Slack water

Slack water, also known as 'the stand of the tide', is a short period in a body of tidal water when the water is completely unstressed, and there is no movement either way in the tidal stream, and which occurs before the direction of the tidal stream reverses.[1] Slack water can be estimated using a tidal atlas or the tidal diamond information on a nautical chart.[2] The time of slack water, particularly in constricted waters, does not occur at high and low water,[3] and in certain areas, such as Primera Angostura, the ebb may run for up to three hours after the water level has started to rise, and the flood may run for three hours after the water has started to fall. Thornton Lecky, writing in 1884, illustrates the phenomenon with an inland basin of infinite size, connected to the sea by a narrow mouth. Since the level of the basin is always at mean sea level, the flood in the mouth starts at half tide, and its velocity is at its greatest at the time of high water, with the strongest ebb occurring conversely at low water.[4]

Implications for seafarers

For scuba divers, the absence of a flow means that less effort is required to swim, and there is less likelihood of drifting away from a vessel or shore. Slack water following high tide can improve underwater visibility, as the previously incoming tide brings clear water with it. Following low tide, visibility can be reduced as the ebb draws silt, mud, and other particulates with it. In areas with potentially dangerous tides and currents, it is standard practice for divers to plan a dive at slack times.

For any vessel, a favourable flow will improve the vessel's speed over the bottom for a given speed in the water. Difficult channels are also more safely navigated during slack water, as any flow may set a vessel out of a channel and into danger.

Combined tidal stream and current

In many locations, in addition to the tidal streams there is also a current causing the tidal stream in the one direction to be stronger than, and last for longer than the stream in the opposite direction six hours later. Variations in the strength of that current will also vary the time when the stream reverses, thus altering the time and duration of slack water. Variations in wind stress also reflect directly on the height of the tide, and the inverse relationship between the height of the tide and atmospheric pressure is well understood (1 cm change in sea level for each 1 mb change in pressure) while the duration of slack water at a given location is inversely related to the height of the tide at that location.


Slack water is a much misused term, often used to describe a period of equilibrium between two opposing streams when the water is anything but slack, but highly stressed. Although there may be no flow in either direction there may be many eddies, and since this so-called slack water occurs before high water while the tide is still rising, the tide may continue to rise even after the direction of the stream has reversed. Conversely, since it occurs after low water while the tide is rising, the tide may also continue to rise during this so-called low water slack period. Such conditions typically occur at river mouths, or in straits open at both ends where their entrances have markedly different physical characteristics. Examples include The Rip between Point Nepean and Point Lonsdale at the entrance to Port Philip Bay, Victoria, Australia; the Menai Strait between Anglesey and Wales; or the Strait of Gibraltar at the entrance to the Mediterranean Sea.

Slack water may also be misused to refer to a process in caves. This occurs when a stream cave, or fluviokarst, is completely filled with water during flooding.

Dodge tides

Some localities have unusual tidal characteristics, such as Gulf St Vincent, South Australia, where the amplitudes of the main semi-diurnal tide constituents are almost identical. At neap tides the semi-diurnal tide is virtually absent, resulting in the phenomenon known as a "dodge tide"[5][6]—a day-long period of slack water—occurring twice a month; this effect is accentuated near the equinoxes when the diurnal component also vanishes, resulting in a period of 2–3 days of slack water.[7][8][9]

See also

Stability theory, a main mathematical concept considering forces in equilibrium.


  1. ^ The American Practical Navigator, Chapter 9:Tides and Tidal Currents, page 139. Accessed 3 September 2011.
  2. ^ Sport Diving, British Sub Aqua Club, ISBN 0-09-163831-3, page 167
  3. ^ The American Practical Navigator, Chapter 9:Tides and Tidal Currents, pp.141-142. Accessed 19 December 2013.
  4. ^ Squire Thornton Stratford Lecky; William Allingham (1918). Wrinkles in Practical Navigation. G. Philip & son. p. 285.
  5. ^ Australian Government > Bureau of Meteorology > National Tidal Unit Glossary Accessed 13 March 2015.
  6. ^ Australian Government > Bureau of Meteorology > Dodge tide Accessed 13 March 2015.
  7. ^ Bye, J. A. T. (1976): Physical oceanography of Gulf St Vincent and Investigator Strait. In: Twidale, C. R., Tyler, M. J. & Webb, B. P. (Eds.), Natural history of the Adelaide Region. Royal Society of SA Inc., Adelaide.
  8. ^ Bye, J.A.T. & Kämpf, J. (2008): Physical oceanography. In: Shepherd, S.A., Bryars, S., Kirkegaard, I.R., Harbison, P. & Jennings, J. T. (Eds.): Natural history of Gulf St Vincent. Royal Society of South Australia Inc., Adelaide.
  9. ^ The American Practical Navigator, Chapter 9:Tides and Tidal Currents, pages 134-5. Accessed 3 September 2011.
Bear Mountain (Carbon County, Pennsylvania)

Bear Mountain, in the Lehigh Valley of Pennsylvania several miles above the Lehigh Gap, is an steep-sided east bank ridgeline running about 9.96 miles (16.03 km) between the hairpin turn in the Lehigh the Lenape Amerindian people (Delaware people) visualized as a Bear's snout, along many water gap gorges, to the steep face dropping down to the Penn Forest Reservoir. The sparsely settled mountain ridge is part of the ridge-and-valley Appalachians, oriented ENE towards the Delaware River climbing rapidly from the Lehigh left bank shoreline from about 580 feet (180 m) over an overhanging knob opposite the mouth of Mauch Chunk Creek to more than 1,200 feet (370 m) in less than 1.01 miles (1.63 km) and to over 1,480 feet (450 m) in just 1.34 miles (2.16 km) the tourism & business district of Jim Thorpe. Bear Mountain is the prominent peak opposite the business district of the tourist attractions of Jim Thorpe in Carbon County, once termed being in the heart of "Switzerland of the United States". The former township and borough of East Mauch Chunk was settled outside the hustle and confusion of cross-river boomtown Mauch Chunk ited on the north Overlooking the long slack water pool of the Lehigh River after it exits the wilderness countryside around the Lehigh Gorge as the Lehigh runs along the northwest flanks of Bear Mountain,


Canals are waterways channels, or artificial waterways, for water conveyance, or to service water transport vehicles. They may also help with irrigation. It can be thought of as an artificial version of a river.

In most cases, the engineered works will have a series of dams and locks that create reservoirs of low speed current flow. These reservoirs are referred to as slack water levels, often just called levels.

A canal is also known as a navigation when it parallels a river and shares part of its waters and drainage basin, and leverages its resources by building dams and locks to increase and lengthen its stretches of slack water levels while staying in its valley.

In contrast, a canal cuts across a drainage divide atop a ridge, generally requiring an external water source above the highest elevation.

Many canals have been built at elevations towering over valleys and other water ways crossing far below.

Canals with sources of water at a higher level can deliver water to a destination such as a city where water is needed. The Roman Empire's aqueducts were such water supply canals.

Carbon County, Pennsylvania

Carbon County is a county located in the U.S. state of Pennsylvania. As of the 2010 census, the population was 65,249. Its county seat is Jim Thorpe, which was founded in 1818 as Mauch Chunk, a company town of the Lehigh Coal & Navigation Company (LC&N) along a new wagon road the company was constructing nine miles long to their coal mine, in the area now known as Summit Hill. LC&N also constructed the Lehigh Canal navigations during this time in the area.In 1827, that wagon road became the nation's second operating railroad, the Summit Hill & Mauch Chunk Railroad which is regarded as the world's first roller coaster, which became its main function between 1873–1931. The area around Mauch Chunk was known as the "Switzerland of America", the long wide slack water pool above the Lehigh's upper dam being surrounded by Mauch Chunk Ridge, Bear Mountain, Pisgah Ridge, Mount Pisgah, Nesquehoning Ridge, Broad Mountain and their various prominences and summits. Another railroad first, the first railway to operate steam locomotives as traction engines and prime movers in the United States was the Beaver Meadows Railroad, which connected from mines west of Beaver Meadows and Weatherly on the opposite side of Broad Mountain along a water path through the Lehigh Gorge at Penn Haven Junction (once supporting five railroads) to the Lehigh Canal opposite Lehighton.

In the 1830s, the first blast furnaces in Northampton County were built by the LC&N in an attempt to make anthracite iron, the foundation of the early industrial revolution in America. The LC&N also built the first wire rope factory in the U.S. in Mauch Chunk.

Carbon County is included in the Allentown–Bethlehem–Easton, PA–NJ Metropolitan Statistical Area, which is also included in the New York–Newark, NY–NJ–CT–PA Combined Statistical Area. However, the Lehigh Valley directly borders the Delaware Valley, and Carbon County is included in Philadelphia's Designated Media Market. It is considered part of the state's Coal Region, though the eastern and northeastern sections are considered part of the Pocono Mountains—since they are east of the Lehigh River, the demarcation arbitrarily separating very similar mountain ridge and valley systems.

Cuan Sound

Cuan Sound is a narrow channel, 200 metres (660 ft) wide, located in Argyll, western Scotland. It separates Seil and Luing and later becomes the Firth of Lorn. It has a very strong current. In Cuan Sound, the north-going stream begins 4.5 hours after high water Oban and sets westward; the south-going stream begins 1.5 hours before high water Oban and sets eastward; the streams attain a rate of 7 knots at springs. This coast from Cuan Sound to Easdale Bay is in many places foul and rocky for 1.5 cables of it. Sgeir na Faoileann, a rock above water, one cable from the shore, and 3.5 cables northward of the entrance to Cuan Sound, is surrounded by foul ground, which extends 1.5 cables south-westward from it. Coirebhreacain and Cuan Sound are seldom attempted except near slack water.

Elk River (Tennessee River tributary)

The Elk River is a tributary of the Tennessee River in the U.S. states of Tennessee and Alabama. The river is about 195 miles (314 km) long.

Falls of Lora

The Falls of Lora is a tidal race which forms at the mouth of Loch Etive when a particularly high tide runs out from the loch. They form white water rapids for two to five days either side of the spring tides.The falls of Lora are generated when the water level in the Firth of Lorn (i.e. the open sea) drops below the level of the water in Loch Etive as the tide goes out. As the seawater in Loch Etive pours out through the narrow mouth of the loch, it passes over a rocky shelf which causes the rapids to form. As the tide rises again there is a period of slack water when the levels are the same on either side. However, due to the narrow entrance to the Loch, the tide rises more quickly than the water can flow into the Loch. Thus there is still considerable turbulence at high tide caused by flow into the Loch. Thus, unlike most situations where slack water is at high and low tides, in the case of the Falls of Lora slack water occurs when the levels on either side are the same, not when the tidal change is at its least. As a result, the tidal range is much greater on the coast than it is inside the loch. A 3 metres (9.8 ft) range at Oban may produce only a 1.3 metres (4.3 ft) range at Bonawe on the loch shore.The loch mouth is spanned by Connel Bridge.

The race is popular with white water kayakers and divers as well as tourists and photographers.

Haisborough Sands

Haisborough Sands (or Haisboro Sands or Haisbro Sands) is a sandbank off the coast of Norfolk, England at Happisburgh. The shoal is 10 miles (16 km) long and 1 mile (1.6 km) wide and lies parallel to the North east coast of Norfolk. The shoal is marked to the north-west by north by the Haisbro Light Buoy, North cardinal. To the south-east by south is a light buoy South cardinal, and to the west by Mid Haisbro light buoy starboard hand. In 1995 there were three drying patches recorded to the north-north east and east-south east of the Mid Haisbro light buoy. Except at slack water their positions are indicated by tidal eddies particularly on the north west, and in slight or moderate seas the swell breaks on the shallower parts of the banks. There are several foul patches on the southern part of the shoal. Over the years this shoal has claimed many ships.


A lacustuary is a freshwater body of water where river and lake waters mix. Lacustuaries are nonmoving or "slack" water, although water in them can ebb and flow due to lake waves moving in and out, and the rising and falling lake water levels.

A lacustuary is that portion of a freshwater river or stream where lake water and river/stream water mix. The upper boundary of the lacustuary is that portion of the river directly affected by rising and falling water levels in the lake. Lacustuaries are generally nonmoving or "slack" water.Lacustuaries are not estuaries. They are far less saline, as lacustuaries are freshwater bodies. Additionally lacustuaries tend to be less saline the closer to the lake, where as estuaries are more saline the closer to the ocean. Lacustuaries, where river and lake waters meet and mix, have different functions than estuaries. Their physical and biological characteristics are also much different from the free-flowing water further upstream, and they play important roles in the ecology of both lake and river ecosystems. Lacustuary vegetation is also markedly different from shoreline and upland habitats.Not all streams which enter a lake have a lacustuary. Man-made ship channels may also be lacustuaries, depending on their slack water status.

Lenticular bedding

Lenticular bedding is a sedimentary bedding pattern displaying alternating layers of mud and sand. Formed during periods of slack water, mud suspended in the water is deposited on top of small formations of sand once the water's velocity has reached zero. Lenticular bedding is classified by its large quantities of mud relative to sand, whereas a flaser bed consists mostly of sand. The sand formations within the bedding display a 'lens-like' shape, giving the pattern its respected name. They are commonly found in high-energy environments such as the intertidal and supratidal zones. Geologists use lenticular bedding to show evidence of tidal rhythm, tidal currents and tidal slack, in a particular environment.

Little Emory River

The Little Emory River rises in Morgan County, Tennessee near the town of Coalfield. It is one of the major tributaries to the Emory River. It crosses into Roane County, where it soon becomes an embayment of Watts Bar Lake several miles upstream of its mouth into the Emory. (Watts Bar Lake is a relatively deep reservoir and causes "slack water" conditions many miles up several Tennessee River tributaries, not just the main stream.)

Middle Mouse

Middle Mouse (Welsh: Ynys Badrig - Patrick's island) is an uninhabited island situated 1 kilometre off the north coast of Anglesey. It is notable as the northernmost point of Wales. The island measures a maximum of 207 metres by 110 metres, with a maximum area of 3.7 acres and has a maximum altitude of 16 metres above sea level. It is one of a chain of three islands off the north of Anglesey, the others being Ynys Amlwch and Maen y Bugail.

Local legend has it that St Patrick was shipwrecked there, giving rise to its Welsh name. He then swam ashore and eventually founded the nearby church of Llanbadrig in about 440 AD, believed to be the oldest Christian site in Wales.Middle Mouse is a favoured place for cormorants, guillemots and razorbills. For visiting scuba divers the attractions are steep underwater cliffs that drop away to 40 metres with abundant marine life. There is very little protection from the fierce tidal flow, so accurate timing for slack water is required.During the 19th century the island was used as a navigational aid to ships sailing into Liverpool. Any vessel that passed the island without signalling, and waiting for, a pilot was liable to incur a fine. The S.S. Liverpool, following a collision with a ship named Laplata, was shipwrecked near Middle Mouse in 1863. She had on board a shipment of tin ingots.In 2005, the island was put up for sale as part of a 168-acre (0.68 km2) estate.

Milton, Brown County, Illinois

Milton is a former settlement in Brown County, Illinois, United States. The town was laid out August 25, 1836, by Lewis Gay, situated on the E. i of the S. E. of section 31, and the W. i of the S. W. I of section 32, near the site of the old Johnson mill, on McKee's creek, but it was a failure. Lewis Gay and William C. Ralls were the proprietors of the town, having laid out the town on McKee creek, five miles from the Illinois river. In the prospectus advertising the sale of town lots, the promoters referred to it as located at the head of slack water navigation.

Obey River

The Obey River is a 47.8-mile-long (76.9 km) tributary of the Cumberland River in the U.S. state of Tennessee. It joins the Cumberland River near the town of Celina, which is generally considered to be the Cumberland's head of navigation. Via the Cumberland and Ohio rivers, the Obey River is part of the Mississippi River watershed.

Near its mouth, the Obey is impounded by the U.S. Army Corps of Engineers Dale Hollow Reservoir, site of a fish hatchery run by the federal government. This dam impounds the Obey for essentially its entire length, causing slack water well up both major tributaries, the East and West Forks. This lake is relatively deep due to the height of the dam and the depth of the gorges through which the Obey and its tributaries flowed; the impoundment also enters Kentucky in its Wolf River and Sulphur Creek embayments.

Below the dam the stream makes two sharp bends before entering the Cumberland. The only major bridge on the Obey, on State Route 53, is located just below the second of these, Peterman Bend.

Port of Saint John

The Port of Saint John is a port complex that occupies 120 hectares (300 acres) of land along 3,900 m (12,800 ft) of waterfront of the Saint John Harbour at the mouth of the Saint John River in the city of Saint John, New Brunswick, Canada. The Port of Saint John, with facilities on both sides of the river, is noted for its extreme tidal range and river currents. Because of the semi-diurnal tides and the river influence, slack water occurs at approximately half tide and not at high or low water as at most other ports.The port is administered by the Saint John Port Authority, a federal agency. Major products shipped through the port include oil, forest products and potash.

Schistura huongensis

Schistura huongensis is a species of ray-finned fish in the most speciose genus of the stone loach family Nemacheilidae, Schistura. It has been recorded from the drainage basins of the Perfume River and Cam Lo in central Vietnam. It can be found in the slack water upstream of riffles in medium sized mountain rivers and streams with a fast current.

Schuylkill Canal

Schuylkill Canal is the common, but technically inaccurate, name for the Schuylkill Navigation, a 19th-century commercial waterway in and along the Schuylkill River in the U.S. state of Pennsylvania. The "canal" was actually a system of interconnected canals and slack-water pools in the river, which is called a navigation. Chartered in 1815, the Schuylkill Navigation opened in 1825 to provide transportation and water power. At the time, the river was the least expensive and most efficient method of transporting bulk cargo, and the eastern seaboard cities of the U.S. were experiencing an energy crisis due to deforestation. It fostered the mining of anthracite coal as the major source of industry between Pottsville and eastern markets. Along the tow-paths, mules pulled barges of coal from Port Carbon through the water gaps to Pottsville; locally to the port and markets of Philadelphia; and some then by ship or through additional New Jersey waterways, to New York City markets.

The Schuylkill was in operation until 1931 and was almost completely filled in the 1950s. Some remaining watered reaches are now used for recreation.

Tidal atlas

A tidal atlas or a tidal stream atlas is used to predict the direction and speed of tidal currents.

A tidal atlas usually consists of a set of 12 or 13 diagrams, one for each hour of the tidal cycle, for a coastal region. Each diagram uses arrows to indicate the direction of the flow at that time. The speed of the flow may be indicated by numbers on each arrow or by the length of the arrow. Areas of slack water may be indicated by no arrows or the words "slack water". UK Admiralty Tidal Atlases show speed in units of tenth of a knot.

An alternative to a tidal atlas is a nautical chart that provides tidal diamonds.

Villosa perpurpurea

Villosa perpurpurea, the purple bean, is a species of freshwater mussel, an aquatic bivalve mollusk in the family Unionidae, the river mussels.

This species is endemic to the United States. Its natural habitat is rivers. It is threatened by habitat loss.

The purple bean is mainly found in northeastern Tennessee and southwestern Virginia. It inhabits small headwater streams to medium-sized rivers. It is also found in moderate to fast-flowing riffles with sand, gravel, and cobble substrates and rarely it is found in deep pools or slack water. It may occur in areas adjacent to water such as willow beds and under flat rocks. The purple bean is a filter-feeder which clings to the bottom. There is no exact number for population but it is estimated to be in the hundreds with the largest population found in the upper Clinch River and Indian Creek. This animal is declining. There was a chemical spill in the Clinch River in 1998 that killed at least 52 specimens. A current threat to the purple bean is a change in turbidity, increased suspended solids, and pesticides. A big impact to the purple bean’s habitat is localized coal mining. There have been five mine tailings pond spills from 1995 to 1999, one of which resulted in a major fish kill. Chemicals that are found in coal fines such as polycyclic aromatic compounds (PAHs) are known to be toxic to mussels and fishes.[1]

Ocean zones
Sea level


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