THE River Ouse downstream of York is getting cleaner - and that's official.

Experts from the Environment Agency said today that the river below Naburn sewage works was of a "good" biological and chemical quality last year, compared with "poor" biological quality in 1995.

Upstream of York and as it flowed through the city, the Ouse was rated to be of "very good" biological quality last year.

Chemical quality was also listed as "good".

Meanwhile, the River Foss, which is slow-flowing and tends to suffer from extensive growth of plants and algae and low dissolved oxygen levels during the summer, also appears to be improving.

The agency said there were high levels of nutrients, particularly phosphate and nitrate, which entered the river through sewage effluents and agricultural activities. But while chemical quality in the lower part of the Foss was "poor" in 2003, this had improved to "fair" by last year. The verdicts came in the agency's annual assessments of water quality in rivers across England.

Across the Yorkshire and Humber region, quality improved over 2003, but was still below the national average.

A little over 65 per cent of rivers in the region had a good or very good biological standard in 2004, compared with the England average of more than 70 per cent. The previous year, only 60.3 per cent of rivers in the region were of good or very good quality.

The chemical quality of river water in the region was slightly better than the national average, with 62.5 per cent scoring "good" or "very good" in 2004, compared with 61.5 per cent across the country.

More than 2,300 kilometres of river were rated as good or very good for biological quality. Only about 300 kilometres were rated either poor or bad - the worst categories. For chemical quality, about 1,700 kilometres were rated good and very good, and nearly 800 kilometres were rated poor or bad.

An agency spokeswoman said the biological standard was based on an examination of the ecology on the river bed, while the chemistry standard focussed on organic matter in the water and how much oxygen was required to break it down.

"It also looks at dissolved oxygen, which is the amount of oxygen actually in the river, and ammonia, which is indicative of poorly treated effluents, such as sewage."

She said new quality standards, to be set under the EU Water Framework Directive, meant rivers, streams and lakes should be of "good" biological and chemical status by 2015.

Updated: 10:20 Tuesday, August 16, 2005