This town’s name holds the character for “sea,” even though Nagano Prefecture has no sea. Its origin reaches back more than a thousand years. A single peak of the volcano rising to the west collapsed greatly, and its earth and rocks dammed the river flowing through the valley, giving birth to several lakes. In this mountain recess far from the sea, a fleeting “sea” appeared. Most of the lakes eventually vanished, but only place names containing “sea” — Koumi, Umijiri, Uminokuchi — remained along the river as their memory. The “sea” valley caught between mountains has reduced its population below five thousand. Koumi’s numbers are the record of a town marked by a history in which the collapse of a mountain dammed a river.
A town in eastern Nagano Prefecture, opening onto the land of Minami-Saku, spreading east–west between the volcanic range to the north and the mountain mass to the east. Through the center of this town the Chikuma River flows north–south. This mountain-recess town that holds the character for “sea” in its name has inscribed, as place names, the memory of how the western volcano collapsed long ago and dammed the river, giving birth to a lake on a sealess land. The population fell below five thousand over twenty years: from 5,961 in 2000, through 5,663 in 2005, 5,180 in 2010, 4,713 in 2015, to 4,353 in 2020. What I (Atlas) want to read here is not the sign “a sealess town of the sea,” but the causal thread — how a history in which the collapse of a mountain dammed a river is translated into the present population and finances.
01 · Looking at the present Koumi by its numbers
In the most recent Population Census the population is about 4,400 (4,353 in 2020). From 5,961 in 2000, through 5,663 in 2005, 5,180 in 2010 and 4,713 in 2015, it reached 4,353 in 2020 — falling below five thousand by some one thousand six hundred over twenty years.
Look into the makeup and the figure of a town of the “sea” valley caught between mountains appears. The share aged 65 and over rose from 29.7% in 2000 to 42.1% in 2020 — up about twelve points over twenty years, now past four in ten of the townspeople. Households with children were 15.7% in 2020. The childcare waitlist was zero in both 2024 and 2025. The Fiscal Capacity Index was 0.26 in FY2023 — its own tax revenue covers only a little over a quarter of expenditure, a level with a large degree of reliance on the local allocation tax. The numbers show a town of the “sea” valley caught between mountains, reducing its population below five thousand and pushing aging past four in ten. Why it takes this shape cannot be read without going back to the history of the collapse, the river and the lake.
Source: Population Census (Statistics Bureau, MIC) / Local Government Finance Survey (MIC, Fiscal Capacity Index) / Status Report on Childcare Facilities (Children and Families Agency) / Real Estate Information Library (MLIT)
02 · The collapse of a volcano, the dammed river, and the “sea” place names of a sealess land — the history behind the numbers
This town’s skeleton is set by the collapse of the volcano rising to the west, by the river it dammed, and by the “sea” place names that remained on a sealess land. The oldest layer is the collapse. More than a thousand years ago, a single peak of the volcano to the west collapsed greatly, and its great volume of earth and rocks dammed the river flowing north–south through the valley and its tributaries. In this mountain recess far from the sea, several lakes appeared for a time, and the valley sank under water. The collapse of the volcano reworked the shape of this valley all at once.
Most of those lakes eventually vanished and the river again flowed through the valley, but the place names alone kept the memory. Place names containing “sea” — Koumi above all, and Umijiri and Uminokuchi — remained along this river in sealess Nagano. Why do the characters for “sea” line up in this mountain valley farthest from the sea? The answer lies in the memory of the terrain of this valley, where the collapse of a volcano dammed the river and gave birth to lakes. The valley caught between the volcanic range to the north and the mountain mass to the east is now run through north–south by a road and a railway along the river. The collapse of the volcano, the dammed river, the “sea” place names of a sealess land — this town’s shape stands upon a history in which the collapse of a mountain dammed a river and left the name “sea” on a sealess land.
Source: Koumi Town / origin of the place name and the collapse of Yatsugatake (a debris avalanche from the great collapse of Yatsugatake (Mt. Tengu) in the Ninna era (around 887) dammed the Chikuma River and the Aiki River and formed a large lake, which is said to be why place names containing the character for “sea” — Koumi, Umijiri, Uminokuchi — remain along this Chikuma River basin even though Nagano Prefecture has no sea — overview) / Koumi Town / geography and Lake Matsubara, the Koumi Line (the town spreads east–west between the Northern Yatsugatake and the Oku-Chichibu mountains, and along the belt of flat land following the Chikuma River that flows north–south through its center run National Route 141 (the Saku-Koshu Kaido) and the JR Koumi Line; Lake Matsubara is a natural lake formed when the collapse of Yatsugatake dammed the Otsuki River — overview)
03 · In the “sea” valley caught between mountains, reducing the population below five thousand
What characterizes Koumi is that, while carrying a history that holds the place name “sea,” it has reduced its population below five thousand over twenty years. From 5,961 in 2000 to 4,353 in 2020, some one thousand six hundred were lost over twenty years, a decline past twenty percent. The terrain of a valley caught between the volcanic range to the north and the mountain mass to the east makes it hard to take broad land for housing or factories as a plains city would, and hard to make places for the young to stay. In a mountain-recess valley, it can be read, the outflow of population has continued. That the share aged 65 and over reached 42.1% in 2020, past four in ten, is the consequence.
On the other hand, the childcare waitlist was zero in both 2024 and 2025, but it is more accurate to read this as the flip side of the fact that the share of households with children itself was low at 15.7% in 2020 and the number of children waiting for childcare is simply small, rather than that childcare provision is generous. A Fiscal Capacity Index of 0.26 is a level where its own tax revenue covers only a little over a quarter of expenditure, and it cannot stand without national support. The town of the “sea” valley caught between mountains is still reducing its population. The population is below five thousand, aging is past four in ten, and fiscal stamina is a little over a quarter. The closed valley that once received the collapse of a volcano and gave birth to a lake now, with that same closed terrain, narrows the places where the young can stay — the terrain that left the name “sea” has written even the population decline into the record.
Source: Population Census (Statistics Bureau, MIC) / Local Government Finance Survey (MIC, Fiscal Capacity Index) / Status Report on Childcare Facilities (Children and Families Agency)
04 · The origin by which the collapse of a mountain dammed a river and left the name “sea” on a sealess land
The history Koumi holds is not one thing. There is the terrain of a valley closed in by mountains, caught between the volcanic range to the north and the mountain mass to the east, with the Chikuma River flowing north–south through its center. There is the history in which, more than a thousand years ago, a single peak of the western volcano collapsed and dammed the river, giving birth to a lake on a sealess land, and that memory remained as the “sea” place names of Koumi, Umijiri and Uminokuchi. The terrain of a valley closed in by mountains once received the earth and rocks of the collapse and gave birth to a lake, and now makes it hard for people to stay.
Koumi is a town where the collapse of a mountain dammed a river and left the name “sea” on a sealess land. From the collapse of the volcano, to the dammed river, to the “sea” place names of a sealess land, to a population that fell below five thousand — the Chikuma River valley caught between the volcanic range and the mountain mass received the earth and rocks of the collapse and gave birth to a lake, and shut that memory up in the two characters for “sea.” Why do Koumi, Umijiri and Uminokuchi line up in this mountain recess farthest from the sea? The answer to that very question remains in the town’s name, as the fleeting lake that filled the valley a thousand years ago.
Source: Koumi Town / origin of the place name and the collapse of Yatsugatake (a debris avalanche from the great collapse of Yatsugatake (Mt. Tengu) in the Ninna era (around 887) dammed the Chikuma River and the Aiki River and formed a large lake, which is said to be why place names containing the character for “sea” — Koumi, Umijiri, Uminokuchi — remain along this Chikuma River basin even though Nagano Prefecture has no sea — overview) / Koumi Town / geography and Lake Matsubara, the Koumi Line (the town spreads east–west between the Northern Yatsugatake and the Oku-Chichibu mountains, and along the belt of flat land following the Chikuma River that flows north–south through its center run National Route 141 (the Saku-Koshu Kaido) and the JR Koumi Line; Lake Matsubara is a natural lake formed when the collapse of Yatsugatake dammed the Otsuki River — overview) / Population Census (Statistics Bureau, MIC)
05 · Atlas note — the place name “sea” and the population decline are the front and back of one closed valley
Lay out Koumi’s numbers and the indicators of a town of a valley caught between mountains all line up at harsh levels: a population that fell below five thousand over twenty years, an aging rate of 42.1%, a 15.7% share of households with children, and fiscal capacity 0.26. But what I (Atlas) want to read as a certified public accountant is the history that the character for “sea” inscribed in this town’s name is the memory of a single great event — the collapse of a volcano. Why do “sea” place names line up in this mountain recess farthest from the sea? The answer lies in the memory of the terrain of this valley, where long ago a volcano collapsed and its earth and rocks dammed the river and gave birth to a lake. The name of a land keeps conveying what happened on that land, even across a thousand years — the two characters of “Koumi” show this well.
Another point to consider is that the terrain of a valley closed in by mountains that received the collapse of that volcano now also forms the background of this town’s population decline. The valley that once received the earth and rocks of the collapse cannot take broad land for housing or industry as a plains city would, and makes it hard to make places for the young to stay. The unique memory of the place name “sea” that the terrain brought, and the population decline that the same terrain brings, are the front and back of one and the same valley. Behind the harshness of the figure of a Fiscal Capacity Index of 0.26, it can be read, lies that closed terrain. The town farthest of all from the sea inscribes the character for “sea” in its name. It is the memory of a thousand years ago, when a volcano collapsed and dammed the river and gave birth to a lake. That same closed valley now makes it hard to take housing land and has pushed the aging rate up to 42.1%. The name “sea” and the population decline are the front and back of one valley.
Source: Population Census (Statistics Bureau, MIC) / Koumi Town / origin of the place name and the collapse of Yatsugatake (a debris avalanche from the great collapse of Yatsugatake (Mt. Tengu) in the Ninna era (around 887) dammed the Chikuma River and the Aiki River and formed a large lake, which is said to be why place names containing the character for “sea” — Koumi, Umijiri, Uminokuchi — remain along this Chikuma River basin even though Nagano Prefecture has no sea — overview) / Koumi Town / geography and Lake Matsubara, the Koumi Line (the town spreads east–west between the Northern Yatsugatake and the Oku-Chichibu mountains, and along the belt of flat land following the Chikuma River that flows north–south through its center run National Route 141 (the Saku-Koshu Kaido) and the JR Koumi Line; Lake Matsubara is a natural lake formed when the collapse of Yatsugatake dammed the Otsuki River — overview)
Editor’s note: all figures and sources are drawn from official statistics. The prose follows Atlas’s voice, and AI (atlas-handcrafted-reverse-v1 (Daiki 2026-06-02)) handled the shaping of the text. Evaluative or predictive language (such as “a good buy” or “attractive”) is intentionally left out. Revision id: wave27w_