• People and the River

Turbidity in Rivers: what is it and how does it impact rivers?

Rivers have become a regular feature in news headlines about the environment as they face a wide range of pressures from human activity. While wastewater often dominates these headlines, it represents only one part of a much bigger picture. Our rivers can be affected by many different pollutants and factors, each with its own sources, behaviours and potential impacts on wildlife and people. Understanding what these pollutants and other factors are, how they enter our waterways, and what levels are considered safe is an important step in protecting and restoring healthy river ecosystems.

 

What is it?

Turbidity occurs naturally during high flows, but human activity has significantly increased the frequency, duration and intensity of turbid conditions in many river systems. Turbidity is described as the cloudiness or haziness of water caused by tiny suspended particles, typically fine sediments, but can also consist of organic matter and pollutants.

Turbidity in rivers can arise from a combination of natural processes and human-driven pressures, such as:

 

  • Agricultural runoff – soil erosion from ploughed fields and poorly vegetated margins can contribute large quantities of fine sediment in runoff
  • Urban runoff – roads, construction sites and drainage systems wash sediment, road debris and organic matter into watercourses
  • Bank erosion – straightened channels, high flows and loss of riparian vegetation destabilise banks, releasing sediment directly into the river
  • Wastewater discharges – storm overflows and treatment works can release suspended solids and organic matter into watercourses
  • Legacy sediments – historic land use, dredging and channel modifications have left large stores of fine material in many lowland rivers, which can be disturbed and re-mobilised during high flows
  • Algal blooms – nutrient enrichment can trigger phytoplankton growth, increasing turbidity even when sediment inputs are low

 

Across the Essex catchment, turbidity is influenced by a combination of diffuse and point-source inputs, including soil erosion, urban runoff, wastewater discharges and the disturbance of legacy sediments stored in lowland channels. As Essex rivers are typically low-gradient and sediment-rich, they can be particularly vulnerable to prolonged periods of elevated turbidity. The pressure of turbidity can be particularly strong in intensively farmed catchments, urbanised tributaries, areas with eroding banks and lowland rivers where fine sediments naturally accumulate. Understanding the relative contribution and impact of each source is essential for designing effective interventions and mitigation.

 

Turbidity in Rivers: what is it and how does it impact rivers? | Essex Rivers Hub

River Blackwater near Horseshoe Hole by Angela Tuff

 

What are the impacts?

High turbidity can pose a significant ecological challenge as it alters the physical, chemical and biological processes of rivers. For example, suspended particles block sunlight from reaching aquatic plants and algae, which can reduce photosynthesis, limit oxygen production and disrupt the base of the food web. In highly turbid rivers, submerged plants may disappear entirely and reduce habitat complexity for fish and invertebrates. It can also induce other stressors on aquatic life, such as clogging the gills of fish with fine sediments and impairing their feeding, respiration and reproduction. Species that rely on visual cues may also struggle to hunt in persistently cloudy waters.

Riverbeds can also be smothered in times of high turbidity when suspended particles settle and blanket gravel beds, filling the spaces between stones. This can reduce spawning habitats for fish like trout and can smother the eggs of other species. Invertebrates that depend on clean, well-oxygenated gravels can also be negatively affected by this.

As fine sediments readily bind to nutrients, pathogens, hydrocarbons and heavy metals, these pollutants are transported further and remain in the water column for a longer period of time during high turbidity. Once deposited, these can accumulate in riverbed sediments and be re-mobilised during storms or dredging.

 

Catchment-scale restoration is key to tackling turbidity as the problem is rarely caused by a single source. A combination of nature-based and engineered solutions can help reduce sediment inputs and improve water clarity. Natural flood management can be used to slow flows and trap sediment upstream, creation of wetland can help capture fine particles and associated pollutants and riparian buffer strips can stabilise banks and intercept runoff. Other solutions include improved soil management to reduce farmland erosion, swales and retention ponds can help control urban runoff and the re-meandering can create more stable and resilient river channels.

Through continued monitoring, partnership working and targeted interventions, turbidity pressures can be reduced and the recovery of sensitive species can be recovered. By understanding how turbidity behaves and its sources, more effective and long-term solutions that can help rivers move closer to good ecological status can be designed and implemented.

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Pressures

The topics below represent the pressures that many waterbodies in the Combined Essex catchment face. They have been divided into six main categories, but it is quite often that these categories can overlap as pressures relate to each other.

Diffuse Pollution

Diffuse pollution occurs as water moves across the land or through the ground and picks pollutants. These pollutants can come from a variety of places, including urban and field run off. The pollutants that enter the river can range from sediment to toxins to excess nutrients, meaning that diffuse pollution can cause a whole range of different issues. The variety in these pressure groups means that it is something that can be quite difficult to tackle. It requires groups of people, business and stakeholders to work together in order to solve this problem.

Fish Passage

Fish should be able to travel up and down a river freely, allowing them to move and breed in the most suitable habitats for them. It is important that fish populations do not become isolated, as this makes them more susceptible to disease and puts pressure on their survival. Unfortunately, there are often many barriers along rivers that prevent fish from being able to migrate up and down stream. Where barriers have been identified, they will be seen as a ‘pressure’ on a waterbody. Thankfully, there are many solutions now that can be put in place to aid fish passage, even over large barriers.

Flow

The flow in a river can vary greatly throughout the year as rainfall and run off can have an effect. This is a natural process. It is when flow is impacted by non-natural processes that it can cause problems. Sometimes, water can be intercepted or removed from a system; this will reduce the flow, therefore changing the habitat conditions. Some species are happy in high energy rivers. This means that when flow is reduced, these species will no longer survive. The opposite of this can occur when excess water is entering a river, for example through increased runoff. Low energy systems then become high energy and displace the species that live there.

Invasive Species

A species that is not meant to be found in a particular area is known as an invasive species. Invasive species can be from a different habitat or a different country altogether. Most of the invasive species that we find on our rivers have come from other countries – plants that people have imported for their gardens or animals that have been released for food or by animal rights activists. Control of invasive species requires a lot of time and effort. We are fortunate that we do have a range of methods to manage most of the invasive species that cause havoc on our rivers, but there are still some which we are still struggling to control.

Physical Modification

Many of our rivers have been heavily modified over the years as rivers have been used for a wide range of purposes. Physical modification is one of the biggest factors that causes our rivers to be unhealthy. The issues that it can cause range from reducing habitat, preventing migration of mobile species, and even have an effect on the water quality. Where structures and modifications are no longer in use or necessary, they should be removed to allow the river to regain its natural state. Unfortunately, this action is not always taken which means that many of our rivers are over straightened and contain redundant structures. It is possible to return a lot of our rivers to their natural state, through one off projects, but in other cases it is not possible as the river has been changed to protect assets or manage flooding. It is recognised that some modifications cannot be removed without having severe negative impacts both socially and economically.

Point Source

This is pollution that comes from a single identifiable source. The pollution entering the river could include a whole range of pollutants. Some point source pollution is known about and licences, for example sewage treatment works. Other sources are not licensed, and therefore work needs to be done with landowners to fix the problems that are allowing the pollution to enter the river. Point source pollution is more easily controlled than diffuse pollution as it often only takes one management approach to solve the issue.