生活大学研究Bulletin of Jiyu Gakuen Vol. 4 College 103～104 of Liberal（2019 ）Arts Vol. 4 103–104 (2019)

Short Note Outline of the Water Circulation Mechanism of the Sakuragawa River Basin Flowing into the Kasumigaura

Shinpei YOSHIKAWA Jiyu Gakuen College

(Received 31 August 2018; Accepted 3 October 2018) In October 2018, The 17th World Lake Conference was held in for the first time in 23 years since 1995. In this paper, we will outline the advanced water circulation mechanism surrounding the Sakuragawa river basin and the Sakuragawa river, which is the inflowing river of , which is the representative lake in this area. Also, it shows an inventory of survey results related to the Sakuragawa river. KeyWords: Lake Kasumigaura, Sakuragawa river, Water circulation mechanism, River environment, River basin management

1. Outline of Lake Kasumigaura area The Lake Kasumigaura area is located in the eastern part of the Kanto region, the southeastern part of Ibaraki prefecture, and the area is 2,157 km2 (Fig. 1). Among them, the area of the lake is 220 km2, the second largest in the after . Lake Kasumigaura is connected downstream to Tonegawa river, in confluence point is set on Hitachigawa watergate. And in the usual time it is a slightly higher water area. On the other hand, the lake is made desalinated by preventing saltwater run-up by the flood gate, making it possible to develop present water resources. In addition, “Lake Kasumigaura” is a generic term such as Lake Nishiura, Lake Kitaura, Hitachitonegawa river etc. In this paper we mainly deal with Nishiura (lake area 172 km2).

2. Outline of Sakuragawa river basin The Sakuragawa river is a class “A” river (Ibaraki prefecture man- agement) with a designated extension of 63.41 km (flow path extension of 55 km) and a watershed area of 350.3 km2 extending across the four cities of Sakuragawa City, Chikusei City, Tsukuba City and City in the southwestern part of Ibaraki Prefecture (Fig. 2). The Sakuragawa river originate in mt. Kuwagara (274 m) in Sakuragawa City, Tsukuba mountain area mt. Tsukuba (877 m), the plains of the western part are flowing southward, towards Lake Kasumigaura (Lake Nishiura) near the central city of Tsuchiura City. Nishiura is a part of Lake Kasumigaura. It is the largest (extension & area) river among the influx rivers.

3. Land use and advanced water circulation mechanism in the Sakuragawa river basin The present time, the plain part of the watershed forms the big agri- cultural area, but in addition to the of ancient river water and groundwater, in addition to the downstream Lake Kasumigaura use dam as a water source, “Kasumigaura-Canal (completed in 1994)” Fig. 1 Kasumigaura River basin map

103 Bulletin of Jiyu Gakuen College of Liberal Arts Vol. 4 (2019)

Fig. 2 Sakuragawa River Fig. 3 Image of advanced water circulation mechanism pipeline network, supported by a stable large-scale irrigation function. On the other hand, the drainage function from the area is carried by rivers in the basin, and the flowing water and substances return to Lake Kasumigaura through rivers. At present, the of Sakuragawa river, it’s not very well. It is considered to be one of pollution load factors of Lake Kasumigaura. Furthermore, the most characteristic is that the complicated and sophisticated artificial hydrological cycle, in which a part of the refluxed water and substance is pumped again from Lake Kasumigaura, in a closed watershed. It can be said that the river basin of the Lake Kasumigaura area (Sakuragawa river and more) has advanced water circulation mechanism nationwide as having “dam lake” downstream of the basin (Fig. 3).

4. The future of the water environment in the Sakuragawa river basin As described above, the drainage from the farmland is accounts for in the river water of the Sakuragawa river. Therefore, in or- der to improve the water quality at a specific point, it is important to reduce the pollutant load amount, but it is not easy. The pres- ent time, “The Kasumigaura water conveyance project” is underway to introduce rich water of the Nakagawa river system to the metropolitan area via Lake Kasumigaura. As this is completed, the water circulation mechanism becomes more complicated, and there is a high possibility that the water of another water system will circulate also to the Sakuragawa river basin. I think that the impact of the project needs to be evaluated not only in Lake Kasumigaura but also in the area where the water is used. In particular there is concern about the spread of alien species. In any case, long-term monitoring of water quality.

References

1) Shinpei Yoshikawa, Chikako Ohtsuka, Masaaki Natsui, Chiharu Kumada, Akihisa Jin, Tetsuya Sumi: Water environment in the Sakuragawa river baisn under the advanced water circulation mechanism of Lake Kasumigaura, Visualization of actual water quality by multipoint electric conduction observation (in Japanese), The 17th World Lake Conference, 2018. 2) Chiharu Kumada, Shinpei Yoshikawa, Chikako Ohtsuka, Masaaki Natsui, Akihisa Jin: Current status and issues of water en- vironment of Sakuragawa River aim for water quality improvement (in Japanese), The 17th World Lake Conference, 2018. 3) Shinpei Yoshikawa, Chikako Ohtsuka, Masaaki Natsui, Akihisa Jin, Chiharu Kumada, Tetsuya Sumi: Sakuragawa River ba- sin under the advanced water circulation mechanism in the Lake Kasumigaura, multipoint electric conductivity observation for visualization of water quality characteristics (in Japanese), The 52nd Annual Conference of JSWE, 2018. 4) Shinpei Yoshikawa, Chikako Ohtsuka, Masaabnhji, Akihisa Jin, Chiharu Kumada, Tetsuya Sumi: Characteristics of water circulation mechanism in Kasumigaura inflow Sakuragawa river basin and Actual condition of water quality visualized by electric conductivity observation (in Japanese), Annual Conference of The Kantou Branch of JSCE, 2018. 5) Chiharu Kumada, About the water system Sakuragawa river as a way of nature and human involvement (in Japanese), Jiyu Gakuen College Graduation Research, 2017.

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