Water quality in the freshwater ecosystem (ponds, lakes, streams and rivers) is made up of many different factors. For example the water can be clear or turbid, it can be acid or alkaline, can have high or low levels of dissolved oxygen and can contain a wide range of other chemicals. The National Rivers Authority assesses chemical water quality in the river Ouzel on a monthly basis (1995 results).
However these measurements are carried out at a single point in time and may miss sudden pollution events which could severely affect water quality for a few days before being washed downstream. To pick up the after effects of these events and get an overall picture of river quality the NRA also carry out biological monitoring. River invertebrates are sampled and identified to family level. Each family tends to live under different oxygen and pollution levels, for example the Oligochaete worms can live in highly polluted, still, turbid waters with little oxygen whereas stonefly larvae require the well oxygenated conditions usually found in fast flowing streams. Almost all families found in UK freshwater have been given a biological score in the range 1 to 10. Families found only in the best conditions have a score of 10 whereas those able to survive under very poor conditions, get a score of 1, there are many families with intermediate values.
Scores for all families found in the river are added to give an overall total. This figure is compared to a standard table and the river can be classed as 'good', 'poor', 'very poor', etc. The river Ouzel is generally classed as good to excellent using the biological monitoring scheme. However this can change if there is a major pollution incident. There are frequent minor pollution incidents in Milton Keynes, the most common being oil spills either by accident or deliberately - some people just pour engine oil down street drains rather than taking it to the recycling points. The NRA use a variety of absorbent wicks to try to prevent this oil getting into the main watercourses where it would kill wildlife.
Further information on water quality and other aspects of aquatic ecology can be found in the Open University Ecology course S328 (which has its own rather nice set of new text books) and water quality part of the Technology T102 course.