An investigation into phosphate levels in the River Nene is being carried out by the Environment Agency.
It is working with the British Geological Survey to evaluate levels of the nutrient which causes excessive weed growth.
Silt samples are being taken from a 60-mile stretch of the river between Badby and Peterborough and then examined.
Rob Price, of the Environment Agency, said: “We will be collecting and analysing silt samples from the river bed.
“This will show us what level of phosphate is trapped in the silt and allow us to model the rate at which it becomes available to algae and plants.
“The information will be used to determine what future works are needed on the river to improve the quality of the environment.
“Rapid and excessive plant growth can have a negative impact on the natural water environment.”
Phosphate is an essential nutrient for plant growth and is available naturally in very small quantities.
It is mined and used in agriculture to help grow crops and other foodstuffs. It is also used with other chemicals in domestic detergents, often to soften water.
Phosphate presence in rivers, lakes and streams is mainly attributed to treated sewage water and run-off from agricultural land.
Mr Price said: “Phosphate readily sticks to other particles such as silt and sediment which transports it downstream.
“Under the right conditions, the nutrient becomes very reactive in water and is taken up by plants and algae which then grow quickly and excessively.
“Too much algae and vegetation stop essential processes within the river’s ecosystem and can cause water quality to decline.
“Excess vegetation also reduces the size of the river channel and can cause issues for boating, fishing and flood risk management.”