All things environmental

Musings about the environment and all it touches, from education to city planning

Portrait of the Alster, an urban river (part 3)

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The Fuhlsbüttel dam on the Alster, with the meander-style fish ladder on the left

Part three: the fish stream

I love the Alster, Hamburg’s urban river.  But I wouldn’t have wanted to be there in late July 2018.  There was a large fish kill; by Fuhlsbüttel dam, hundreds of carcasses of pike, tench and other fish were floating belly up, stinking.  This was not a result of toxic pollution.  Simply, the weather had been dry and hot, the Alster and its tributaries had slowed down, warmed up, and become somewhat stagnant.  Fast flowing, cool waters are rich in dissolved oxygen; stagnant water, not.  The level of dissolved oxygen dipped below 4 parts per million in the main stem, even lower in tributaries like the Tarpenbek.  The fish, basically, suffocated. City worked armed with hooks and nets removed about five tonnes of dead fish. And it wasn’t just the Alster, it wasn’t just Hamburg; this scenario repeated itself in small streams throughout northwest Germany.

(And if that wasn’t embarrassing enough, Hamburg’s triathlon, the world’s largest, had to be cancelled.  The swim segment take place in the lake, but the warm stagnant waters had been invaded by a bloom of blue-green algae.)

Hamburg climatologists have warned that climate change is producing warmer and drier summers. The fish kill is an instance of what the city can expect to happen more often.  Quite a challenge.

A map of the Alster watershed. The river is in blue, city limits dotted line, the built-up area in grey, and the combined sewer outlets in red

It may be just a temporary setback, though, because the quality of fish habitat in Hamburg has remarkably improved since the 80’s.

Much of the deterioration of the water quality can be traced to the development of the sewer system: from 100 kilometers of pipe in 1865, there was almost 5000 kilometers by 1985.  Much of the older pipes discharged directly into the Alster and its tributaries.  Already in the late 19th century the lake had become green with slimy algae, and the invasive pondweed Elodea Canadensis was impeding barge traffic.  But the completion of the sewerage net by the late 80s finally improved the situation: the new main sewers intercept the sewage before it flows into the Alster system, sending it to the central treatment plant.  It is only when there is a deluge that this works only partially with the excess overflowing into the lake and river.  The city has also built 15 very large sewage storage tanks to address this issue.  As a result, the water quality is now much better.  Mayflies and other critters that require good water quality have reappeared.  To ensure that this trend continues, the city has installed nine automated water quality monitoring stations that the public can access online in real time – Hamburg believes in transparency and accountability.

A different issue for the Alster fish habitat, however, is fragmentation.  Following the completion of the original dam that created the lake in 1235, eight dams were built upstream on the main stem of the Alster.  The river had always been used to float logs, but the peat extracted upstream and the building stones had to be shipped down by barge; it was easier to construct dams and locks than roads in what was then up-river wilderness.  Eventually an 8-kilometer trench was dug to connect the Alster with the Best river, creating the first canal for barge transport between Hamburg and Lübeck, 91 km long, completed in the early 1500s.  Barge traffic continued on the small canal well into the 19th century.

But of course, these dams fragmented the waters and prevented fish migration, affecting eel and salmon, in particular.  This is something that the city has decided to remedy: as of 2015, it has completed the construction of three fish ladders, two for the downtown locks (Rathausschleuse and Mühlenschleuse), one upstream of the lake at the Fuhlsbüttel dam. Three more are in the works (Poppenbüttel, Wohldorf, and Saseler Damm).  By 2022 fish will be able to swim from the Elbe all the way across the city to Schleswig-Holstein, should they want to.

The fish ladder at the Fuhlsbüttel dam is the most interesting.  In 2000, the city installed a small hydroelectric power plant (110 kW, enough for 200 households) to take advantage of the 4-meter drop at the dam.  In order to protect the fish (so that it does not get entrained into the turbine), the inlet was oversized to ensure slow flow towards the turbine.  The fish ladder itself has generous dimensions; a full flow of 0.5 m³/s is allocated for it, even if that reduces the flow available to generate hydropower (a maximum of 3.6 m³/s, thus a sacrifice of potential power of 12% – fish do matter).

There are dams and weirs across many of the tributaries, and some have fish ladders as well.  For instance, the old grain mill on the Ammersbek, now a heritage site, has been refitted with a small hydroelectricity generator.  The site has a simple fish ladder and an education centre.  When I visited in June 2017, there was a good flow in the ladder. A sign says it is used by trout, eel, burbot, pike, perch, roach, and gudgeon.

Detail of the fish ladder on the Fuhlsbüttel dam

The gentler fish ladder on the Ammersbek













This is all great – it shows that there is public and government support for fish protection, even within a city.  Northern Germans may not be fish people like Westcoasters are salmon people; but they eat large quantity of fresh water fish as well as seafood, many people fish in the city streams, many people volunteer their time restoring the stream banks or leading educational trips for kids. Hamburg cares about fish, and doesn’t stint on measures to protect them.

And yet, there was a huge fish kill.  Would the completion of the fish ladders have prevented it?  Unlikely.  Fish ladders do help connect the habitat, and fish may migrate away from the bad spots.  But the dead fish at Fuhlsbüttel were collected mere meters away from a ladder.  None of these systems can replace fresh rain and cool water.

But there is something else that may also help the fish: groundwater.  Cool groundwater gradually seeps into surface streams, helps maintaining streamflow during dry periods.  Anything that contributes to recharging groundwater helps fish, indirectly.  But groundwater recharge, in the form of green infrastructure, is also a key component of flood prevention. This is what the last section of this four-post series describes.

The fish ladders so far


Written by enviropaul

January 14, 2020 at 4:51 pm

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  1. […] This is just one example of how surface water bodies – lakes, ponds, creeks, wetlands – are effective in storing excess waters and preventing floods.  This is a part of what Hamburg does well: green infrastructure.  Green infrastructure means providing storage for excess rain, and that can be done by turning the city into a sponge: green spaces, green roofs, floodable lands and detention ponds all help.  Hamburg is aggressively working on expanding these measures simultaneously.  And of course, acting like a sponge also means that there is more water gently released to the streams during the dry spells, helping the fish habitat. […]

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