Restoring Chalk Rivers: The Importance of Sediment, Science and Communication

Understanding how river restoration works – and helping people understand what they see

Our chalk rivers are among the most distinctive freshwater ecosystems in the world. They are also rivers that have been shaped by people for hundreds, and in some places thousands, of years.

Across the Test, Itchen, Hampshire Avon and their tributaries, generations of engineering have left a legacy of weirs, hatches, sluices, mill structures, water-meadow carriers and artificially modified channels. Many of these structures were created for perfectly understandable reasons – to power mills, control water levels, irrigate water meadows, supply watercress beds or manage land. But every structure changes the way a river behaves. It can alter the speed and depth of the water, interrupt fish movement and change the way gravel and finer sediment move through the catchment.

As more restoration projects are planned and delivered across Hampshire, understanding how restoration can change the way a river behaves – and explaining those changes to the people who live, work and fish alongside our rivers – is becoming increasingly important. Sediment is an important part of that story.

Understanding sediment

Sediment is often described as a problem in our rivers. But sediment is a natural and essential part of a functioning river system.

Sediment comes from many different sources. Soil can be washed from fields and riverbanks during rainfall and flood events. Fine particles can be released through natural erosion, while gravel, sand and smaller stones can be moved by higher flows. Leaves and other organic material also enter the river from the surrounding landscape. The important question is not simply whether sediment is present, but how much there is, where it is deposited and how naturally it moves through the river system.

Gravel – the foundation of spawning habitat

Gravel is particularly important in chalk rivers. Many of our native salmonids, including brown trout and Atlantic salmon, use clean gravel beds for spawning. Fish excavate nests, or redds, in the gravel where eggs are deposited and subsequently develop. The spaces between appropriately sized stones allow water to flow through the gravel, supplying oxygen to developing eggs helping to maintain suitable conditions for juvenile fish. A healthy gravel bed is therefore much more than simply a layer of stones on the riverbed. It is a functioning part of the river’s life cycle.

The movement of gravel is also important. During higher flows, individual stones can be mobilised and moved downstream, helping to maintain and renew spawning areas. Where this natural movement is interrupted, gravel can become trapped in one location while other parts of the river become starved of the material they need.

The importance of larger stones

Larger flints and stones also have an important role to play. They provide physical structure within the river, creating areas of variable flow, providing shelter for juvenile fish. These larger stones can act as ‘home stones’, offering cover from the current and helping young fish find areas where they can feed while using relatively little energy to maintain their position. They also provide habitat for other river life, including our native white-clawed crayfish, which use stones and the spaces beneath and between them for shelter and protection.

The variety of stone sizes within a river is therefore important. A riverbed made up of a mixture of gravels, cobbles and larger flints can provide a much greater range of habitats than a channel dominated by one type of material.

Fine sediment has a role too

It is easy to think of fine sediment as something that should simply be removed from a river. That is not the case. Fine sediment is a natural component of river systems and, in the right place and at the right concentration, can form part of a functioning river ecosystem. Fine particles are transported downstream during higher flows and deposited in areas where water velocity reduces. Organic material and fine sediments can also contribute to the food web and provide both habitat and food for many aquatic organisms. The problem arises when the balance is disrupted. Too much fine sediment can settle between and over the gravels that salmon and trout need for spawning. It can reduce the spaces through which oxygenated water flows and potentially smother the habitat on which fish and aquatic invertebrates depend. This is why the movement of sediment is so important. A river does not need to be free of sediment. It needs sediment to be moving through the system in a way that reflects the river’s natural processes. This is one of the reasons why understanding river restoration requires more than simply looking at the structure that is being removed or modified. We need to understand the river as a connected system.

What happens when a river is slowed down?

Imagine a river flowing naturally over a gravel bed. Water moves downstream, gravel is occasionally shifted during higher flows, and finer material is transported through the system. Now put a weir or hatch across the river. The water upstream becomes deeper and slower. Material that might previously have continued downstream begins to settle. Year after year, more sediment accumulates. Over decades – and sometimes centuries – a structure can therefore create a store of sediment that has become part of the river’s history. This becomes particularly important when we start to remove, modify or bypass historic structures. Once water begins to flow more naturally again, some of that stored sediment may move. The result can be temporarily increased turbidity – water that appears cloudy or discoloured – and sediment being transported downstream. That does not automatically mean that something has gone wrong. But neither should we assume that because sediment movement is a natural process, it does not matter. The important questions are:

How much sediment is likely to move?
Where will it go?
How long might the effect last?
What could be the consequences for the river and the people who use it?

The River Anton – a useful example

Current restoration work on the River Anton provides a useful example of why understanding sediment matters.

Geomorphological surveys identified historic deposits of ultra-fine, silty sediment within the channel, creating a recognised risk that some of this material could be mobilised during restoration.

The project has therefore incorporated measures to manage that risk, including sediment traps, silt curtains, controlled working methods and water-quality monitoring.

Monitoring can provide information about what is happening in the river during the works, including changes in turbidity and other water-quality parameters. But there is another important part of the process: communication.

People downstream need to understand what is happening and why.

A sediment plume does not stop at a project boundary. It does not recognise a fishing beat, a landowner’s boundary or the end of one organisation’s responsibility. What happens upstream can affect fisheries, anglers, landowners and other river users downstream.

That is why restoration needs to be considered at the level of the whole river, rather than simply as an individual project.

Restoration is about more than removing a weir

Restoring a chalk river does not necessarily mean removing every structure. Some structures have historical, cultural or practical importance. Some may be extremely difficult to remove. In other situations, modifying a structure or creating a bypass may provide a better solution. The objective should be to understand what the river needs and then consider the most appropriate solution. For fish, connectivity can be particularly important. Atlantic salmon and other migratory species need to be able to move through the river system. Wild trout and other species also depend upon suitable spawning habitat, clean gravels, appropriate flows and functioning river processes. Restoration therefore needs to look at the river as a connected system. That creates some important questions:

What happens to sediment when a barrier is removed?
What happens to the habitat upstream?
What happens downstream?
When is the best time to carry out the work?
How will anglers and fisheries be affected?
What does the landowner need to know?
How will the work be monitored?

And perhaps most importantly: How do we make sure that the people who care about the river understand what we are trying to achieve?

The importance of timing

There is rarely a perfect time to carry out in-channel restoration. Autumn has traditionally been seen as a practical period for some river works, but it is also an increasingly important time for grayling fishing. At the same time, autumn, winter and early summer (grayling) are important periods in the life cycle of salmonids, with spawning activity taking place in many chalk-stream systems. That means restoration needs to be planned carefully.

Sometimes it may be possible to phase a project, work on different sections at different times or agree specific working windows. The earlier those conversations begin, the more opportunities there are to find sensible solutions. This is another area where good communication can make a practical difference. Explaining why work is taking place at a particular time – and acknowledging the competing interests involved – can help people understand the decisions being made.

A conversation about the future of our rivers

Every river, every structure and every community is different.

There is also a much bigger conversation to be had.

What should our chalk rivers look like in the future?
How much should we allow natural river processes to return?
Which historic structures still have a purpose?
Where should we intervene, and where should we allow the river to do more of the work itself?
How do we balance the needs of wild fish, fisheries, landowners, wildlife and people?

There are unlikely to be simple answers. But we do not need to agree on everything before we begin the conversation.