Overall, rivers in Northern Ireland and Wales consistently had the cleanest water. In addition, with the exception of Wales, all the regions showed improvements in their water quality over the period shown.

With regards to the two leading nations, in 1990, roughly 96% of rivers in Wales were classified as having good chemical levels, noticeably higher than Northern Ireland's 83%. In the following years, Wales's river cleanliness remained relatively stable until 1998, before declining sharply to around 80% by 2002. In contrast, Northern Ireland's river quality gradually improved, with its purity rising to nearly 100% in 2000, and then slightly falling to 96% by 2002, having surpassed Wales since 1999.

Regarding the remaining countries, in 1990, Scotland's rivers were of the lowest quality in terms of chemical composition, with only 20% of river water there up to par, less than half of England's 45%. Subsequently, the latter's purity rose sharply to a peak of 70% by 1996, and then dramatically fell back to a low of 30% in 1998, followed by a significant recovery to 50% by 2002. Similarly, the percentage of clean water in Scotland rose steadily, reaching 60% in 2000 before declining to 40% over the final two years.