and engineers a simple method of measuring the volumetric flow rate in small to medium-sized streams/rivers or in industrial discharge locations. Since

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836-583: The current genus for Pacific salmon and Pacific trout. The presence of these species so far inland established that Oncorhynchus was not only present in the Pacific drainages before the beginning of the Pliocene (~5–6 mya), but also that rainbow and cutthroat trout , and Pacific salmon lineages had diverged before the beginning of the Pliocene. Consequently, the split between Oncorhynchus and Salmo (Atlantic salmon and European trout) must have occurred well before

874-414: The design of a weir that ensure that fish can bypass the barriers and access upstream habitats. Unlike dams, weirs do not usually prevent downstream fish migration (as water flows over the top and allows fish to bypass the structure in that water), although they can create flow conditions that injure juvenile fish. Recent studies suggest that navigation locks have also potential to provide increased access for

912-446: The farmland. The most popular method to do this is with pivots. 28°01′S 24°43′E  /  28.017°S 24.717°E  / -28.017; 24.717 This Northern Cape location article is a stub . You can help Misplaced Pages by expanding it . Diversion weir A weir / w ɪər / or low-head dam is a barrier across the width of a river that alters the flow characteristics of water and usually results in

950-444: The first one out. The canals divide into all of the blocks and the streets. There are a total of 6 plots per street and each plot also has a number from one to six. To reference a specific plot, you take the plot number, follow it up with the block letter and then the street number. e.g. 3 G 13. Each plot feeds off of the canal into their own dams with their own hatch from the canal. The water then goes through pumps to be sprinkled across

988-694: The flow at outlets of lakes, ponds, and reservoirs. Salmonids Salmonidae ( / s æ l ˈ m ɒ n ɪ d iː / , lit. "salmon-like") is a family of ray-finned fish that constitutes the only currently extant family in the order Salmoniformes ( / s æ l ˈ m ɒ n ɪ f ɔːr m iː z / , lit. "salmon-shaped"), consisting of 11 extant genera and over 200 species collectively known as " salmonids " or " salmonoids ". The family includes salmon (both Atlantic and Pacific species), trout (both ocean-going and landlocked), char , graylings , freshwater whitefishes , taimens and lenoks , all coldwater mid- level predatory fish that inhabit

1026-413: The flow of water pulled into the filtering system. The word likely originated from Middle English were , Old English wer , a derivative of the root of the verb werian, meaning "to defend, dam". The German cognate is Wehr , which means the same as English weir. Commonly, weirs are used to prevent flooding , measure water discharge, and help render rivers more navigable by boat. In some locations,

1064-449: The genome is still partially-tetraploid. Around half of the duplicated protein-coding genes have been deleted, but all apparent miRNA sequences still show full duplication, with potential to influence regulation of the rainbow trout's genome. This pattern of partial tetraploidy is thought to be reflected in the rest of extant salmonids. The first fossil species representing a true salmonid fish ( E. driftwoodensis ) does not appear until

1102-602: The geometry of the top of the weir is known and all water flows over the weir, the depth of water behind the weir can be converted to a rate of flow. However, this can only be achieved in locations where all water flows over the top of the weir crest (as opposed to around the sides or through conduits or sluices) and at locations where the water that flows over the crest is carried away from the structure. If these conditions are not met, it can make flow measurement complicated, inaccurate, or even impossible. The discharge calculation can be summarised as where However, this calculation

1140-423: The maximum height a species can jump or creates flow conditions that cannot be bypassed (e.g., due to excessive water velocity) effectively limits the maximum point upstream that fish can migrate. In some cases this can mean that huge lengths of breeding habitat are lost, and over time this can have a significant impact on fish populations. In many countries, it is now a legal requirement to build fish ladders into

1178-427: The middle Eocene. This fossil already displays traits associated with extant salmonids, but as the genome of E. driftwoodensis cannot be sequenced, it cannot be confirmed if polyploidy was present in this animal at this point in time. This fossil is also significantly younger than the proposed salmonid divergence from the rest of the teleost fishes, and is the earliest confirmed salmonid currently known. This means that

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1216-413: The rate at which water moves downstream even slightly, a disproportionate effect can be had on the likelihood of flooding. On larger rivers, a weir can also alter the flow characteristics of the waterway to the point that vessels are able to navigate areas previously inaccessible due to extreme currents or eddies . Many larger weirs will have construction features that allow boats and river users to "shoot

1254-427: The river bottom) that reduces the water oxygen content and smothers invertebrate habitat and fish spawning sites. The oxygen content typically returns to normal once water has passed over the weir crest (although it can be hyper-oxygenated), although increased river velocity can scour the river bed causing erosion and habitat loss. Weirs can have a significant effect on fish migration . Any weir that exceeds either

1292-441: The salmonids have a ghost lineage of approximately 33 million years. Given a lack of earlier transition fossils, and the inability to extract genomic data from specimens other than extant species, the dating of the whole-genome duplication event in salmonids was historically a very broad categorization of times, ranging from 25 to 100 million years in age. New advances in calibrated relaxed molecular clock analyses have allowed for

1330-410: The terms dam and weir are synonymous, but normally there is a clear distinction made between the structures. Usually, a dam is designed specifically to impound water behind a wall, whilst a weir is designed to alter the river flow characteristics. A common distinction between dams and weirs is that water flows over the top (crest) of a weir or underneath it for at least some of its length. Accordingly,

1368-515: The water level can rise and submerge the weir without any alterations made to the structure. A polynomial weir is a weir that has a geometry defined by a polynomial equation of any order n . In practice, most weirs are low-order polynomial weirs. The standard rectangular weir is, for example, a polynomial weir of order zero. The triangular (V-notch) and trapezoidal weirs are of order one. High-order polynomial weirs are providing wider range of Head-Discharge relationships, and hence better control of

1406-540: The weir" and navigate by passing up or down stream without having to exit the river. Weirs constructed for this purpose are especially common on the River Thames , and most are situated near each of the river's 45 locks . Because a weir impounds water behind it and alters the flow regime of the river, it can have an effect on the local ecology . Typically, the reduced river velocity upstream can lead to increased siltation (deposition of fine particles of silt and clay on

1444-609: Was thought to be the reason for the variation within Salmonidae. Current evidence done with molecular clock analyses revealed that much of the speciation of the group occurred during periods of intense climate change associated with the last ice ages, with especially high speciation rates being observed in salmonids that developed an anadromous lifestyle. Together with the closely related orders Esociformes ( pikes and mudminnows ), Osmeriformes ( true smelts ) and Argentiniformes ( marine smelts and barreleyes ), Salmoniformes comprise

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