Abstract
The Senkawa-josui irrigation canal is a small artificial watercourse of treated wastewater from a sewage plant. The simple structure of the channel (linear flowing, concrete banks and uniform riverbed) and the presence of a large number of carp seemed to prohibit distribution of diverse species. In order to improve the situation, the concrete bank (80 m long) was removed and two semi-stagnant sections which large carp could not enter were made in April 1996. Aquatic plants were also planted in the area. The aquatic fauna and flora were researched for three years after the channel improvement.
Many species were found soon after the improvement. Juvenile fish and tadpoles grew in the stagnant sections. Male dragonflies also created their territories in the stagnant sections and the females laid eggs in them. The number of species of benthic macroinvertebrates increased. Spotbill ducks and little egrets were also found in the improved area. Diverse relations between aquatic plants and insects were observed. Species diversity of aquatic organisms was successfully raised in the imprgved area, showing that changing simple channel structures to complicated ones can help increase the number of species in artificial watercourses and urban streams.
