Floodplain: invasive species along Tsurumi River

City of Yokohama has three main rivers, Tsurumi 鶴見川, Katabira 帷子川, and Ohoka 大岡川, pouring into Tokyo Bay. Tsurumi River is the longest, with the largest basin. Its source is in Machida City of Tokyo. From there, it runs through northern Wards of Yokohama, i.e., Aoba 青葉区, Tsuzuki 都筑区, Midori 緑区, Ko’hoku 港北区, and Tsurumi 鶴見区. The river basin of Tsurumi has a good size. A large part of Tsurumi Ward is made of it. It created the space in Ko’hoku Ward for Shin-Yokohama Park with the Nissan Stadium where 2002 FIFA World Cup Final was held (and will be the main venue for football / soccer in 2020 Tokyo Olympics). Nowadays, especially its middle basin maintains “natural” riverbeds which are enough to have petit forests. From Shin-Yokohama Park to upstream, the river side has lots of wild Japanese walnut trees (Juglans ailantifolia) that I guess are the survivors of rampant urbanization of Tama Hill 多摩丘陵 during the last 150 years (; my post on October 28, 2016). This area is a pleasant place. Many enjoy relaxed walking, jogging, or cycling with refreshing winds from the river. In summer, kids play riverside laughing. In a very urban environment, the floodplain of Tsurumi River is a precious breathing space for us, residents of Yokohama.

Japanese walnuts in Yokohama as of July 2019

Floodplain environment receives frequent floods that wash over the previous living things and brings some other ones from upstream. It’s a peculiar, fast moving place. Japanese archipelago is destined to have lots of natural disasters, such as storms. Our fathers have spent huge amount of money and energy to lessen the impact of force majeure. Japanese hydrologically engineered our rivers for hundreds of years in order to control floods. This has made riverbeds very “artificial” place, especially in Kanto Region 関東地方. It is rare to find rivers in Megalopolis Tokyo area without undergoing concreted riverbeds or heavy dredging. Yeah. Thanks to that, we has not have yearly flooding of, say, Tsurumi River that was once seasonal things some 70 years ago. There is a price to pay. In Kanagawa Prefecture, we have Aster cantoensis kitam, which is Japanese endemic and endangered species (IB category). The reason why the chrysanthemum it has reached thus far is, we concreted the riverbeds for controlling the flow. It drastically reduced its habitat of sufficiently gravelly sand in floodplain. OK, that’s what I’ve heard. I’ve never seen Aster cantoensis kitam. The discussion has been something of abstraction. Then, one day while I strolled along Tsurumi River, I had a kind of Eureka! moment.

Although it looks natural, I noticed the vegetation in the riverbed for Tsurumi River has lots of invasive species. Their presence is definitely larger than, say, in Niiharu Citizen Forest which situates near the middle basin as one of the water sources for Tsurumi River. Disclaimer: in Niiharu, we Lovers are sentinels to spot and stop the invasion of non-endemic species that can run over precious endemic flora. Thanks to that, I’ve become somehow watchful about the existence of invasive ones. Yap, the floodplain of Tsurumi River is often undergoing drastic treatments. Of course, several times during typhoon season every year, the place is inundated with muddy river water, probably containing polluted materials from upstream cities of Tokyo. In addition to concreted banks here and there, once in a year or so, the municipality controls the growth of its vegetation with extensive mowing and chemical weed killers. Perhaps thanks to global warming, the frequency the plain receives such extreme is, it seems to me, increasing. No matter, the grasses and trees come back every time and cover the riverbed quickly. That’s itself impressive. And the survivors of the harsh condition are truly tough ones, I presume. And in the list of stayers, the species which come to Japan not long ago stand out with their energy.

Invasive species 1:
 English plantain (Plantago Lanceolata)

Invasive species 2:
 A colony of Wandering jew (Tradescantia fluminensis) …
 I’ve just noticed its English name suggests something …

I’m not sure if we can call this invasive …
 Orange Daylily (Hemerocallis fulva var. kwanso)
 originating from China,
 and came here in prehistoric time.
 (We have 8^th century poems describing this flower.)
 What is non-endemic, then?

That’s telling something. Those that can remain in an environment different from their home town are … strong. Can we really manage and preserve our endemic species with such formidable new comers that try to expand its territory no matter what? To begin with, the reason of their thriving along Tsurumi River is our city development, excavating old forest floor, pouring concrete over it, building factories and houses, bringing in foreign species from somewhere, etc. etc. Nature conservation … what does it mean?

If you find an environmental issues in Kanagawa Prefecture, please make a contact with Kanagawa Natural Environment Conservation Center 神奈川県自然環境保全センター

657 Nanasawa, Atsugi City, 243-0121 〒243－0121　厚木市七沢657
Phone: 046-248-0323

You can send an enquiry to them by clicking the bottom line of their homepage at http://www.pref.kanagawa.jp/div/1644/