1. A Blocked River Becomes a Lake: Lake Hibara
In July 1888, Mount Bandai in Fukushima Prefecture had a steam-blast eruption. One of its peaks collapsed, and a huge avalanche of rock and soil buried villages and killed 477 people [1]. Debris also blocked the Nagase River, and Japan's tourism agency reports that more than 300 lakes and ponds formed behind the debris, including Lake Hibara [3]. Sources differ on the day of the eruption (July 15 or July 21), so this article gives only the month and year.
A lake like this is called a dammed lake: a river is blocked, and the valley behind the block fills with water. The blocking material does not have to be volcanic. Landslides and lava flows can do the same job. Because we know the year Hibara began forming, we can say its age out loud, which is rare for a lake. So what if no dam is involved at all?
2. A Sinking Piece of Crust Makes a Hollow: Lake Biwa and Lake Baikal
Lake Biwa did not appear because something blocked a river. A 1959 paper from Kyoto University describes it as a tectonic basin, formed at least 3 million years ago when part of the earth's crust dropped along faults [2]. The hollow came first, and water collected in it. Today Biwa covers roughly 670 square kilometers and is about 103 to 104 meters deep at its deepest, and it is classed as a graben, a block of land that has dropped between faults [4]. For comparison, Lake Hibara is about 10 to 11 square kilometers, so Biwa is roughly 60 times larger by area (a calculation using sources [1] and [4], and the Hibara area is from Wikipedia).
The same kind of lake exists on the other side of the world. Lake Baikal in Siberia is often described as a rift lake, meaning it lies in a crack where the crust is pulling apart, though I could not read a source for that detail. UNESCO describes it as the oldest lake in the world, at about 25 million years, and the deepest, at about 1,700 meters [5]. Counting a lake's age is tricky, though. The answer depends on whether you count when the basin began or when the water reached its present shape, which is one reason different books give different ages for Biwa.
3. A Volcano's Bowl Can Hold a Lake: Lake Mashu
Volcanoes can also dig the hollow instead of blocking the river. Lake Mashu in Hokkaido sits inside a volcanic caldera, a big bowl left by a volcano. According to Wikipedia, it has no major river flowing in and no river flowing out [7]. With no big rivers in or out, the water must come and go in other ways, such as rain, snow, and seepage. This is the main difference from Hibara: at Mashu, the volcano made the bowl itself, while at Hibara the volcano made a dam.
Mashu is also known for clear water. It was reported to have a transparency of 41.6 meters in 1931, but readings since the 1950s have usually been 20 to 30 meters [7]. A record like that is a measurement from one day, not a permanent fact. These numbers come from a Wikipedia summary and were not checked against the original surveys. Next, a lake whose hollow was left behind by a river.
4. A River That Changes Course Leaves an Oxbow Lake
A river crossing a flat plain does not flow straight. It bends into loops called meanders. The narrow strip of land between the two sides of a loop is called a neck, and during floods the river can erode the neck away and take the shortcut, which is called a cutoff [6]. The old loop is now cut off from the main channel, and it becomes a crescent-shaped lake called an oxbow lake [6]. A study of the White River in Arkansas, USA, followed three cutoffs at different stages of turning from a river bend into an oxbow lake [8].
Here the order is reversed again. The hollow was a river channel first, and the lake is what is left over. Where meandering rivers cross wide plains, several oxbow lakes can sit side by side, each marking an older path of the river. But what about lakes beside the sea?
5. Sand Can Cut Off Part of the Sea and Make a Lagoon: Lake Saroma
On the coast, waves and currents can build a long ridge of sand across the mouth of a bay. The bay behind it can be cut off from the open sea and become a lagoon. Lake Saroma in Hokkaido is an example. Wikipedia describes it as a coastal lagoon of brackish (slightly salty) water, about 150 square kilometers, the third largest lake in Japan by area [7]. Lake Kasumigaura, in eastern Japan, is a related case, where land built up by a river is described as closing off a former bay. I did not find an English-language source I could read for that one, so treat it as an example to look up.
Japanese sources sometimes use different words for lagoons and for former bays that became lakes, and the line between them is not fixed. Whatever the name, the pattern is the same: the sea once reached the spot, and something built a wall of sand or soil that closed the door. So do lakes last?
6. Do Lakes Last Forever, or Do They Fill In?
Lakes are not permanent. Rivers and rain carry soil and plant material into them, and over time that material settles on the bottom. A textbook from Ohio University notes that some oxbow lakes drain or silt up from deposits left during floods [6]. What is left behind is called a meander scar, and wetlands and marshes often grow there [6]. Biwa and Baikal show that a lake can last for millions of years [2] [5], so filling in is a general idea, not a schedule: how fast a lake fills depends on the lake.
So the five types are really five answers to the same two questions: what made the hollow, and what keeps water in it? A blocked river and a river's leftover loop show how fast it can happen. A sinking crust or a volcano's bowl can last for much longer. River bends and sandbars are easy clues to look for on a map or on a visit, so the last two types are good ones to start with.
7. How to Test These Ideas with Sand and a Map
First, try a sand model outdoors, in a place where spilled water cannot run into anything it could harm, and with an adult nearby. Pile damp sand into a tray or a corner of a sandbox, pour a thin stream of water to carve a channel, then block the channel with sand. Water backs up behind the block, just like at Hibara. A real dam is far larger and messier, so this only shows the basic idea.
Second, open an atlas or an online map and pick several lakes near you. For each one, look for rivers flowing in or out, the coast, and the hills around it. Then use those clues to guess which of the five types it might be. A map can show you where things are, but not how a lake began, so check your guess with a geology page from a university or a national geological survey. If you want more, the original sources below are a good place to start. They give different numbers for the same lake in several cases, so compare them instead of trusting only one.