Rules of Engagement: 4
A crucial aspect of the hierarchical assessment approach is the concept of ‘reference’. In a European context, pristine, truly natural, river environments are extremely rare. Even the most remote rivers are affected to some degree by human actions. Furthermore, there is no time in the past for which detailed information is available that can be considered to represent pristine conditions for the present. Indeed, the mid-20th century, which is often used for assessing past conditions (mainly because it marks the time when aerial photograph and high resolution map cover became widely available) was a time when most rivers were more heavily degraded by human activities than at present. This degradation was often indirect, reflecting a time when forest clearance was at a peak and agricultural activity was intense and was rarely based on conservation principles.