KITA-KYUSHU—A museum here offers a rare glimpse of when underwater worlds collide, but actually seeing the phenomenon depends mainly on luck.

Through an acrylic window at the municipal River Museum standing near the mouth of the Murasakigawa river, a “salt wedge” can become visible if the conditions are right.

A salt wedge is the boundary wall that separates a mass of a river’s fresh water from a body of salty seawater in an estuarine zone. Salt wedges can be seen only near river mouths where fresh water exists alongside seawater.

The discovery of the phenomenon in the river, which flows through downtown Kita-Kyushu, was made possible after decades of water quality improvement efforts.

The submerged window measures 2.3 meters tall, 7.2 meters wide and 25.5 centimeters thick.

On one recent day, as seen through the window, wavy lines abruptly emerged parallel to the water surface of the 5-meter-deep Murasakigawa river. The lines grew thicker and thinner, gradually connecting into a long single line that separated the water into two volumes.

“You were so lucky,” Masahiko Uchimura, the museum’s director, told me. “The phenomenon cannot always be seen. It has the scientific name of a ‘Helmholtz wave,’ but we customarily call it a ‘wavery zone’ or a ‘salt wedge.’”

Uchimura, 63, explained that the heavier seawater usually lies beneath the river’s fresh water. When the tide rises, the seawater ascends while the fresh water descends, creating a clear boundary.

In a cross-sectional view, it looks like a wedge of seawater driven in from beneath, hence the name salt wedge.

Salt wedges are usually parallel to the water surface, but they can stand upright or lie sideways.

They are more common on the Sea of Japan coast than on the Pacific coast because of tide-current conditions, Uchimura said.

Some establishments in Japan have observation windows for underwater views of rivers, including the Chitose Aquarium facing the Chitosegawa river in Hokkaido, and the salmon museum on the Miomotegawa river in Murakami, Niigata Prefecture.

But Kita-Kyushu’s River Museum is the first in Japan with an observation window in an estuarine area, which allows for views of salt wedges, Uchimura said.

“Our museum probably has very few, if any, other parallels elsewhere in the world,” he said.

Wavery zones can be spotted in the clear Choshigawa river in Mie Prefecture, but only after observers dive into the water.

The River Museum allows viewers to stay dry, Uchimura said.

However, he said the museum cannot extensively advertise the salt wedges because of their unpredictability.

Museum officials are keeping daily observation records to pinpoint conditions that favor the emergence of the phenomenon.

Kaiki Fukuda, an official with the museum’s project section, and his colleagues believe that salt wedges form because of a large salinity gap between the upper and lower water layers.

They hypothesize that salt wedges are prone to emerge over two to three hours near full tide two or three days after a daily continuous rainfall of 30 millimeters or more, regardless of the season.

But the phenomenon sometimes does not occur even if all these conditions are met.

“So much more needs to be understood,” Fukuda, 27, said.

Saltwater fish avoid crossing salt wedges into the fresh water above.

“Certain fish, such as Japanese sea bass, may end up in the upper layer when they chase prey, but they never stay long in the layer,” Fukuda said.

Fish movements, in fact, can show the presence of the separate layers.

‘DEAD RIVER’ REVIVED

The Murasakigawa, which flows through an industrial zone, had previously been called a “dead” or “black” river because it was seriously contaminated by wastewater and other pollutants.

For about 50 years, the industrial, government and private sectors have developed sewage systems, conducted cleanups and taken other measures to improve the river’s water quality.

The River Museum opened in July 2000 to educate the public on that history and to urge them to reflect on how water is linked with people’s livelihoods.

“Our focus was on the environment,” Uchimura said. “This salt wedge thing was a chance by-product. We knew nothing about it in the beginning.”

The museum advises people not to visit during the COVID-19 pandemic. But it hopes that record keeping will enable it to eventually present date and time forecasts for salt wedges to attract visitors after the pandemic is over.

‘WHAT A MIRACLE’

Laksamana Rayhan Utomo, a 29-year-old from Indonesia, was in the University of Kita-Kyushu’s master’s program for environmental engineering when he visited the River Museum in 2018 to learn about water quality improvement in the Murasakigawa river.

He happened to witness a salt wedge.

One revelation described in the Koran says, “And He is the one who merges the two bodies of water: one fresh and palatable and the other salty and bitter, placing between them a barrier they cannot cross” (25:53).

“The phenomenon I saw with my own eyes proved that the teaching of the sacred scripture is true,” Utomo said. “I wondered if a miracle had taken place. Perhaps the place is like no other in the world. What I saw there convinced me that the Koranic revelations are true, which further consolidated my faith.”

Utomo has since taken more than 30 people to the museum, including Indonesian students studying in Japan and Indonesian politicians on environmental inspection tours.

Although they could not always see a salt wedge there, Utomo said his recollections were enough to draw cries of, “What a miracle,” from his compatriots.

Utomo, who currently works for a company based in Kurume, Fukuoka Prefecture, said he will continue to use social media and other means to spread word about the phenomenon.

“Muslims are not just in Indonesia,” he said. “I hope all those who are familiar with the holy scripture will take a look at the wall in the water with their own eyes.”