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No. 128 “The Maruko Bridge and Its Vicinity”

Reports from our Seasonal newsletters: Special Edition on the Annual Report about the Foundation’s Projects; No. 128 “The Maruko Bridge and Its Vicinity”; The Foundation Newsletter, Tamagawa (English Version)

■The Maruko Bridge and Its Vicinity■

The area around the Maruko Bridge, located near the Tamagawa Station on the Tokyu Toyoko Line, is one of the most popular places along the Tama River, with large parks and walking trails along the river. On holidays and sunny days, many people enjoy fishing at leisure, taking a walk along the riversides. On the wide stretch of the right bank of the river (on the Kawasaki City side), there are people playing sports such as rugby and soccer, or even practicing golf at the range. On such days, the parks become packed with people in the early morning.
At a little distance above the bridge, you can see the Chofu Intake Weir located close to a railway underpass of the Tokyu Toyoko Line. The weir was constructed by the national government in 1936 to supply drinking water, but is currently used to provide water for industry.
It also functions as a high-tide weir. It is said that Goby fish live there. A water pollution monitoring station is placed on the left river bank.

Photo ＆ Text Hidehiko Endo

[Opening Article]

Rivers and Ecological Networks

by Akira Kameyama

Professor Emeritus, Tokyo University of Agriculture and Technology

Winner of the 2nd Tokyu Foundation for Better Enviroment Social Contribution in Academia Award.

With the promulgation and enforcement of Japan’s Basic Act on Biodiversity, quite a few prefectures and municipalities are now seeking to formulate their regional biodiversity strategies. They include biodiversity-related information gathering and assessment of its current state, policies and their challenges, measures to deal with such issues, major initiatives including action plans and leading projects, and mechanisms to promote these initiatives. From the biodiversity conservation viewpoint, the most important activity is to prevent species from extinction. To accomplish this, the most urgent is to create plans for ecological networks that are essential as habitats for living organisms.

To establish ecological networks, the following will be needed: (1) protection of existing habitats, (2) improvement of habitat environment that has been degraded, restoration of lost habitats, and creation of new habitats, and (3) securing of corridors that unite habitats.

Since most big cities in Japan are located in the basins of large rivers, the roles of these rivers in the construction of ecological networks are expected to be great. Let’s think about it here, taking Tokyo as an example.

We can gather information about riparian species from the Census of Rivers and Riparian Areas conducted by The Ministry of Land, Infrastructure, Transport and Tourism. Since there are three large rivers running through the Tokyo area, the Edo River, the Ara River, and the Tama River, we looked at the findings from the recent census on these three rivers. In the Edo River basin, 11 species of mammals were found living; in the Ara River 16, and in the Tama River 17. Mammals commonly found in all three rivers include Japanese shrew mole (Urotrichus talpoides), small Japanese mole (Mogera imaizumii), hare, Japanese grass vole, large Japanese field mouse, harvest mouse, badger, fox and weasel. The inhabitation of Japanese grass vole and large Japanese field mouse, both of which are indicator species of grassland or woodland, as well as harvest mouse that mostly live in reed beds in wetlands shows that these basins are rich in environmental diversity as mammal habitats. Presence of hares and badgers implies that the expansion and connection of habitats are being secured. The presence of foxes and weasels that are predators high in the food chain shows that the large habitat environment for wildlife remains in the basins. Badgers can be seen almost everywhere along the Tama River from the source to the estuary during all seasons. The numbers of raptor species found are 5, 7, and 8, in the Edo River, the Ara River and the Tama River basins, respectively. Northern goshawks, common buzzards, falcons, and common kestrels are identified in all three basins. The reasons why these high-ranked birds of prey can live in these basins are the same as the reasons for mammals.

Looking down upon the Tokyo area from the sky, you can see the three several-hundred meter wide rivers traverse the Kanto Plain from the mountains, hilly areas and plateau, to their eventual outlets to the sea. These rivers and their basins with much larger land areas than the water areas provide animals with two functions: their habitats and corridors. Distinct features of these rivers include (1) largeness of areas, (2) interaction with surrounding ecosystems, (3) diversity in their location and vegetation, and (4) adequate management by humans, such as that found in Satoyama（community-based woods）. These large rivers connect small and medium size tributaries, and play an important role of connecting habitats for living organisms.

For the conservation of biodiversity, it is necessary to create an ecological networking plan for the habitats of living organisms without delay. To accomplish this, information gathering on the living conditions of species and environment of their habitats is a must. I introduced here some species inhabiting the rivers and their basins in Tokyo hoping that people would profoundly understand the importance of living organisms oriented thinking in this age of biodiversity.

[Special Article]

Issues of the Tama River from the Study of Fishway

by Prof. Yoichi Yasuda

Environmental Engineering and Hydraulics Laboratory,
Department of Civil Engineering,College of Science and Technology, Nihon University

Twelve years have passed since I began my study of fishways. The academic encounter with Prof. Miya at Nagasaki University, now retired, and Prof. Hamano of the National Fisheries University, who is now at Tokushima University, led me to the study of fishways. The study focused upon facilitating the upstream and downstream migrations of crustaceans including freshwater shrimps and crabs. I had an opportunity to assist in a project to install a fishway with eco-steps, which created an environment that facilitates the migration of shrimps, crabs, gobies, shellfish and swimming fishes such as sweetfish and dark chubs. Later, I accomplished a project of creating an environment suitable for upstream migration by temporarily installing a portable fishway at the Chofu Intake Weir in the lower Tama River on the side of Kanagawa Prefecture, where the migration had been blocked in the past. I received a grant from the Tokyu Foundation for Better Environment for this project. In both of these projects, the crux of creating the environment that facilitates the migrations was the slowing of the water flow which was made possible by tilting walls at the edge of the bank.

I advanced the study of fishways and proposed a pool type fishway with a trapezoidal section. This fishway succeeded in facilitating upstream and downstream migrations of not only shrimps, crabs, gobies and shellfish but various fishes from over 80 centimeter-long salmon to 5 centimeter-long juvenile dark chubs. This was not the only effect of my proposed fishway. It also provided a structure enabling drift gravel as big as 20cm and driftwood that accumulated at flood stages to be carried away by the flood water flow within the pool. I also jointly developed a trapezoidally-sectioned stone masonry type fishway which has notches in its center and succeeded in creating the environment that facilitates upstream and downstream migrations even under the condition of the water volume changing from 30.03 ?/s to 10?/s. This study is still underway.

In the past decade, half cone type fishways have been installed at the weirs upstream of the Nikaryo Weir of the Tama River. This half cone type fishway functions normally with water flowing in only half the width of the fishway. If the water volume exceeds this amount, the fishway won’t be able to provide a suitable environment for migrations. This means that the water volume flowing in the fishway is less than the volume flowing in the Tama River. As a result, more fish are disoriented. We cannot disregard the situation where some fish are losing their way in a pond dammed by the weir and cannot swim downstream. The environment around the Shukugawara Weir facilitates migrations through the installed fishways at both sides of the river. However, juvenile fish often drop through the gate at the center of the weir. Since the flow from the secondary dam hits the steps, the potential impact death incidence is high in this environment. I hope that this situation will be corrected so that fish can safely migrate upstream and downstream without being disoriented.