Thames Landscape Strategy - Hampton to Kew -

River bed survey

It is that wonderful time of the year again when the river is left to its natural state exposing large areas of foreshore that would normally be covered with water. Called the ‘draw down’ these periods take place annually, usually in November between Teddington and Richmond Locks. During this period the gates at Richmond Lock which are usually closed during low tide to facilitate navigation are kept open to allow essential maintenance works on the lock, weirs and sluices. The weir being lifted allows the river to drain naturally at low tide leaving much more of the foreshore exposed than normal – although high tides are unaffected and can still be very large so watch out if you walking along the river.

This year we continued to use this unique opportunity to access the bottom of the river to find out just how many alien species are invading our waterways. Scientists from the Zoological Society of London, Thames Landscape Strategy and the Marine Conservation Society have been working for the past week looking for a number of native and non-native species. From this survey it is hoped to find out whether these “alien” species (some from as far away as China) are having an effect on the rare native populations.

One factor that we didn’t account for in the preparation of the survey however, was last week’s tidal surge that made its way down the North Sea and subsequently up the Thames Estuary whilst we were actually down on the riverbed carrying out the project.

This tidal surge affected the tidal Thames for three reasons: There was a spring tide in the Thames area, a high pressure was sitting over much of the Atlantic and there was a deep depression above Scotland which generated strong northerly winds blowing down the North Sea. When these conditions are in place it can effectively lift a huge body of water upwards of three metres and push it down the North Sea towards Holland. As this body of water nears the mouth of the Thames it is naturally channelled up the river. Happily we have the Barrier to protect us which last week was closed twice on the high tides to stop the surge devastating low lying areas.
Although the Thames Barrier was shut and London wasn’t inundated, the effect of the surge was still clearly felt. Fortunately it reached our bit of the Thames around low tide – quite safe but – …amazing in how it affected the movement of the water. I have never known the tide change so quickly and come in so fast. One minute we were surveying the exposed river bed and the next we were knee deep in water. That was the end of that day’s sampling but a great reminder of the natural power of the river and the forces that control it.

The survey resumed safely the next morning and so far we have found many invasive species including the Chinese mitten crab, zebra mussels and Asiatic clam, all of which pose a threat to Britain’s indigenous river life like swan mussels and the river limpet. Invasive species are particularly adaptable to life in the Thames and will readily out-compete our native wildlife for food and space. Our scientific team has also been on a special lookout for the endangered depressed river mussel (named after the shape of its shell and one of the UK’s most endangered creatures) and European eel that hides under rocks once the shore is exposed. Both of these have been spotted although sadly not in any great number. No wonder the depressed river mussel is sad being outcompeted by so many invasive species! We also saw several kingfishers and a large flounder near Teddington Lock.

It is intended that the survey will become an annual event. This will reveal trends and help conservationists to identify problems and threats that could one day be used to find a possible management solution to halt the decline of the native river life. Such is the importance of the survey that the Marine Conservation Society has already asked us to find volunteers to help in next year’s survey. Anyone who would like to help should contact the Thames Landscape Strategy and we will pass on your names. The survey will take place next November and full training will be provided.

Above the surface of the water a more obvious consequence of an alien species affecting native populations is Dutch elm disease. This disease devastated the landscape in the 1970’s and was described by some historians as the greatest change to the countryside since the Enclosure Acts. Although I am too young to remember what the landscape looked like before the disease took hold I do remember the lines of tall (up to 40m), dead ‘skeleton’ trees that appeared so out of place during the summer. In total a quite staggering 25 million elm trees died in the UK alone.

The Dutch elm disease that took hold in the 1970’s was a particularly virulent form of a fungal disease of elm trees which is spread by the native elm bark beetle. Although believed to be originally native to Asia, the fungus moved via North America to Europe (carried in a shipment of logs), where it quickly took hold as the native English elm population had no resistance to the disease.

Once the fungus has taken hold the afflicted tree attempts to block its spread by plugging all of the tubes (called xylem) which take water and nutrients up the trunk to feed the rest of the plant. As might be imagined this will eventually prove fatal. The first symptom of infection is seen in the upper branches of the tree during the summer. Leaves turn prematurely yellow and fall leading to die-back in the branches, quickly spreading down to the roots.

Not all the roots die and will with time put up small suckers. These grow up for some years into small elm trees but after a decade or so the new trunks become large enough to support the bark beetles usually at 3m in height or when the trunk reaches 10cm in diameter. When this happens the fungus returns and the new tree dies. Unfortunately, there is nothing that can be done to stop this and that is why there are so many dead elm trees still found along the towpath and in hedges. Once the tree has died it usually needs to be removed and where possible the dead wood is re-used close by as habitat for insects and beetles.

Before they die back however, the elm suckers are an important habitat for a range of wildlife. The problem is how to sustain this before the elm beetle has a chance to spread the fungus and kill the tree. Research has shown that where elm has been regularly clipped it has remained healthy for up to 40 years so one of the best ways to manage elm is to coppice the suckers incorporating the tree into a low native hedge.

View a video of the survey being carried out on the riverbed

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