River Foss - life on a knife edge

By John Littlewood
Fisheries Technical Officer for the Environment Agency

People are normally only aware of the River Foss when it floods

Historically the River Foss has played an important role in the city of York, from the days of the Romans to modern times, but is always overshadowed by its bigger, better known brother, the River Ouse.

Most people walk or drive past the Foss and never give it or its inhabitants a second thought unless it threatens to flood.

The natural flow of the Foss is low and the river has been heavily engineered by man over its lifetime. Its higher reaches in rural areas are at risk from agricultural pollution, while downstream is urban, where water runs off from roads and sewage treatment works.

Pumping water into the Foss protected the river's delicate eco system

Where it meets the Ouse, the river's passage is blocked by lock gates and a flood defence barrier.

The Foss exists on a knife edge, due to its high nutrient levels and lack of diluting flows. It has good populations of fish species, some protected by European legislation, such as bullhead and brook lamprey, as well as chub, bream, brown trout, roach and barbel.

Threat to wildlife

In summer of 2009 various problems coincided, leading to fish deaths at the river mouth. The very dry weather made river levels fall even lower than normal, meaning less dilution of any contamination in the water.

Organic material in the river is being broken down constantly by bacteria, which strips dissolved oxygen from the water and gives out carbon dioxide. At the same time this summer, weed growth worsened leading to dramatic swings in dissolved oxygen levels through the natural process of photosynthesis.

The natural flow of the Foss has been heavily engineered by man

The warm weather also raised the temperature of the very slow-flowing water - as water warms its ability to hold dissolved oxygen decreases.

This combination of events initially caused fish to move to where more oxygen was available but large numbers perished. The Environment Agency worked to improve conditions for fish by installing two large pumps in the Foss Basin, where the river joins the Ouse, and pumping water over the lock gates back into the Foss.

The cascading effect of the water into the river raised dissolved oxygen levels nearby and 48 hours' pumping brought the river back to normal levels and saved many fish.

Rain had gone some way to reducing problems by raising natural water flow. It also brought colder air temperatures, which increased the water's ability to hold dissolved oxygen.

Other river inhabitants have the option of walking, flying or crawling to safety but fish don't and need water of a minimum quality to survive. It is no wonder fish are described as canaries of the water environment.