November: Impact of Sediment on Aquatic Life
(English follows Japanese)
[Gathering Materials]
It is the season when the autumn leaves of the surrounding trees are beautiful. However, we are not just admiring the autumn leaves; it is also the time when we are busy
gathering fallen leaves. The reason is that we use a large amount of fallen leaves for
shigara-dodome (wattle fence erosion control). (We leave the fallen leaves on the soil as they are, and collect those in places where they cannot decompose naturally, such as on concrete or asphalt.)
Aiming for environmental restoration from the entire watershed to the sea by taking care of the land area
Despite this grand goal, the activity sites are not private land but rivers and retarding basins under administrative management, so we cannot change the flow of the river or modify concrete structures. Therefore, as work that can be done with the time, labor, and tools available to the current activity members, we have been prioritizing building wattle fence erosion control to reduce the inflow of mud from the surrounding slopes into the water area.
[Securing Materials]
For this "wattle fence erosion control construction," what takes more time and effort than the actual work is securing and preparing materials. Because we use a large amount of branches of various thicknesses and fallen leaves, we ask acquaintances for them, collect branches and fallen leaves, separate thin branches from thick ones for the long pruned branches transported by truck from landscaping companies, and store the fallen leaves in large bags to transport them from the material storage area to the work site by car and on foot.
[Wattle Fence Erosion Control Construction Work]
Continuing the "wattle fence erosion control construction" in the area that ended halfway last month due to a lack of materials.
On the slope, the invasive species Tradescantia fluminensis is growing all over, so we remove what is growing in the work area. Even though it is an invasive species, treating it as a nuisance by "exterminating" or "removing" it makes me feel guilty...😓
Because it has extremely strong reproductive power, it is said that if you leave it around, it will grow again from there, so we take it home in large bags and take time to let it wither.
Two weeks later, we got a lot of thick branches, so we will stack them further on the previous location to increase the height.
The part we made last year with thin branches because we didn't have thick ones is decaying quite a bit, but it is collecting dead leaves. Since decay is within expectations, this is originally a structure
for the soil to stabilize and plants to grow, so that their roots support the slope, but even though we pruned the Aucuba, maybe because it is a tall tree and there is no sunlight, the grass is not growing...😢 I want to rebuild it more firmly with thick branches.
Even though we carry so many branches and fallen leaves that I wonder, 'Will we ever use all of this?', we end up using them all in no time at all...
We also sometimes take out and use branches and fallen leaves that have accumulated too much in the river.
[The Impact of Sediment on Aquatic Life]
While researching biological surveys conducted in rivers within the city in the past, I found the following text in a report compiled four years ago by a curator at the Yokosuka City Museum of Humanities and Nature.
https://y-suisitu.com/wp-content/uploads/2021/12/3_20211116.pdf
・Decrease in aquatic insects
→Inflow of sediment into riversdue todepletion of larvae before emergence, and there is a possibility that reproduction of emerged adults is being hindered by strong winds, salt damage, etc.
・Semisulcospira libertina (freshwater snails) could not be confirmed
→Inflow of sediment into riversnot onlyaffects the habitat of the snails themselves, but also leads to a decrease inattached algaewhich serve as their food source.
・Many Shimayoshinobori and Kuroyoshinobori (goby species) were noticeably emaciated → They are omnivorous and eat animal matter such as
aquatic insects and earthworms, as well as plants such as algae, but all of these food organisms are easily affected by the inflow of sediment.
In this way, the cause cited for the decline in organisms is in every case 'inflow of sediment into rivers'.
During heavy rain, the water from the river to the sea turns deep brown, so I assume the impact on the ocean is the same. While sediment flowing from slopes into rivers is something that happens even without human influence, areas where soil accumulates along rivers become
'fertile land', and at river mouths, they become tidal flats, both of which I believe are places that nurture diverse life. However, currently, they are divided by concrete for human convenience, disaster prevention, and economic reasons, and because the 'boundary area between land and water = a place rich in biodiversity where both exist' has disappeared, I feel that the situation has become one where only negative impacts are being caused...
Although it looks like children's play, I have reaffirmed that this is important work.
I will continue to work steadily in the future.
<Apology>
The talk event scheduled to be co-hosted on the 29th had to be canceled due to the poor health of the main speaker...😭
The work members were also running around at the grassroots level asking for flyers to be placed and were looking forward to the preparations, so it is truly regrettable, but I would like to hold it again in the future.
(Text: Mariko Miki / Organizer of 'Water Cycle Connection Activity: chotto')
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.
【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…
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.
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.
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 riverbanks 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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