particularly in combination with areas of fast flow, such as riffles [22,23]. In a natural river system this heterogeneity enables the establishment of stable communities. However,
in rivers with impoundments, such as those in East Anglia, ponded areas are destroyed once flows increase with autumn rains, by mill gates being opened, or by extra water being introduced in inter-basin transfer schemes [24]. Disturbed silt and organic debris may then be washed downstream, increasing BOD.