Editing Aller (Germany) (section)

== Hydrology ==
The [[river]] is the biggest tributary of the Weser. The [[drainage basin|catchment area]] of the Aller covers {{convert|15744|km2|sqmi}}, about one third that of the Weser itself. The water flow gauge at Rethem below the Aller's confluence with the Leine registers an average volumetric flow of {{convert|114|m3|cuft}} per second.

The Aller receives via the [[Oker]] about half the run off from the [[Harz mountains]] (the other half drains into the Elbe via the [[Saale]] and other tributaries). Consequently, the Aller was and is frequently flooded by its tributaries. Since the commissioning of the [[dam]]s in the Harz, it is now possible to limit the impact, especially of seasonal floods, e.g. by delaying and slowing down the release of meltwaters on the Oker and [[Ecker]], the [[Innerste]] and the [[Grane (river)|Grane]] in spring. As a result, the discharge of the Aller into the Weser can be largely controlled and, although the level varies, it can be kept within boundaries.

Very little of the Aller is shaded by woods (apart from the section along the [[Aller Canal]]). Direct sunlight on much of the river encourages the growth of [[aquatic plant]]s and, in the absence of shady trees along its banks, the water becomes quite warm during the summer months.

=== Water velocity ===
The water velocity of the Aller is very slow in places. In its upper reaches, at the height of the [[Drömling]], the water compares to a sluggish canal and, in some sections, it is almost like a stretch of still water. Its sluggishness is due to the very gradual slope of the channel, which in Lower Saxony averages a mere {{convert|10|–|20|cm/km}}. Even in the upper course of the river, the flow across the whole of the stream bed is uniformly slow, due to the river having been widened and straightened. Here, the bed is covered by a mixture of sand and mud.

=== Sewage ===
[[Sewage]] from about half a million people is piped into the upper reaches of the Aller between its source and Müden/Aller, both directly and via its tributaries. The sewage is treated by 40 large [[sewage plant]]s. Sewage from the town of Wolfsburg, the only city along the river, is not discharged into the river, but dispersed by [[sewage farm]]s. The chemical composition of the Aller shows that the sewage discharged into it has generally been treated sufficiently well.

=== Heavy metal pollution ===
The pollution of the Aller by [[heavy metal (chemistry)|heavy metal]]s has resulted from centuries of [[Mining in the Upper Harz|mining in the Harz mountains]]. Harz rivers, such as the Aller tributary of the [[Oker]], picked up heavy metals from the mines and their spoil heaps. These were mainly cadmium, zinc and lead compounds that were concentrated into the [[suspended solids]] in the river. In this way, these chemicals were transported by the Oker into the Aller and Weser where they were deposited in the slacker-moving sections as [[sediment]]s.

In 1999, investigations were carried out into the heavy metal content of the suspended material in the Aller near Verden. They showed that the Aller was polluting the Weser disproportionally with [[lead]], [[cadmium]], [[zinc]] and [[mercury (element)|mercury]].<ref>Dieter Steffen: ''Schwermetallfrachten der Aller und deren Auswirkung auf die Weser-Bilanzierung auf der Basis von Schwebstoffuntersuchungen des Jahres 1999''</ref>

=== Water quality ===
The Lower Saxon water quality report <ref>''Gewässergütebericht Aller / Quelle 2004'' des [[Niedersächsischen Landesbetriebs für Wasserwirtschaft, Küsten- und Naturschutz]]</ref> of 2004 assessed the chemical [[water pollution]] level of the Aller overall as moderately polluted (quality level II). Some individual indicators were rated as quality level I (unpolluted to very lightly polluted), some however showed heavy levels of pollution (quality levels III-IV). Heavy pollution is predominantly caused by [[nitrate]]s. Pollution by [[ammonium]] in places is ascribed to the unsatisfactory operation of certain sewage works. Raised [[phosphate]] values were put down to intensive arable farming near the source of the river. Its [[salt]] content today is above the critical level for [[aquatic plant]]s. Since 1990, the salt content has reduced considerably, which may be connected with lower discharges from the former [[East German]] area following the ''[[Die Wende|Wende]]'' in 1989. Biological [[Freshwater environmental quality parameters|water quality]], based on the research into its [[Saprobe|saprobian systems]], is generally rated as unpolluted to very lightly polluted. The only area that is critically contaminated is the region of Wolfsburg, where deposits of digested [[sludge]] in the river are responsible.

=== Flooding and historical flood protection ===
[[File:Droemling Hochwasser-02.jpg|thumb|Spring flood in 1987 in the [[Drömling]]: the Aller at the level of the poplar avenue]]
[[File:Aller Ohre Kanal.jpg|thumb|Start of the Aller spillway (right) near Grafhorst leading to the [[Ohre]]]]
In former times, there were frequently spring floods in the Aller depression that stayed for a long time. This was mainly due to the very gentle slope of the river from its entry into the ice age glacial drainage channel of the Breslau-Magdeburg-Bremen [[glacial valley]]. The river had to accept large quantities of water from the rivers [[Leine]] and [[Oker]] following [[snowmelt]] in the [[Leine Uplands]] and the [[Harz]]. In the upper courses the Aller filled the flat, swampy basin of the [[Drömling]] from which the water drained only slowly. In addition the [[Ohre]] river flowed diffusely through the Drömling. Because the [[Drainage divide|watershed]] between [[Weser]] and [[Elbe]] lay in the middle of the Drömling, the river could even change its course so that the waters of the Aller flowed into the Elbe. The [[Prussia]]n king, [[Frederick the Great]], had the Prussian-owned eastern part of the Drömling drained between 1780 and 1796 and cleared for settlers (''Kolonisten''). His neighbours in western Drömling, in the shape of the [[Duchy of Brunswick]] and [[Electorate of Brunswick-Lüneburg|Hanover]] did not join the drainage project and only began to drain and control the Aller from 1860 onwards. They built the {{convert|20|km|mi|adj=mid|-long}} Aller Canal from a place near [[Grafhorst]] to the area of [[Calvörde]], through which the waters of the Aller could flow into the Ohre. Another canal built to prevent floods was the{{snd}}equally named{{snd}}[[Aller Canal]], finished in 1863, which protected the Aller depression near [[Gifhorn]]. The construction of the [[Mittelland Canal]] in the 1930s enabled surplus water from the Aller to be drained off. That is achieved near Grafhorst by the Aller relief channel, a {{convert|3|km|mi|adj=mid|-long}} canal. In spite of these measures there were floods in the Aller valley even in the 20th century that led to heavy losses for agriculture in the region.

=== Current flood protection ===
In the years 1954–1962, there was increased flooding along the Aller, which caused considerable damage to its water meadows. The flooded areas in the Aller valley varied between {{convert|300|and|5000|m|ft}} wide, mainly due to the uneven flow of the river. In 1961, the [[Landtag of Lower Saxony]] decided to regulate the Aller by widening it. The aim was to protect settlements as well as the agricultural economy of the villages and towns along the course of the river which were generally poorly protected from flooding. Most of the measures to regulate the Aller were established in the 1960s. They were followed by the construction of a {{convert|15|ha|acre|adj=on}} [[retention basin]] near Gifhorn in the 1970s. By contrast, plans for a retention basin on the bend in the Aller near [[Grafhorst]] were scrutinised between 1993 and 1996 on environmental grounds.<ref>
[http://www.wpk-gbr.de/allerknie.htm Allernknie] {{webarchive|url=https://web.archive.org/web/20050824151819/http://www.wpk-gbr.de/allerknie.htm |date=2005-08-24 }}</ref> The planned {{convert|12.5|km2|sqmi|adj=on}} "Fahle Heide" basin west of Gifhorn was never built due to lack of funding. As part of the widening of the Aller the banks were reshaped along large stretches. The state purchased strips of land up to {{convert|12|m|ft}} wide from those living along its banks. The banks were made flatter and lined with stones. Willows, alder and reed beds were planted. The Upper Aller was widened to {{convert|15|m|ft}}, the Middle Aller to {{convert|30|m|ft}} and the Lower Aller from its confluence with the Leine to {{convert|50|m|ft}}. Flood embankments were built along the Lower Aller near [[Rethem (Aller)]], [[Westen (Dörverden)|Westen]] and [[Häuslingen]].

==== Flood protection in Celle region ====
[[File:AllerAltencelle.jpg|thumb|The canal-like widened Middle Aller near [[Altencelle]]]]
As early as the 1980s the town of Celle had drawn up a "Plan for Flood Protection of the Celle Region".<ref>[http://www.celle.de/media/custom/342_6602_1.PDF Rahmenentwurf zum Hochwasserschutz der Region Celle]</ref> This described a combination of widely varying measures, such as excavation of the river shore, flood basins and dykes. On 6 June 2005 the town received authority to start the first stage of the flood protection project. In 2006 work was begun.<ref>[http://www.nlwkn.niedersachsen.de/master/C14994627_L20_D0_I5231158_h1.html NLWKN: Start der Hochwasserschutzprojekte in Celle]{{dead link|date=July 2017 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> This first part of the work entailed excavation of the river shore between Boye and the confluence with the river Fuhse, which were intended to increase the cross-section of the Aller channel during high water periods.<ref>[http://cdl.niedersachsen.de/blob/images/C17829250_L20.pdf Übersichtskarte der Vorlandabgrabungen in der Region Celle]{{dead link|date=July 2017 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> The flood basins have an area of about {{convert|15|ha|acre}} and are {{convert|1.5|m|ft}} deep, requiring about {{convert|240000|m3|ft3}} of earth to be washed away. The cost of the project came to around {{Euro|2{{nbsp}}million}}. Only two years later, it was determined that these first interventions of the river system had resulted in sand being permanently deposited in the Aller producing shoals. In May 2009, the shipping channel was dredged for the first time in the vicinity of the new flood basins so that ships could once again pass through. Now,{{when? |date=November 2024}} it is planned to redesign the basins and, in some cases to fill them in, in order to reduce the future dumping of sand.

=== Negotiability for fish ===
In straightening the Aller in the 1960s the slope increased as the shortest route was taken. Ledges (''Sohlabstürze'') were built on the river bed to reduce erosion; these have since been converted to rock ramps (''Sohlgleiten'') in order to enable water organisms to pass more easily. There is no longer a barrier to fish at the [[weir]] near [[Grafhorst]]—designed to raise the water level during summer droughts—since it has been given a [[fish ladder]]. The migration of fish is however prevented by weirs near Gifhorn and [[Müden (Aller)]] and the locks of the [[Mittelland Canal]] near Wolfsburg-[[Wendschott]]. The [[Elbe Lateral Canal]] near [[Osloß]] flows freely under the Aller, however.
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