DO declined significantly (po0:05) in 22 stretches in all rivers for at least 20 years to 1998, and in some cases where data were available, for 40 years (Table 1). A typical example is illustrated in Fig. 2. All except six stretches (CH5, CO5, D2, D3, G3, ST4 –see Fig. 1 for river codes) with declining DO were in the middle or lower reaches of rivers, where they either entered estuaries or joined another watercourse. In all affected
stretches, the flow was held upby a mill or ford, or ponded above a weir. Only two affected stretches were sampled close to a sewage treatment works (STW), but all flowed through arable or industrial land. DO increased significantly in three stretches, all of which showed significant trends of increase in temperature and chloride. The remaining 24 stretches showed no significant change in DO.