A storm surge, storm flood, tidal surge or storm tide is a coastal flood or tsunami-like phenomenon of rising water associated with low pressure weather systems, the severity of, affected by the shallowness and orientation of the water body relative to storm path, as well as the timing of tides. Most casualties during tropical cyclones occur, it is a measure of the rise of water beyond what would be expected by the normal movement related to tides. The two main meteorological factors contributing to a storm surge are a long fetch of winds spiraling inward toward the storm, a low-pressure-induced dome of water drawn up under and trailing the storm's center; the deadliest storm surge on record was the 1970 Bhola cyclone, which killed up to 500,000 people in the area of the Bay of Bengal. The low-lying coast of the Bay of Bengal is vulnerable to surges caused by tropical cyclones; the deadliest storm surge in the twenty-first century was caused by the Cyclone Nargis, which killed more than 138,000 people in Myanmar in May 2008.
The next deadliest in this century was caused by the Typhoon Haiyan, which killed more than 6,000 people in the central Philippines in 2013 and resulted in economic losses estimated at $14 billion. The Galveston Hurricane of 1900, a Category 4 hurricane that struck Galveston, drove a devastating surge ashore; the highest storm tide noted in historical accounts was produced by the 1899 Cyclone Mahina, estimated at 44 ft at Bathurst Bay, but research published in 2000 concluded that the majority of this was wave run-up because of the steep coastal topography. In the United States, one of the greatest recorded storm surges was generated by Hurricane Katrina in 2005, which produced a maximum storm surge of more than 25 ft in southern Mississippi, with a storm surge height of 27.8 ft in Pass Christian. Another record storm surge occurred in this same area from Hurricane Camille in 1969, with a storm tide of 24.6 ft at Pass Christian. A storm surge of 14 ft occurred in New York City during Hurricane Sandy in October 2012.
At least five processes can be involved in altering tide levels during storms: The atmospheric pressure effect The direct wind effect The effect of the Earth's rotation The effect of waves near the shore The rainfall effect. The pressure effects of a tropical cyclone will cause the water level in the open ocean to rise in regions of low atmospheric pressure and fall in regions of high atmospheric pressure; the rising water level will counteract the low atmospheric pressure such that the total pressure at some plane beneath the water surface remains constant. This effect is estimated at a 10 mm increase in sea level for every millibar drop in atmospheric pressure. Strong surface winds cause surface currents at a 45° angle to the wind direction, by an effect known as the Ekman Spiral. Wind stresses cause a phenomenon referred to as "wind set-up", the tendency for water levels to increase at the downwind shore and to decrease at the upwind shore. Intuitively, this is caused by the storm blowing the water toward one side of the basin in the direction of its winds.
Because the Ekman Spiral effects spread vertically through the water, the effect is proportional to depth. The pressure effect and the wind set-up on an open coast will be driven into bays in the same way as the astronomical tide; the Earth's rotation causes the Coriolis effect, which bends currents to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. When this bend brings the currents into more perpendicular contact with the shore, it can amplify the surge, when it bends the current away from the shore it has the effect of lessening the surge; the effect of waves, while directly powered by the wind, is distinct from a storm's wind-powered currents. Powerful wind whips up strong waves in the direction of its movement. Although these surface waves are responsible for little water transport in open water, they may be responsible for significant transport near the shore; when waves are breaking on a line more or less parallel to the beach, they carry considerable water shoreward.
As they break, the water particles moving toward the shore have considerable momentum and may run up a sloping beach to an elevation above the mean water line, which may exceed twice the wave height before breaking. The rainfall effect is experienced predominantly in estuaries. Hurricanes may dump as much as 12 in of rainfall in 24 hours over large areas and higher rainfall densities in localized areas; as a result, surface runoff can flood Streams and rivers. This can increase the water level near the head of tidal estuaries as storm-driven waters surging in from the ocean meet rainfall flowing downstream into the estuary. In addition to the above processes and wave heights on shore are affected by the flow of water over the underlying topography, i.e. the configuration and bathymetry of the ocean bottom and affected coastal area. A narrow shelf, for example, or one that has a steep drop from the shoreline and subsequently produces deep water in proximity to the shoreline, tends to produce a lower surge but a higher and more powerful wave.
This situation is well exemplified by the southeast coast of Florida. The edge of the Floridian Plateau, where the water depths reach 91 metres, lies just 3,000 m offshore of Palm Beach; the 180 m depth contour followed southward from Palm Beach County
Föhr pronunciation is one of the North Frisian Islands on the German coast of the North Sea. It is part of the Nordfriesland district in the federal state of Schleswig-Holstein. Föhr is a popular destination for tourists. A town and eleven distinct municipalities are located on the island; the climate is oceanic with moderate winters and cool summers. Being a settlement area in neolithic times, Föhr had been part of mainland North Frisia until 1362; the coastline was destroyed by a heavy storm flood and several islands were formed, Föhr among them. The northern parts of Föhr consist of marshes. From the middle-ages until 1864, Föhr belonged to the Danish realm and to the Duchy of Schleswig, but was transferred to Prussia as a result of the Second Schleswig War. Seafaring has long been the most popular trade, but farming and tourism became the most important economic factors after the end of the Age of Sail; the island can be reached via an airstrip. Apart from German, a dialect of the North Frisian language, Fering, is spoken on Föhr.
Several authors and poets have written in Fering. Föhr is situated southeast of Sylt. Among those German islands which are accessible only by ship or airplane Föhr is the most populous and has the largest surface. Föhr is called "The Green Island" due to being sheltered from the storms of the North Sea by its neighbouring islands Sylt and Amrum, so that Föhr's vegetation is thriving compared to other islands, it is 12 km long. While the northern parts are marshland, the south consists of higher geestland; the highest elevation measures 13 m above mean sea level and is located on the geestland ridge between the villages of Nieblum and Midlum. The geest makes up about two fifths of Föhr's total area and most villages are located there. In the marshlands, a number of solitary farmsteads can be found, which were moved out of the villages during the 1960s; until the Grote Mandrenke flooding in 1362, Föhr had been part of the mainland, connected by deep tidal creeks. Föhr, like its neighbour islands, is a popular tourist resort.
From the ferry terminal a sandy beach of about 15 km length extends all along Föhr's southern shore and halfway up the western coast. North and northwest of Föhr the Reserved Area I of the Schleswig-Holstein Wadden Sea National Park is located. Föhr's population counts 8,592; the only town on the island is Wyk on its south-eastern coast, a popular seaside resort. In addition there are sixteen small villages on Föhr which are distributed among eleven municipalities, they adhere to the Amt Föhr-Amrum: Alkersum Borgsum Dunsum, comprising Lesser and Greater Dunsum Midlum Nordseebad Nieblum with its neighbourhood Goting Oevenum Oldsum with the districts of Toftum and Klintum Süderende Nordseebad Utersum with the hamlet of Hedehusum Witsum Wrixum A local peculiarity is that all place names end with the suffix -um, which means "home". Föhr features a moderate oceanic climate; the beneficial effects of the local climate and seawater on certain medical conditions inspired the physician Carl Haeberlin from Wyk to develop treatments for climatotherapy and thalassotherapy at the beginning of the 20th century.
He became the pioneer of these disciplines in Germany. The higher geestland cores of the North Frisian islands, scattered between ample marshlands, attracted settlers when the sea level rose at the end of the Neolithicum. Gravesites and several minor artifacts found on Föhr bear witness to this; when the Frisians colonised the area of modern Nordfriesland during the 7th century, their first settlements were erected on Föhr, according to archaeological findings. The sparsely inhabited island witnessed a steep rise of population. A rather large amount of jewellery originating from Scandinavia, found in graves of the time points out a vivid connection to northern Europe. From the age of the Vikings, several ring walls, the Lembecksburg among them, are preserved; the Danish Census Book of King Valdemar II of Denmark tells of two Harden on Föhr, which were territorial subdivisions of the time. The Westerharde Föhr was at times the refuge of a pirate serving the Danish. In 1368 the Westerharde, which included Amrum, was transferred to the Counts of Holstein under the supervision of the knight Klaus Lembeck, bailiff of Ribe.
In 1400 the Harde remained within Ribe County. Until 1864 the western part of Föhr, together with Amrum, belonged to the Danish Enclaves in North Frisia while Osterland and Wyk belonged to the Duchy of Schleswig since it had seceded from the Danish Kingdom in the 1420s. Together with the Wiedingharde, the Bökingharde, the isle of Strand and Sylt, Osterland in 1426 signed the "Compact of the Seven Hundreds" with Duke Henry IV of Schleswig, which stated that the Hundreds intended to keep their judicial autonomy. In 1523 the northern marshlands of Föhr were shut off against the sea by dikes and 22 hectacres of new farming land were won. Beginning in 1526, the Protestant Reformation began to introduce the Lutheran confession on Föhr, completed in 1530. In the 17th century a private navigation school was established in Süderende by pastor Richardus Petri, the first of its kind on the island, it improved the situation of the seafaring population and soon other navi
Sylt is an island in northern Germany, part of Nordfriesland district, Schleswig-Holstein, well known for the distinctive shape of its shoreline. It is the largest island in North Frisia; the northernmost island of Germany, it is known for its tourist resorts, notably Westerland and Wenningstedt-Braderup, as well as for its 40-kilometre-long sandy beach. It is covered by the media in connection with its exposed situation in the North Sea and its ongoing loss of land during storm tides. Since 1927, Sylt has been connected to the mainland by the Hindenburgdamm causeway. In latter years, it has been a resort for the German jet set and tourists in search of occasional celebrity sightings. With 99.14 square kilometres, Sylt is the fourth-largest German island and the largest German island in the North Sea. Sylt is located from 9 to 16 kilometres off the mainland, to which it is connected by the Hindenburgdamm. Southeast of Sylt are the islands of Föhr and Amrum, to the north lies the Danish island of Rømø.
The island of Sylt extends for 38 kilometres in a north-south direction. At its northern point at Königshafen, it is only 320 metres wide, its greatest width, from the town of Westerland in the west to the eastern Nössespitze near Morsum, measures 12.6 kilometres. On the western and northwestern shore, there is a 40-kilometre-long sandy beach. To the east of Sylt, is the Wadden Sea, which belongs to the Schleswig-Holstein Wadden Sea National Park and falls dry during low tide; the island's shape has shifted over time, a process, still ongoing today. The northern and southern spits of Sylt are made up of infertile sand deposits, while the central part with the municipalities of Westerland, Wenningstedt-Braderup and Sylt-Ost consists of a geestland core, which becomes apparent in the form of the Red Cliff of Wenningstedt; the geestland facing the Wadden Sea turns into fertile marshland around Sylt-Ost. Sylt has only been an island since the Grote Mandrenke flood of 1362; the so-called Uwe-Düne is the island's highest elevation with 52.5 metres above sea level.
The island in its current form has only existed for about 400 years. Like the mainland geestland, it was formed of moraines from the older ice ages, thus being made up of a till core, now apparent in the island's west and centre by the cliff and beach; this sandy core began to erode as it was exposed to a strong current along the island's steep basement when the sea level rose 8000 years ago. During the process, sediments were accumulated south of the island; the west coast, situated 10 kilometres off today's shore, was thus moved eastward, while at the same time the island began to extend to the north and south. After the ice ages, marshland began to form around this geestland core. In 1141, Sylt is recorded as an island, yet before the Grote Mandrenke flood it belonged to a landscape cut by tidal creeks and, at least during low tide, it could be reached on foot, it is only since this flood that the creation of a spit from sediments began to form the current characteristic shape of Sylt. It is the northern and southern edges of Sylt which were, still are, the subject of greatest change.
For example, Listland was separated from the rest of the island in the 14th century and from the 17th century onwards the Königshafen began to silt up as the "elbow" spit began to form. In addition to the constant loss of land, the inhabitants during the Little Ice Age were constrained by sand drift. Dunes shifting to the east threatened settlements and arable land and had to be stopped by the planting of marram grass in the 18th century. Though, material breaking off the island was washed away and the island's extent continued to decrease. Records of the annual land loss exist since 1870. According to them, Sylt lost an annual 0.4 metres of land in the north and 0.7 metres in the south from 1870 to 1951. From 1951 to 1984, the rate increased to 0.9 metres and 1.4 metres while shorelines at the island's edges at Hörnum and List are more affected. Severe storm surges of the last decades have endangered Sylt to the point of breaking in two, e.g. Hörnum was temporarily cut off from the island in 1962.
Part of the island near Rantum, only 500m wide is threatened. Measures of protection against the continuous erosion date back to the early 19th century when groynes of wooden poles were constructed; those were built at right angles into the sea from the coast line. They were replaced by metal and by armoured concrete groynes; the constructions did not have the desired effect of stopping the erosion caused by crossways currents. "Leeward erosion", i.e. erosion on the downwind side of the groynes prevented sustainable accumulation of sand. In the 1960s breaking the power of the sea was attempted by installing tetrapods along the groyne bases or by putting them into the sea like groynes; the four-armed structures, built in France and many tons in weight, were too heavy for Sylt's beaches and were unable to prevent erosion. Therefore, they were removed from the Hörnum west beach in 2005. Since the early 1970s the only effective means so far has been flushing sand onto the shore. Dredging vessels are used to pump a mixture of sand and water to a beach where it is spread by bulldozers.
Thus storm floods would only erase the artificial accumulation of sand, while the shoreline proper remains intact and erosion is slowed down. Thi
A moraine is any glacially formed accumulation of unconsolidated glacial debris that occurs in both and glaciated regions on Earth, through geomorphological processes. Moraines are formed from debris carried along by a glacier and consisting of somewhat rounded particles ranging in size from large boulders to minute glacial flour. Lateral moraines are formed at the side of the ice flow and terminal moraines at the foot, marking the maximum advance of the glacier. Other types of moraine include ground moraines, till-covered areas with irregular topography, medial moraines which are formed where two glaciers meet. Moraines may be composed of debris ranging in size from silt-sized glacial flour to large boulders; the debris is sub-angular to rounded in shape. Moraines may be on the glacier’s surface or deposited as piles or sheets of debris where the glacier has melted. Moraines may form through a number of processes, depending on the characteristics of sediment, the dynamics on the ice, the location on the glacier in which the moraine is formed.
Moraine forming processes may be loosely divided into active. Passive processes involve the placing of chaotic supraglacial sediments onto the landscape with limited reworking forming hummocky moraines; these moraines are composed of supraglacial sediments from the ice surface. Active processes form or rework moraine sediment directly by the movement of ice, known as glaciotectonism; these form push moraines and thrust-block moraines, which are composed of till and reworked proglacial sediment. Moraine may form by the accumulation of sand and gravel deposits from glacial streams emanating from the ice margin; these fan deposits may coalesce to form a long moraine bank marking the ice margin. Several processes may combine to form and rework a single moraine, most moraines record a continuum of processes. Moraines can be classified either by origin, location with respect to a glacier or former glacier, or by shape; the first approach is suitable for moraines associated with contemporary glaciers—but more difficult to apply to old moraines, which are defined by their particular morphology, since their origin is debated.
Some moraine types are known only from ancient glaciers, while medial moraines of valley glaciers are poorly preserved and difficult to distinguish after the retreat or melting of the glacier. Lateral moraines are parallel ridges of debris deposited along the sides of a glacier; the unconsolidated debris can be deposited on top of the glacier by frost shattering of the valley walls and/or from tributary streams flowing into the valley. The till is carried along the glacial margin; because lateral moraines are deposited on top of the glacier, they do not experience the postglacial erosion of the valley floor and therefore, as the glacier melts, lateral moraines are preserved as high ridges. Lateral moraines stand high because they protect the ice under them from the elements, causing it to melt or sublime less than the uncovered parts of the glacier. Multiple lateral moraines may develop as the glacier retreats. Ground moraines are till-covered areas with irregular topography and no ridges forming rolling hills or plains.
They are accumulated at the base of the ice as lodgment till, but may be deposited as the glacier retreats. In alpine glaciers, ground moraines are found between the two lateral moraines. Ground moraines may be modified into drumlins by the overriding ice. Rogen moraines or ribbed moraines are a type of basal moraines that form a series of ribs perpendicular to the ice flow in an ice sheet; the depressions between the ribs are sometimes filled with water, making the Rogen moraines look like tigerstripes on aerial photographs. Rogen moraines are named after Lake Rogen in Härjedalen, the landform’s type locality. End moraines, or terminal moraines, are ridges of unconsolidated debris deposited at the snout or end of the glacier, they reflect the shape of the glacier's terminus. Glaciers act much like a conveyor belt, carrying debris from the top of the glacier to the bottom where it deposits it in end moraines. End moraine size and shape are determined by whether the glacier is advancing, receding or at equilibrium.
The longer the terminus of the glacier stays in one place, the more debris accumulate in the moraine. There are two types of end moraines: recessional. Terminal moraines mark the maximum advance of the glacier. Recessional moraines are small ridges left. After a glacier retreats, the end moraine may be destroyed by postglacial erosion. Recessional moraines are observed as a series of transverse ridges running across a valley behind a terminal moraine, they form perpendicular to the lateral moraines that they reside between and are composed of unconsolidated debris deposited by the glacier. They are created during temporary halts in a glacier's retreat. A medial moraine is a ridge of moraine, it forms when two glaciers meet and the debris on the edges of the adjacent valley sides join and are carried on top of the enlarged glacier. As the glacier melts or retreats, the debris is deposited and a ridge down the middle of the valley floor is created; the Kaskawulsh Glacier in the Kluane National Park, has a ridge of medial moraine 1 km wide.
Supraglacial moraines are created by debris accumulated on top of glacial ice. This debris can accumulate due to ice flow toward the surface in the ablation zone, melting of surface ice or from debris that falls onto the glacier from valley sidewalls. Washboard moraines known as minor or corrugated moraines, are low-amplitude ge
A marsh is a wetland, dominated by herbaceous rather than woody plant species. Marshes can be found at the edges of lakes and streams, where they form a transition between the aquatic and terrestrial ecosystems, they are dominated by grasses, rushes or reeds. If woody plants are present they tend to be low-growing shrubs; this form of vegetation is what differentiates marshes from other types of wetland such as swamps, which are dominated by trees, mires, which are wetlands that have accumulated deposits of acidic peat. Marshes provide a habitat for many species of plants and insects that have adapted to living in flooded conditions; the plants must be able to survive in wet mud with low oxygen levels. Many of these plants therefore have aerenchyma, channels within the stem that allow air to move from the leaves into the rooting zone. Marsh plants tend to have rhizomes for underground storage and reproduction. Familiar examples include cattails, sedges and sawgrass. Aquatic animals, from fish to salamanders, are able to live with a low amount of oxygen in the water.
Some can obtain oxygen from the air instead, while others can live indefinitely in conditions of low oxygen. Marshes provide habitats for many kinds of invertebrates, amphibians and aquatic mammals. Marshes have high levels of biological production, some of the highest in the world, therefore are important in supporting fisheries. Marshes improve water quality by acting as a sink to filter pollutants and sediment from the water that flows through them. Marshes are able to absorb water during periods of heavy rainfall and release it into waterways and therefore reduce the magnitude of flooding; the pH in marshes tends to be neutral to alkaline, as opposed to bogs, where peat accumulates under more acid conditions. Marshes differ depending on their location and salinity. Both of these factors influence the range and scope of animal and plant life that can survive and reproduce in these environments; the three main types of marsh are salt marshes, freshwater tidal marshes, freshwater marshes. These three can be found worldwide and each contains a different set of organisms.
Saltwater marshes are found around the world in mid to high latitudes, wherever there are sections of protected coastline. They are located close enough to the shoreline that the motion of the tides affects them, sporadically, they are covered with water, they flourish where the rate of sediment buildup is greater than the rate at which the land level is sinking. Salt marshes are dominated by specially adapted rooted vegetation salt-tolerant grasses. Salt marshes are most found in lagoons, on the sheltered side of shingle or sandspit; the currents there carry the fine particles around to the quiet side of the spit and sediment begins to build up. These locations allow the marshes to absorb the excess nutrients from the water running through them before they reach the oceans and estuaries; these marshes are declining. Coastal development and urban sprawl has caused significant loss of these essential habitats. Although considered a freshwater marsh, this form of marsh is affected by the ocean tides.
However, without the stresses of salinity at work in its saltwater counterpart, the diversity of the plants and animals that live in and use freshwater tidal marshes is much higher than in salt marshes. The most serious threats to this form of marsh are the increasing size and pollution of the cities surrounding them. Ranging in both size and geographic location, freshwater marshes make up the most common form of wetland in North America, they are the most diverse of the three types of marsh. Some examples of freshwater marsh types in North America are: Wet meadows occur in areas such as shallow lake basins, low-lying depressions, the land between shallow marshes and upland areas, they occur on the edges of large lakes and rivers. Wet meadows have high plant diversity and high densities of buried seeds, they are flooded but are dry in the summer. Vernal pools are a type of marsh found only seasonally in shallow depressions in the land, they can be covered in shallow water, but in the summer and fall, they can be dry.
In western North America, vernal pools tend to form in open grasslands, whereas in the east they occur in forested landscapes. Further south, vernal pools form in pine flatwoods. Many amphibian species depend upon vernal pools for spring breeding. An example is the endangered gopher frog. Similar temporary ponds occur in other world ecosystems. However, the term vernal pool can be applied to all such temporary pool ecosystems. Playa lakes are a form of shallow freshwater marsh that occurs in the southern high plains of the United States. Like vernal pools, they are only present at certain times of the year and have a circular shape; as the playa dries during the summer, conspicuous plant zonation develops along the shoreline. Prairie potholes are found in the northern parts of North America as the Prairie Pothole Region; these landscapes were once covered by glaciers, as a result shallow depressions were formed in great numbers. These depressions fill with water in the spring, they provide important breeding habitats for many species of waterfowl.
Some pools only occur seasonally. Many kinds of marsh occur along the fringes of large rivers; the different types are produced by factors such as water level, ice scour, waves. Large tracts of marshland have been embanked and ar
The Elbe is one of the major rivers of Central Europe. It rises in the Krkonoše Mountains of the northern Czech Republic before traversing much of Bohemia Germany and flowing into the North Sea at Cuxhaven, 110 km northwest of Hamburg, its total length is 1,094 kilometres. The Elbe's major tributaries include the rivers Vltava, Havel, Schwarze Elster, Ohře; the Elbe river basin, comprising the Elbe and its tributaries, has a catchment area of 148,268 square kilometres, the fourth largest in Europe. The basin spans four countries, with its largest parts in the Czech Republic. Much smaller parts lie in Poland; the basin is inhabited by 24.4 million people. The Elbe rises at an elevation of about 1,400 metres in the Krkonoše on the northwest borders of the Czech Republic near Labská bouda. Of the numerous small streams whose waters compose the infant river, the most important is the Bílé Labe, or White Elbe. After plunging down the 60 metres of the Labský vodopád, or Elbe Falls, the latter stream unites with the steeply torrential Malé Labe, thereafter the united stream of the Elbe pursues a southerly course, emerging from the mountain glens at Jaroměř, where it receives Úpa and Metuje.
Here the Elbe enters the vast vale named Polabí, continues on southwards through Hradec Králové and to Pardubice, where it turns to the west. At Kolín some 43 kilometres further on, it bends towards the north-west. At the village of Káraný, a little above Brandýs nad Labem, it picks up the Jizera. At Mělník its stream is more than doubled in volume by the Vltava, or Moldau, a major river which winds northwards through Bohemia. Upstream from the confluence the Vltava is in fact much longer, has a greater discharge and a larger drainage basin. Nonetheless, for historical reasons the river retains the name Elbe because at the confluence point it is the Elbe that flows through the main, wider valley while the Vltava flows into the valley to meet the Elbe at a right angle, thus appears to be the tributary river; some distance lower down, at Litoměřice, the waters of the Elbe are tinted by the reddish Ohře. Thus augmented, swollen into a stream 140 metres wide, the Elbe carves a path through the basaltic mass of the České Středohoří, churning its way through a picturesque, deep and curved rocky gorge.
Shortly after crossing the Czech-German frontier, passing through the sandstone defiles of the Elbe Sandstone Mountains, the stream assumes a north-westerly direction, which on the whole it preserves right to the North Sea. The river rolls through Dresden and beyond Meißen, enters on its long journey across the North German Plain passing along the former western border of East Germany, touching Torgau, Dessau, Magdeburg and Hamburg on the way, taking on the waters of the Mulde and Saale from the west, those of the Schwarze Elster and Elde from the east. In its northern section both banks of the Elbe are characterised by flat fertile marshlands, former flood plains of the Elbe now diked. At Magdeburg there is a viaduct, the Magdeburg Water Bridge, that carries a canal and its shipping traffic over the Elbe and its banks, allowing shipping traffic to pass under it unhindered. From the sluice of Geesthacht on downstream the Elbe is subject to the tides, the tidal Elbe section is called the Low Elbe.
Soon the Elbe reaches Hamburg. Within the city-state the Unterelbe has a number of branch streams, such as Dove Elbe, Gose Elbe, Köhlbrand, Northern Elbe, Southern Elbe; some of which have been disconnected for vessels from the main stream by dikes. In 1390 the Gose Elbe was separated from the main stream by a dike connecting the two then-islands of Kirchwerder and Neuengamme; the Dove Elbe was diked off in 1437/38 at Gammer Ort. These hydraulic engineering works were carried out to protect marshlands from inundation, to improve the water supply of the Port of Hamburg. After the heavy inundation by the North Sea flood of 1962 the western section of the Southern Elbe was separated, becoming the Old Southern Elbe, while the waters of the eastern Southern Elbe now merge into the Köhlbrand, bridged by the Köhlbrandbrücke, the last bridge over the Elbe before the North Sea; the Northern Elbe passes the Elbe Philharmonic Hall and is crossed under by the old Elbe Tunnel, both in Hamburg's city centre.
A bit more downstream the Low Elbe's two main anabranches Northern Elbe and the Köhlbrand reunite south of Altona-Altstadt, a locality of Hamburg. Right after both anabranches reunited the Low Elbe is passed under by the New Elbe Tunnel, the last structural road link crossing the river before the North Sea. At the bay Mühlenberger Loch in Hamburg at kilometre 634, the Northern Elbe and the Southern Elbe used to reunite, why the bay is seen as the starting point of the Lower Elbe. Leaving the city-state the Lower Elbe passes between Holstein and the Elbe-Weser Triangle with Stade until it flows into the North Sea at Cuxhaven. Near its mouth it passes the entrance to the Kiel Canal at Brunsbüttel before it debouches into the North Sea; the Elbe has been navigable by commercial ve
The Cloppenburg Geest is a geest region near the town of Cloppenburg in North Germany and the centre of the Saalian glaciation Upper Pleistocene terrain of the Ems-Hunte Geest region. The meltwater sands of the advancing ice sheet covered the old terrain with outwash sands. Woldstedt spoke about underlying sands that, in the "Cloppenburg-Bassum Geest", belonged to the Elster glaciation. A covering of boulder clay was deposited over the outwash sands during the Saale glaciation, or more the Drenthe stage. A series of meltwater valleys characterises the surface of the Cloppenburg Geest, something, vital to the emergence of the river network. "Numerous parallel, flat channels cross the terrain and so create a landscape of parallel ridges" writes Woldstedt. There are two opposing theories for the formation of the rivers. Hausfeld put their emergence down to large cracks in the Drenthe ice sheet, through which meltwaters flowed as the glacier thawed, cutting through the ground moraines and down into the outwash sands.
Woldstedt spoke of channels in another connexion. The advancing ice followed the depth contours and deepening them; when the ice sheet retreated, dead-ice remained deep in these channels. During the marine regression of the Weichselian glaciation that ended about 12,000 years ago, in which the northwest German plain was not covered by ice, the rivers of the Cloppenburg Geest cut into the valley sands. At that time the windborne and dune sands were formed as the area around the perimeter dried out; the climate of the post-glacial period was warmer. The rise in sea level, the base level for river erosion led to a rise in the water table in the geest depression. In the valleys vast fen peats formed, whilst on the valley edges and the larger basins raised bogs were formed. R. Hausfeld: Die Entwicklung der Hümmlingsbäche und ihre heutige Pflanzenwelt. Jahrbuch des Emsländischen Heimatbundes, Bd. 29, 1983, S.244-266. R. Hausfeld: Die Vegetation nordwest-niedersächsischer Bachtäler in Abhängigkeit von landwirtschaftlicher Nutzung und wasserbaulichen Eingriffen.
In: Fließgewässer und ihr Einzugsgebiet. Hrsg.: Biologische Schutzgemeinschaft Hunte Weser-Ems. Wardenburg: BSH-Vlg. 1984, S.137-170.. R. Hausfeld: Das Markatal, eines der letzten naturnahen erhaltenen Bachtäler Nordwestdeutschlands. Jahrbuch für das Oldenbuger Münsterland, 1984, S.184-207. G. Roeschmann: Die Böden der nordwestdeutschen Geestlandschaft. Mitteilungen der Deutschen Bodenkundlichen Gesellschaft, Bd. 13, H.1, 1971, S.155-231. P. Woldstedt: Die Geschichte des Flußnetzes in Norddeutschland und angrenzende Gebiete. Eiszeitalter und Gegenwart, Bd. 7, 1956, S.5-12. P. Woldstedt: Nordwestdeutschland und angrenzende Gebiete im Eiszeitalter. Stuttgart 1955.. Heinz-Josef Lücking: Ökologische Bewertung des Soestetals zwischen Cloppenburg und Stedingsmühlen aus der Sicht des Naturschutzes unter besonderer Berücksichtigung der Vegetation, Gewässergüte und des ökomorphologischen Gewässerzustandes. BSH/NVN naturspecialREPORT 1995, ISBN 3-923788-29-0 Heft 21. Diplomarbeit im Fach Geographie an der Justus-Liebig-Universität, Gießen, 1992