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68-526: Bala Lake , or Llyn Tegid ( [ˈɬɨ̞n ˈtɛɡɨd] ), is a large freshwater glacial lake in Gwynedd , Wales. The River Dee , which has its source on the slopes of Dduallt in the mountains of Snowdonia , feeds the 3.7 miles (6.0 km) long by 0.5 miles (0.8 km) wide lake. It was the largest natural body of water in Wales even before its level was raised by Thomas Telford to provide water for

136-547: A desert climate often face physical water scarcity. Central Asia , West Asia , and North Africa are examples of arid areas. Economic water scarcity results from a lack of investment in infrastructure or technology to draw water from rivers, aquifers , or other water sources. It also results from weak human capacity to meet water demand. Many people in Sub-Saharan Africa are living with economic water scarcity. An important concern for hydrological ecosystems

204-529: A collection of 17 interlinked global goals designed to be a "blueprint to achieve a better and more sustainable future for all". Targets on fresh water conservation are included in SDG 6 (Clean water and sanitation) and SDG 15 (Life on land). For example, Target 6.4 is formulated as "By 2030, substantially increase water-use efficiency across all sectors and ensure sustainable withdrawals and supply of freshwater to address water scarcity and substantially reduce

272-541: A constant and sufficient supply of water. In the 1950s, these sluices were removed when the Dee and Clwyd River Authority constructed a new outlet channel and sluices to better control flooding of the upper Dee caused by uncontrolled releases of water from the lake. The operation of these sluices enables the lake to operate as water storage or water capacitance in the Dee system and thus allow water to be abstracted at Huntington near Chester, England in order to supply fresh water to

340-402: A drinking water supply it remains vital to protect due to its ability to carry contaminants and pollutants from the land into lakes and rivers, which constitute a significant percentage of other people's freshwater supply. It is almost ubiquitous underground, residing in the spaces between particles of rock and soil or within crevices and cracks in rock, typically within 100 m (330 ft) of

408-579: A gauging station near Chirk . This location was chosen as it was on a section of the river where flow could be readily measured and above the very flat stretch that meanders into Cheshire. The current rules state that when flow exceeds 10 cubic metres per second (860,000 m /d) no intervention is required. In practice some adjustment of Bala sluices may take place to increase storage in Llyn Celyn and conversely some releases may be made from Llyn Celyn for recreational or power generation purposes. When

476-414: A glacial valley along the fault line between Bala and Tal-y-Llyn . Towards the end of the last ice age , the receding Dee valley glacier left a recessional moraine , effectively damming the valley so resulting in the establishment of the lake, and on which the town of Bala now stands. The lake has abundant pike , perch , brown trout , roach , and eel . It also contains the gwyniad , a fish unique to

544-428: A larger salt content. Freshwater habitats can be classified by different factors, including temperature, light penetration, nutrients, and vegetation. There are three basic types of freshwater ecosystems: Lentic (slow moving water, including pools , ponds , and lakes ), lotic (faster moving water, for example streams and rivers ) and wetlands (areas where the soil is saturated or inundated for at least part of

612-465: A section of former trackbed from the former Ruabon–Barmouth line . Gerald of Wales records the lake in his 12th century Itinerarium Cambriae under the name Penmelesmere . In his 1804 translation of Gerald's work, Sir Richard Colt Hoare states that the lake was also referred to as Pymplwy meer deriving from "pum plwyf" (five parishes). This name refers to the parishes of Llandderfel , Llanfawr, Llanycil , Llanuwchllyn and Llangywer . This name

680-519: A single factor. Groundwater showed greater resilience to climate change than expected, and areas with an increasing threshold between 0.34 and 0.39 aridity index exhibited significant sensitivity to climate change. Land-use could affect infiltration and runoff processes. The years of most recharge coincided with the most precipitation anomalies, such as during El Niño and La Niña events. Three precipitation-recharge sensitivities were distinguished: in super arid areas with more than 0.67 aridity index, there

748-409: A wide range of chemical parameters supplemented by daily fixed site monitoring with analysis provided in near real-time by a dedicated laboratory service. The results of the analysis are made available to the four participating organisations on a routine daily basis. For each of the critical water quality parameters alert levels and action levels have been set based on past experience. If an alert level

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816-449: Is consumed through human activities than is naturally restored, this may result in reduced fresh water availability (or water scarcity ) from surface and underground sources and can cause serious damage to surrounding and associated environments. Water pollution also reduces the availability of fresh water. Where available water resources are scarce, humans have developed technologies like desalination and wastewater recycling to stretch

884-416: Is critical to the survival of all living organisms . Many organisms can thrive on salt water, but the great majority of vascular plants and most insects , amphibians , reptiles , mammals and birds need fresh water to survive. Fresh water is the water resource that is of the most and immediate use to humans. Fresh water is not always potable water , that is, water safe to drink by humans . Much of

952-460: Is exceeded an immediate alert (DEEPOL 1) is issued to all participants. As contamination levels increase or the contaminant plug nears an abstraction point the DEEPOL level rises to DEEPOL 2 and finally DEEPOL 3 at which time the affected abstractions are closed until the river quality returns to normal. This quality management system was developed following a severe Phenol pollution of

1020-452: Is extracted for human consumption. Agriculture uses roughly two thirds of all fresh water extracted from the environment. Fresh water is a renewable and variable, but finite natural resource . Fresh water is replenished through the process of the natural water cycle , in which water from seas, lakes, forests, land, rivers and reservoirs evaporates, forms clouds , and returns inland as precipitation. Locally, however, if more fresh water

1088-544: Is insufficient to maintain flows and in such cases drought provisions are agreed which progressively reduce the maintained flow at Manley Hall. The overall effect of this regulation has a marked impact on the hydrograph of the River Dee. In dry years the hydrograph flat-lines at 10 m³/s whilst dry weather continues such as in 1990 The River Dee regulation system also manages a water quality monitoring and alerting system that includes real-time quality monitoring for

1156-711: Is often given as "Bala Lake" in English, after the nearby town of Bala . However, the word "bala" means "outlet of a lake", giving Bala Lake an impossible meaning. The usage of the English name in preference to the Welsh name is considered controversial by many. In 2023, the Eryri National Park Authority voted to use an all-Welsh list of standardised names for its lakes , thus favouring Llyn Tegid in English-language usage. The lake formed in

1224-636: Is only a short distance from the lake exit. This provided 18 million cubic metres of stored water in Bala Lake that could be controlled and used on a seasonal basis for low-flow regulation. This enables continuous abstraction from the River Dee of 235,000 cubic metres per day by six statutory water undertakings and British Waterways Board . An additional benefit was a reduction in flooding events downstream of Bala as Bala Lake ( 52°53′31″N 3°37′05″W  /  52.892°N 3.618°W  / 52.892; -3.618  ( Llyn Tegid (Bala) ) )

1292-483: Is particularly crucial in Africa, where water resources are often scarce and climate change poses significant challenges. Saline water in oceans , seas and saline groundwater make up about 97% of all the water on Earth . Only 2.5–2.75% is fresh water, including 1.75–2% frozen in glaciers , ice and snow, 0.5–0.75% as fresh groundwater. The water table is the level below which all spaces are filled with water, while

1360-826: Is securing minimum streamflow , especially preserving and restoring instream water allocations . Fresh water is an important natural resource necessary for the survival of all ecosystems . Water pollution (or aquatic pollution) is the contamination of water bodies , with a negative impact on their uses. It is usually a result of human activities. Water bodies include lakes , rivers , oceans , aquifers , reservoirs and groundwater . Water pollution results when contaminants mix with these water bodies. Contaminants can come from one of four main sources. These are sewage discharges, industrial activities, agricultural activities, and urban runoff including stormwater . Water pollution may affect either surface water or groundwater . This form of pollution can lead to many problems. One

1428-420: Is the degradation of aquatic ecosystems . Another is spreading water-borne diseases when people use polluted water for drinking or irrigation . Water pollution also reduces the ecosystem services such as drinking water provided by the water resource . Uses of water include agricultural , industrial , household , recreational and environmental activities. The Sustainable Development Goals are

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1496-537: The Amazon River . The atmosphere contains 0.04% water. In areas with no fresh water on the ground surface, fresh water derived from precipitation may, because of its lower density, overlie saline ground water in lenses or layers. Most of the world's fresh water is frozen in ice sheets . Many areas have very little fresh water, such as deserts . Water is a critical issue for the survival of all living organisms. Some can use salt water but many organisms including

1564-473: The Devil had found a way to open it. In any event, the water burst forth from the spring and completely drowned a nearby town, and this is how the lake was formed. "It was said Llyn Tegid (now called Bala Lake) was bottomless. Centuries ago an expert diver tried it, but was terribly frightened by his experience. He asserted that a dragon was coiled up at the bottom of the lake, and if he had not been very careful

1632-530: The Ellesmere Canal (later Llangollen Canal ). The town of Bala , which was once an important centre for the North Wales woollen trade, is located on the north-eastern end of the lake. The 3-mile (4.8 km) narrow gauge Bala Lake Railway , between the town and Llanuwchllyn (whose name means "church ['llan'] above ['uwch'] the lake ['llyn']"), runs along the lake's south-eastern shore using

1700-663: The Green Sahara periods) and are not appreciably replenished under current climatic conditions - at least compared to drawdown, these aquifers form essentially non-renewable resources comparable to peat or lignite, which are also continuously formed in the current era but orders of magnitude slower than they are mined. Fresh water can be defined as water with less than 500 parts per million (ppm) of dissolved salts . Other sources give higher upper salinity limits for fresh water, e.g. 1,000 ppm or 3,000 ppm. Fresh water habitats are classified as either lentic systems , which are

1768-455: The Wirral, England . The sluices allows water stored in the lake between 159.2m OD and 163.5m OD (4.3m height difference) to be utilised in managing the flow in the Dee. This body of water is estimated to be some 18,000,000 m Bala has been a tourist destination since the early 19th century. With the advent of the railways, Bala saw a growth in visitor numbers. This continued with the invention of

1836-514: The earth 's fresh water (on the surface and groundwater) is to a substantial degree unsuitable for human consumption without treatment . Fresh water can easily become polluted by human activities or due to naturally occurring processes, such as erosion. Fresh water makes up less than 3% of the world's water resources, and just 1% of that is readily available. About 70% of the world's freshwater reserves are frozen in Antarctica . Just 3% of it

1904-461: The 1920s to supply Birkenhead with water. In the industrial revolution many rivers in industrial areas became too polluted by effluents to be usable for water supply. The Dee however remained clean with relatively few polluting effluents in the Dee catchment upstream of Chester. Consequently, the City of Chester has been able to directly abstract Dee water since the first Chester Waterworks Company

1972-550: The Bala Lake Scheme was promoted to increase the available water for abstraction in the River Dee. Telford's original sluices were by-passed and the natural lake outlet was lowered. New sluice gates were constructed downstream of the confluence with the Afon Tryweryn ( 52°54′26″N 3°35′01″W  /  52.9071°N 3.5835°W  / 52.9071; -3.5835  ( Bala outlet sluice ) ), which

2040-520: The Dee Steering Committee to accurately predict when any release of pollutant would reach any of the main abstraction points on the river. The principle model flow data was provided from long-term data sets from Manley Hall gauging station which lies just upstream of the long, almost flat, serpentine section of the river path. The transit time through this stretch of the river can take several days in low-flow conditions. Additional data

2108-481: The Mersey were rehabilitated. However, the natural flow of the River Dee during most summers is insufficient to sustain any significant abstractions. To overcome this problem, a series of reservoirs have been constructed to store the excess water available in the winter time and release it back into the River Dee during the drier months. This is the principle of low-flow regulation . This was used by Thomas Telford at

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2176-531: The River Brenig valley - Llyn Brenig ( 53°04′59″N 3°31′59″W  /  53.083°N 3.533°W  / 53.083; -3.533  ( Llyn Brenig ) ). This reservoir was first filled in 1979 providing an additional 60 million cubic metres (49,000 acre⋅ft) of storage. This increased the potential for abstraction from the river in the lower reaches to nearly 900,000 cubic metres per day (200 × 10 ^  imp gal/d). By 2002

2244-548: The River Dee in the 1980s which resulted in contaminated water being supplied to large areas of Liverpool and the Wirral. Contamination of raw water is a more significant problem on the River Dee because the River Dee is normally of exceptionally good quality and as a result the abstractions have been constructed directly from the river rather than via bank-side storage reservoirs as is more common when taking water from Rivers of less reliable quality. The first Water Protection Zone

2312-624: The Wirral far exceed the locally available sources of clean water. The River Dee runs mainly in North Wales before flowing through Chester , England, and then returning to Wales in a man-made channel constructed to gain land from the Dee Estuary . The Dee is the largest relatively clean river left near to the North West conurbation and without water from the Dee much of Liverpool would be without water, unless local rivers such as

2380-494: The area above this level, where spaces in the rock and soil contain both air and water, is known as the unsaturated zone. The water in this unsaturated zone is referred to as soil moisture. Below the water table, the entire region is known as the saturated zone, and the water in this zone is called groundwater. Groundwater plays a crucial role as the primary source of water for various purposes including drinking, washing, farming, and manufacturing, and even when not directly used as

2448-412: The authorised abstractions had been taken over by three statutory undertakings and British Waterways Board with a total licensed abstraction of 850,000 cubic metres per day (190,000,000 imp gal/d). In addition a residual flow of at least 364,000 cubic metres per day is maintained over Chester Weir in all but the most testing of droughts , safeguarding the passage of migratory fish and limiting

2516-421: The available supply further. However, given the high cost (both capital and running costs) and - especially for desalination - energy requirements, those remain mostly niche applications. A non-sustainable alternative is using so-called " fossil water " from underground aquifers . As some of those aquifers formed hundreds of thousands or even millions of years ago when local climates were wetter (e.g. from one of

2584-597: The beginning of the nineteenth century in order to guarantee a supply of water to the Ellesmere Canal . Telford constructed sluices at the outlet of Bala Lake to control the flow downstream so that there was always sufficient water to supply the canal where it started at Horseshoe Falls . The River Dee has also been used for direct drinking water supply with the Alwen Reservoir ( 53°03′47″N 3°33′36″W  /  53.063°N 3.560°W  / 53.063; -3.560  ( Llyn Alwen ) ), built in

2652-479: The creature would have swallowed him." Folk-Lore and Folk Stories of Wales , Marie Trevelyan 1909. Many of Wales' largest lakes feature in Welsh mythology , often associated with lake monsters , called afancod . The legend of such a creature in this lake was recorded in 1909, when the folklorist Marie Trevelyan collated a number of local legends including the story of a coiled dragon-like creature living in

2720-400: The creature's existence has been found. The lake is already known to be home to a unique and extant species, the gwyniad . The survival of the gwyniad has led to discussions as to whether the lake could support a larger predator, and whether such a creature could survive in its isolated environment into modern times. Supporters for the potential existence of an afanc creature suggest that both

2788-406: The flow at Manley Hall decreases towards 10 m³/s, additional flow is released from Bala Lake. If that is insufficient, flow from Llyn Celyn is used to maintain 10 cubic metres per second (860,000 m /d) at Manley Hall. In extreme situations where the flow from Llyn Celyn is insufficient to maintain the flow, releases of water from Llyn Brenig are made. Circumstances can arise where even this

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2856-535: The freshwater flow to be measurably contaminated both by insoluble solids but also by the soluble components of those soils. Significant quantities of iron may be transported in this way including the well-documented transfer of iron-rich rainfall falling in Brazil derived from sand-storms in the Sahara in north Africa . In Africa, it was revealed that groundwater controls are complex and do not correspond directly to

2924-547: The great majority of higher plants and most mammals must have access to fresh water to live. Some terrestrial mammals, especially desert rodents , appear to survive without drinking, but they do generate water through the metabolism of cereal seeds, and they also have mechanisms to conserve water to the maximum degree. Freshwater ecosystems are a subset of Earth's aquatic ecosystems . They include lakes , ponds , rivers , streams , springs , bogs , and wetlands . They can be contrasted with marine ecosystems , which have

2992-412: The ingress of saline water over Chester Weir during high tides. The operating rules for the current system are agreed with all the participants and define the circumstances in which flow will be precisely managed. The key measuring location is at Manley Hall ( 52°57′58″N 2°58′16″W  /  52.966°N 2.971°W  / 52.966; -2.971  ( Manley Hall Gauging Station ) ),

3060-460: The lake some centuries earlier. Modern reports of sightings have been recorded since the 1920s. These include strange disruptions of the water's surface, disturbance of watercraft and the increase in local belief that a beast was living below the lake's surface. Of these sightings, the most notable was reported by the lake's manager Dowie Bowen in the 1970s. Bowen described seeing a crocodile-like creature, about eight feet (2.4 m) in length emerge from

3128-967: The lake's size (40 metres deep and almost 6 km long) and the abundance of potential prey within it (pike, perch, brown trout and eels) would be able to support a single large predator or even a breeding population. Freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids . The term excludes seawater and brackish water , but it does include non-salty mineral-rich waters , such as chalybeate springs. Fresh water may encompass frozen and meltwater in ice sheets , ice caps , glaciers , snowfields and icebergs , natural precipitations such as rainfall , snowfall , hail / sleet and graupel , and surface runoffs that form inland bodies of water such as wetlands , ponds , lakes , rivers , streams , as well as groundwater contained in aquifers , subterranean rivers and lakes . Water

3196-476: The level at its outflow is automatically controlled. Depending on flow conditions and the level of water in Llyn Celyn , water can flow either into or out of the lake at the normal outflow point. Controls on the level of water in the lake were first constructed around 1840. Sluices designed and built by Thomas Telford and William Jessop were installed to ensure that the newly constructed Ellesmere Canal had

3264-453: The locality and listed as critically endangered by the IUCN due to the introduction of the invasive and non native ruffe ; and the very rare mollusc Myxas glutinosa (the glutinous snail). According to legend, whilst the Dee itself flows through the lake, the waters never mix. However this was not confirmed by the detailed limnological work undertaken from the 1990s, to understand and manage

3332-420: The motor vehicle. The lake remains popular; it has two sailing clubs, and a number of companies provide kayaks , yachts and various other types of boats for hire. There was a belief in Wales that certain lakes are offended when mortals attempt to measure their depths. Two men went out in a boat to the deepest part of the lake to measure it with a plummet and line. They were angrily warned by the lake to return to

3400-467: The number of people suffering from water scarcity ." Another target, Target 15.1, is: "By 2020, ensure the conservation, restoration and sustainable use of terrestrial and inland freshwater ecosystems and their services, in particular forests, wetlands , mountains and drylands , in line with obligations under international agreements." Subnotes River Dee regulation system The water demands of North West England including Liverpool and

3468-556: The occurrence of algal blooms on the lake. In the 1990s the lake suffered from blooms of blue-green algae which indicated a significant and worrying eutrophication of the lake. Investigation by the Environment Agency , in partnership with the water industry, the farming community and others, has put in place a plan for reducing pollution inputs to the lake. The lake forms part of the River Dee regulation system and

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3536-413: The others as well. Water scarcity (closely related to water stress or water crisis) is the lack of fresh water resources to meet the standard water demand. There are two types of water scarcity. One is physical. The other is economic water scarcity . Physical water scarcity is where there is not enough water to meet all demands. This includes water needed for ecosystems to function. Regions with

3604-450: The scheme, some of the stored water was specifically set aside to make special releases to help fisheries , to provide recreational opportunities (canoeing and white-water rafting on the Afon Tryweryn ) and to disperse pollution events should they occur. A four megawatt hydro-electric station at the dam was also included in the scheme in 1965. Further statutory powers were gained in 1973 to construct another major regulating reservoir in

3672-421: The sea if windy conditions have lifted drops of seawater into the rain-bearing clouds. This can give rise to elevated concentrations of sodium , chloride , magnesium and sulfate as well as many other compounds in smaller concentrations. In desert areas, or areas with impoverished or dusty soils, rain-bearing winds can pick up sand and dust and this can be deposited elsewhere in precipitation and causing

3740-499: The shore or face destruction. The mythological figure Tegid Foel (English: Bald Tegid ) is associated with the lake. He was the husband of the goddess or witch Ceridwen and the place where his court stood is now beneath the waters of the lake. According to Taliesin , the 6th century CE early Brittonic poet of Sub-Roman Britain whose work survived in a Middle Welsh manuscript, the Book of Taliesin , Tegid Foel's entire court

3808-420: The stillwaters including ponds , lakes, swamps and mires ; lotic which are running-water systems; or groundwaters which flow in rocks and aquifers . There is, in addition, a zone which bridges between groundwater and lotic systems, which is the hyporheic zone , which underlies many larger rivers and can contain substantially more water than is seen in the open channel. It may also be in direct contact with

3876-666: The surface, and soil moisture, and less than 0.01% of it as surface water in lakes , swamps and rivers . Freshwater lakes contain about 87% of this fresh surface water, including 29% in the African Great Lakes , 22% in Lake Baikal in Russia, 21% in the North American Great Lakes , and 14% in other lakes. Swamps have most of the balance with only a small amount in rivers, most notably

3944-490: The time and remains politically fraught to this day , since the construction of the reservoir involved flooding the Tryweryn Valley and the village of Capel Celyn and twelve farms. Local people saw this a destruction of part of the Welsh culture in order to supply England with water. This caused a great deal of controversy, resentment and protest. To try to offset some of the environmental concerns associated with

4012-469: The time). Freshwater ecosystems contain 41% of the world's known fish species. The increase in the world population and the increase in per capita water use puts increasing strains on the finite resources availability of clean fresh water. The response by freshwater ecosystems to a changing climate can be described in terms of three interrelated components: water quality, water quantity or volume, and water timing. A change in one often leads to shifts in

4080-657: The underlying underground water. The original source of almost all fresh water is precipitation from the atmosphere , in the form of mist , rain and snow . Fresh water falling as mist, rain or snow contains materials dissolved from the atmosphere and material from the sea and land over which the rain bearing clouds have traveled. The precipitation leads eventually to the formation of water bodies that humans can use as sources of freshwater: ponds , lakes , rainfall , rivers , streams , and groundwater contained in underground aquifers . In coastal areas fresh water may contain significant concentrations of salts derived from

4148-416: The water. Bowen's report was soon followed by another sighting in 1979, when a fisherman visiting the lake also described seeing a large hump-backed beast at the water's surface. By the 1990s the various sightings had attracted film crews and investigators. One Japanese crew spent three days investigating and filming the lake using specialist diving equipment and a submarine. However, no substantial evidence for

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4216-407: Was completed in 1965 by Liverpool Corporation, and designed to operate in conjunction with the Bala Lake Scheme. This enables additional Dee abstractions of 327,000 cubic metres per day (3.78 m /s) together with additional flood control storage. In summer time the impact was to increase threefold the dry-weather flow for most of the length of the river. This development was hugely controversial at

4284-420: Was able to contain the worst of the winter flood peaks. As demand for water increased it was necessary to increase the storage on the River Dee and the next development was Llyn Celyn ( 52°57′00″N 3°41′35″W  /  52.95°N 3.693°W  / 52.95; -3.693  ( Llyn Celyn ) ), a new 81,000,000-cubic-metre capacity regulating reservoir within the Bala Lake catchment area. This

4352-489: Was also recorded by other English writers as "Pimble-mere" (in the 1813 Cambrian Travellers' Guide) and "Pemble Mere". The Modern Welsh name "Llyn Tegid" first appears in the Cronica Walliae , published in 1568. The name is translated by George Borrow as "Lake of Beauty", where the name "Tegid" derives from teg , a common Welsh place name element meaning "fair" (as in pleasant or fine scenery). The lake's name

4420-461: Was constant recharge with little variation with precipitation; in most sites (arid, semi-arid, humid), annual recharge increased as annual precipitation remained above a certain threshold; and in complex areas down to 0.1 aridity index (focused recharge), there was very inconsistent recharge (low precipitation but high recharge). Understanding these relationships can lead to the development of sustainable strategies for water collection. This understanding

4488-464: Was drowned in one night. Although in legend, its lights and the little town around it can still be seen on moonlit nights. The drowning of the court of Tegid Foel also survives in a variant folktale . Near the lake was a walled spring that had to be secured and locked every night with a lid so that the spring water would not be corrupted by supernatural influences. For whatever reason, the man responsible for this task neglected his duty, though some say

4556-560: Was formed in 1826. A project to review the pressures and opportunities for the Dee abstractions was undertaken and published in 1996. One of the outcomes of this report was the development and implementation of the River Dee Water Protection Zone. In order to better manage flow and the substantial abstractions of water from the river, a mathematical model was developed by the University of Lancaster and

4624-400: Was obtained from gauges below the major reservoirs and from the sluices at Bala Lake . With the benefit of frequent data updates and with the addition of data from real incidents together with real-time data from Manley Hall, it has become possible to predict arrival times of pollutants at any one point downstream of Manley Hall within a window of a few minutes at low flows. In the late 1950s

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