The ViewWinter 2014

River Invaders

When the graphic images of flooded homes on the Somerset Levels dominated the news last winter, the impact rivers can and do have on communities became all too clear.

Questions were asked of politicians and policy makers – why is this happening and what can be done to stop it? Alison Barlow visited Professor Stephen Rice from the University’s Department of Geography to explore the issues and find out about some interesting Loughborough research.

Professor Rice sets the scene: “The build-up of sediment in rivers is regarded as a key issue in flooding because it can reduce the ability of a river to transport flood water downstream. Dredging is seen by some as a solution but as a strategy it really only addresses the symptoms of the problem, not the ultimate cause of excess sediment.

“This is often attributed to agricultural practices like ploughing and ditching which can move sediment from across the landscape into rivers. But research we have undertaken at Loughborough indicates that there may be another factor to consider – the impact animals have on the build- up of sediment in our rivers.”

The quantities involved are substantial – in one small stream only a few metres wide, crayfish were responsible for moving over 1.6 tonnes of sediment in a single month.

The idea that plants and animals change their environment to their own benefit is called ecosystem engineering. 

For geomorphologists like Professor Rice an interesting question is what such engineering does to affect sediment transport in rivers, in particular the recruitment, movement and deposition of sand and silt. This fine sediment is of broad concern because it has a number of negative environmental consequences, including increased flood risk.  

Professor Rice’s research suggests that one of the reasons there is so much fine sediment in certain rivers is because of the activities of the Signal crayfish.

Signal crayfish are river invaders. They were introduced to the UK in the 1970s and 1980s for aquaculture. “The idea was that there would be a market for crayfish meat and lots of fisheries were set up. But there was no market. As the farms went bust the crayfish escaped or were actively dumped and ended up in our water courses. Since then they have spread and had a major impact on freshwater ecosystems in the UK,” outlines Professor Rice.

So how do these small creatures make a difference to the transport of sediment in rivers? 

“It has been a challenge to investigate the links between fine sediment in rivers and crayfish activity,” says Professor Rice.

“But our work suggests that there are strong links. Crayfish recruit sediment to the river when they dig burrows in river banks. Where there are crayfish in high densities the river bank can look like honeycomb and these banks eventually collapse introducing large amounts of fine sediment to the river.”

He continues: “As well as adding sediment the crayfish also mobilise sediment from the river bed as they swim, forage and fight with each other.  Critically, once this sediment is in a river it can be moved by normal, relatively low river flows. In rivers infested with crayfish, sediment therefore moves between floods, when normally it would be stable on the river bed. This causes an increase in the annual sediment load carried by the river to downstream depositional sites.”

Establishing the impact of the crayfish required a combination of field work and laboratory experiments.  In the field the team use sophisticated equipment to measure turbidity and other characteristics of water, minute by minute. They also capture and radio tag crayfish so they can monitor crayfish activity levels.

With colleagues from the Environment Agency (EA) they use three- dimensional acoustic imaging equipment to ‘watch’ the crayfish in action. “What we see is a very strong daily cycle of sediment movement which peaks at night when our tracking confirms that crayfish are most active,” Professor Rice comments. 

“This is very consistent. We do not see any significant increases in the water flow that could otherwise explain this daily pattern of sediment movement. Our lab work backs up the field observations showing that adult crayfish are capable of mobilising large quantities of fine sediments,” he adds.

Professor Stephen Rice from the University’s Department of Geography Professor Stephen Rice from the University’s Department of Geography

With several years of data now at their disposal the team are able to model how much sediment movement is due to crayfish activity and how much is due to the regular river flow.

The quantities involved are substantial – in one small stream only a few metres wide, crayfish were responsible for moving over 1.6 tonnes of sediment in a single month.

“Our estimates for the summer months are that between 25% and 45% of sediment movement in the infested rivers can be attributed to the impact of crayfish. We expect that the annual impact will be less, because floods are prevalent in the winter, but the large summertime impact is important because this is a time of year when very little sediment would otherwise by moving,” said Professor Rice.   

As you might expect the research undertaken at Loughborough has attracted the interest of managers and policy makers, in particular the EA*.

“Officers at the EA are keen to build up a complete picture of how fine sediment moves in UK rivers partly because of their statutory responsibilities to manage pollutants, but also because they spend significant sums de-silting and dredging flood-prone river stretches. We are therefore working actively with them to establish the magnitude and spatial extent of this crayfish effect. 

“The impact of our research to date is that we have added significantly to debate about fine sediment management and the implications for flooding, habitat rehabilitation and pollutant transfer. Policy and funding currently focuses on managing fine sediment delivery from farming.

Whilst this effort remains important, we should perhaps be directing some resources toward the management of invasive crayfish too if we want to get the maximum benefit out of the investment,” concludes Professor Rice. 

*EA are funding a current PhD student