These results show that environmental variables have a much greater influence on river invertebrate assemblages in drought years than in wet years. Flow rate is crucial in dry years, when water ponded at mills or behind other impoundments may act as a sink for nitrate and phosphorus, providing a site for denitrification and a stable habitat for macrophyte and/or algal growth, leading to organic silt build-up. Fauna tolerant of low DO will become the dominant species in the ponded areas. Although these areas may appear stagnant, or even become anoxic beneath mats of floating macrophytes or filamentous algae, they provide increased patchiness in the system and allow for a more diverse community of vegetation and macroinvertebrates