11月：泥の堆積が水辺の生物に与える影響　November Work Report:Impact of Sediment on Aquatic Life

2025年11月30日 02:06

(English follows Japanese)

【資材集め】

周囲の木々の紅葉が美しい季節。
でも、私たちは紅葉を愛でるばかりではなく「落ち葉集め」に奔走する時期でもあります。
理由は「しがら土留め」に大量の落ち葉を使うから。
（土の上にある落ち葉はそのままにして、コンクリートやアスファルトの上など自然に分解できない場所にあるものを集めます）

「陸域の手入れをすることによって、流域全体〜海中までの環境再生を目指す」

という壮大な目的はありながらも、活動場所は私有地ではなく行政の管理下にある河川や遊水池なので、川の流れを変える、コンクリート構造物に手を加えるなどはできず、今現在の活動メンバーが費やせる時間、労力、持っている道具でできる作業として「周囲の斜面地から水域への泥の流入を軽減させるしがら土留めづくり」を優先して行ってきています。

【資材の確保】

この「しがら土留めづくり」、実際の作業よりも時間と労力がかかるのが「資材の確保・準備」。
いろいろな太さの枝と落ち葉を大量に使うため、知り合いに声をかけまくって枝と落ち葉を集め、造園やさんからトラックで運んでもらった長いままの剪定枝は、太い枝から細い枝を切り分け、資材置き場から作業場所まで車＋徒歩で運ぶために落ち葉は大きな袋に入れて保管。

知人の敷地で出た剪定枝をもらいに行く。自家用車のトランクで運ぶので何往復もする。

【しがら土留めづくり作業】

先月、資材切れで中途半端に終わっている箇所の「しがら土留めづくり」の続き。

斜面には、外来種のトキワツユクサが一面に繁茂しているため、作業箇所に生えているものを除去。外来種とは言え「駆除」「除去」と邪魔者扱いは罪悪感、、、😓

ものすごく繁殖力が強いため、その辺に置くとすぐそこかからまた生えてくるとのことで、大きな袋に入れて持ち帰り、時間をかけて枯らします。

２週間後、太い枝がたくさん手に入ったので、前回の場所にさらに重ねて高さを出します。

去年、太い枝がなく細い枝で作った箇所は、だいぶ朽ちてきているものの、枯葉が溜まってくれています。
朽ちることは想定内なので、本来であれば土壌が安定して草木が育ち、その根で斜面を支えてもらうための造作ですが、アオキの剪定をしたとは言え高木で陽が当たらないせいか草が育っていません、、、😢
太い枝でもっとしっかり作り直したい。

枝も落ち葉も「こんなに使い切れるかな？」と思うくらい運ぶものの、あっという間に使い切ってしまう、、、

川の中に溜まりすぎている枝や落ち葉を少し取り出して使ったりも。

【水生生物への土砂の影響】

過去の市内の河川で行われた生物調査について調べていて、4年前に横須賀市人文・自然博物館の学芸員の方がまとめられたレポートに下記の文章を見つけました。
https://y-suisitu.com/wp-content/uploads/2021/12/3_20211116.pdf

・水生昆虫類の減少
　→河川への土砂の流入による羽化前の幼虫の減耗、強風や塩害などによる羽化した成虫の繁殖の阻害などが生じている可能性

・カワニナが確認できなかった
　→河川への土砂流入がカワニナ自身の生息に影響するばかりでなく、餌となる付着藻類の減少をまねく。

・シマヨシノボリ、クロヨシノボリが、やせ細った状態のものが目立った
　→雑食性で、水生昆虫やミミズ類などの動物質のほか、藻類などの植物も食べるが、いずれの餌料生物も土砂の流入の影響を受けやすい。

と、生物の減少の原因と挙げられているのがいずれも「河川への土砂の流入」。

大雨の際には川〜海まで真茶色になっているので、海中への影響も同様だと推測します。

斜面から土砂が川へ流れ込むことは人間の影響がなくても起きることですが、川沿いに土が溜まった場所は「肥沃な土地」として、河口では干潟となり、共に多様な生物を育む場所となるのだと思いますが、現在は人間の利便性、防災、経済のためにコンクリートで分断され、「陸と水域の境界域＝両方ある生物多様性が豊かな場所」が消失したためにマイナスの影響しか与えない状況になっているのかなと、、、

子供の遊びのように見える作業ですが、大切な作業だと再確認。
今後も地道に継続していきます。

＜お詫び＞
29日に共催で開催予定だったトークイベントは、メインの登壇者の方の体調不良により開催中止となってしまいました、、、😭
作業メンバーも草の根でチラシ置き依頼に奔走したり、楽しみに準備を進めていたので本当に残念ですが、来年以降にリベンジ開催したいと思っています。

（テキスト：三木真理子/「水の環つなぎ活動:chotto」主宰）

November Work Report

It’s the season when the leaves on the surrounding trees turn beautiful shades of red and yellow. But for us, it’s not just a time to admire the autumn colors—it’s also the season when we’re busy gathering fallen leaves.
（We leave the fallen leaves that are on the soil as they are, and collect only those that are on concrete, asphalt, or other surfaces where they cannot decompose naturally.)

The reason?
We use huge amounts of them to build our "shigara", hand-built erosion-control structures created by staking branches on the ground and weaving additional branches between them, then filling the structure with fallen leaves to slow water flow and trap sediment.

Our overarching goal is “to restore the environment from the land upstream all the way to the ocean.”

But since our activity sites are rivers and retention basins managed by local government—not private land—we are not allowed to alter the flow of the river or modify concrete structures. So, given the time, energy, and tools our current volunteer members can realistically commit, we’ve prioritized building "shigara" fences to reduce the flow of sediment from the surrounding slopes into the water.

【Securing Materials】

When it comes to making these shigara fences, what actually takes more time and effort than the building itself is gathering and preparing the materials. Because we need huge quantities of branches of various thicknesses and lots of fallen leaves, we reach out to everyone we know for help collecting them.
The long pruning branches we receive by truck from landscaping companies have to be sorted—cutting smaller branches from the larger ones—and the leaves are stored in large bags so that we can transport from our material storage area to the worksite by car and then on foot.

Picking up pruning branches from a friend’s property. Since I transport them in my car trunk, it takes several trips.

Large branches are cut down and separated into branches and leaves.

【Building "shigara" fences】

We continued building shigara fences in an area where large amounts of soil have been washing down the slope into the river, narrowing its width.

The slope is completely overgrown with the invasive Tradescantia fluminensis, so we removed the plants growing in the work area.
Even though it’s an invasive species, calling it “removal” or “eradication” makes me feel a bit guilty…

We used up all the branches and leaves we brought quickly

Two weeks later, we received some thick branches from our friends, so we added another layer to the ones we made last time and increased its height.

Since there are roots near the tree trunk and we can’t drive stakes in, we place stones instead.

We pack in a large amount of branches and fill the gaps with lots of fallen leaves.

The section we built last year with only thin branches—because we didn’t have thick ones—has started to decay quite a bit, but it has still managed to accumulate plenty of fallen leaves.
Since eventual decay is expected, the structure is originally intended to stabilize the soil so that grasses and trees can grow and their roots can support the slope. However, grasses are not growing yet because tall trees are blocking the sunlight even after we pruned Aoki trees (Japanese Aucuba) on the slope … 😢
Still, we would like to rebuild it properly using thicker branches.

The upper section is the one we built last year.

We transport what feels like an almost excessive amount of branches and leaves, wondering, “Can we really use all of this?”—but somehow we go through it all in no time.

We also sometimes pull out some of the branches and leaves that have piled up too much in the river and put them to use.

Impact of Sediment on Aquatic Life

While reviewing past biological surveys conducted in rivers within the city, I came across the following observations in a report compiled four years ago by a curator at the Yokosuka City Museum:
https://y-suisitu.com/wp-content/uploads/2021/12/3_20211116.pdf

Decrease in aquatic insects
→ Sediment flowing into the river may be causing high mortality among larvae before they emerge, and strong winds or salt damage may be hindering the reproduction of adults after emergence.
No kawanina snails were found
→ Sediment inflow not only affects the snails’ ability to inhabit the area but also reduces the periphyton they feed on.
Many Shima-yoshi-nobori (R. nagoyae) and Kuro-yoshi-nobori （R. brunneus）gobies were found in an emaciated state
→ These omnivorous fish eat aquatic insects, worms, and other animal matter as well as algae, but all of these food sources are easily affected by sediment entering the river.

In each case, the decline in these species was linked to the inflow of sediment into the river.

When it rains heavily, the river all the way to the sea turns completely brown, so I assume the impact on the marine environment is similar.

I believe that even without human influence, soil from slopes would naturally flow into rivers and form riverbank which is a critical zone for ecological interaction, and at river mouths, these areas would form tidal flats that both nurture diverse forms of life.
However, nowadays, for the sake of human convenience, disaster prevention, and economic priorities, these areas have been divided by concrete. As a result, the “border zones between land and water— places rich in biodiversity” have disappeared, leaving us with a situation where the effects are only negative…

To prevent things from getting any worse, we’ll continue steadily building shigara sediment-control fences.

(Text: Mariko Miki, organizer)

*It’s very unfortunate, but the talk event we had planned to hold on Nov. 29th has been canceled due to the main speaker’s illness. We hope to reschedule it sometime next year or later.

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