Sun Moon Lake: Taiwan’s Heart, Submerged and Revealed beneath an 18.18-Meter Rise in Water Level

30-Second Overview:
Sun Moon Lake has a dual identity: Taiwan’s most popular tourist lake, and the largest pumped-storage battery supporting the national power grid. In 1934, Japanese electric-power works caused the water level to surge by 18.18 meters, completely submerging the Thao people’s old settlement and fertile fields, while also opening Taiwan’s century-long history of hydroelectric generation. Today, it supports Taiwan’s largest pumped-storage hydropower system. Between late night and daytime, lake water moves back and forth across an elevation difference of 380 meters, providing the island with its most stable reserve power.

In June 1934, with the completion of the first Sun Moon Lake power station (today’s Daguan Unit 1), water from the Zhuoshui River was diverted into this once-quiet natural lake. At that moment, Sun Moon Lake’s water level began to climb slowly but steadily, ultimately reaching a full 18.18 meters above the original lake surface. This 18.18-meter rise was a dramatic transformation of geography and culture: a lake whose natural area had been about 5.75 square kilometers and whose deepest point was only about 4.8 meters expanded, after the water level was raised, to roughly 7.73 to 8.4 square kilometers at full pool, with a maximum depth of 27 meters and a storage capacity increased by about 6.72 times. The “old Shiyin settlement” (or Shuishe community), where the Thao people had lived for generations, and the fertile “floating fields” sank forever beneath the water. ^{1 2 3}

The Vanished Floating Fields and a Displaced People

For most visitors, the “beauty” of Sun Moon Lake comes from its broad expanse of blue-green water. For the Thao, however, it was a forced farewell. Before the water level rose, the Thao cultivated crops on the lake surface by layering grass and mud, forming a distinctive landscape of “floating fields.” These floating fields could be as large as several thousand square meters or as small as 1 square meter, either fixed to the water surface or drifting with the wind. In the Qing period, the literatus Lan Dingyuan marveled in Record of Shuili Shalian: “All around the shore is water; the local people plant grasses as fields, floating on the water surface, moving back and forth with the wind.” ^{4 5}

Yet to supply Taiwan-wide demand for electricity, this “wind-borne” secluded paradise was sacrificed. The rising water level submerged the Thao old settlement and floating fields, and also forced the Thao people to shift away from their original farming, fishing, and hunting lifeways toward tourism performances and handicrafts. ⁶ During recent droughts, such as in 2021 when the water level fell to around 738 meters, the remains of the Shuishe settlement, submerged for nearly a century, reemerged. Surviving stone walls and implements appeared, and even Qing-period tombstones from the Daoguang era, 178 years earlier, as well as large Thao dugout canoes and other remains surfaced, silently recounting those submerged memories and the broad distribution of settlement ruins. ^{7 8}

📝 Curator’s Note:
When we stand on the shore admiring the lake and mountains, what lies beneath our feet is in fact another people’s loss. The “beauty” of Sun Moon Lake is, at its core, an engineering miracle built on sacrifice.

Taiwan’s Largest “Pumped-Storage Battery”

If you visit Sun Moon Lake late at night, you may notice the lake water quietly “rising”: Taiwan’s largest pumped-storage hydropower system is operating, quietly sending water from downstream reservoirs back into the lake for storage. Sun Moon Lake and the lower Mingtan and Minghu reservoirs together form a vast energy-circulation system.

The Sun Moon Lake hydroelectric project, begun in 1919 and completed in 1934, was an important milestone in Taiwan’s modernization. The works included damming the upper Zhuoshui River at Wujie Dam and excavating an approximately 15-kilometer underground diversion tunnel through Mount Shuishe to bring water into Sun Moon Lake. After generation, the tailwater was then discharged into the Shuili River. To transport the materials needed to build the power plant, the Jiji railway line was born. ⁹ The Shuishe Dam, 30.3 meters high, and Toushe Dam, 10.08 meters high, on the lakeshore are the principal facilities that raised the water level. In addition, the spillway shaft on the lakeshore near Shuishe Dam, shaped like an “eagle-eye skylight,” is an important safety facility: when the water level is too high, excess water is discharged through it to prevent overflow of the dam body. ¹⁰

During off-peak late-night hours, the system uses surplus electricity to pump water from the lower reservoir back to Sun Moon Lake, the upper reservoir. During daytime peak demand, water is released to generate electricity. The installed capacity of Mingtan Power Plant reaches 1,602 MW, even exceeding that of the First Nuclear Power Plant. ¹¹ This “up-and-down” operation causes Sun Moon Lake’s water level to fluctuate by nearly 2 meters each day. The well-known “Nine Frogs Stack,” originally designed to promote ecological awareness, has now become Taiwan’s most intuitive “water-shortage indicator” and “power-generation pulse.” ¹²

It is worth noting that Daguan Unit 1 was damaged by U.S. bombing in 1944, temporarily halting power generation. In recent years, Taipower has further promoted a “pumped-storage hydropower 2.0” upgrade plan, raising the downstream Minghu Reservoir dam by 1.9 meters and adding nearly 1 million metric tons of reservoir capacity, for a full capacity of 7.9 million metric tons. The project is expected to generate an additional 222.3 million kWh of electricity per year, further strengthening Sun Moon Lake’s role as “Taiwan’s largest battery.” Daguan Unit 2 and Mingtan Power Plant still have a combined installed capacity of about 2.6 GW; Mingtan alone has 1,602 MW, with annual generation of about 2.4 billion kWh. Pumped-storage operations cause the daily water-level variation in Mingtan Reservoir, the lower reservoir, to reach about 28 meters, producing a localized “tidal” landscape. ^{13 14}

Thao Culture and Ancestral-Spirit Homeland: The Transformation of Lalu Island

Sun Moon Lake is the ancestral homeland of the Thao people, and their culture and legends are closely tied to this lake. The best known is the “white deer pursuit” legend: the Thao ancestors originally lived on the Chianan Plain or near Alishan, and while hunting chased a white deer across the Central Mountain Range, eventually arriving at Tuziting (Puzi). The white deer leapt into the lake; the people discovered abundant fish and shrimp and fertile land, and settled there. ¹⁵

Lalu Island (formerly Zhuzai Islet and Guanghua Island) holds supreme status in Thao culture. It is the highest ancestral-spirit site of the Thao, and the bishop wood tree on the island symbolizes the male ancestral spirits. Its history is filled with change: Thao and Han people once lived there; in 1879, an epidemic caused the people to disperse, while another account says a Han-built hexagonal pavilion damaged the site’s feng shui. During the Japanese colonial period, Tamashima Shrine was built on the island. After the water level rose, the island’s area shrank, and after the 921 earthquake part of it sank. The Thao actively fought to regain control of their ancestral-spirit site, ultimately restoring its name as Lalu and moving the Old Man Under the Moon statue formerly on Lalu Island to Longfeng Temple on the shore. The island is now entirely closed to ordinary visitors, as a sign of respect for the ancestral spirits. ^{16 17}

Thao culture has many other distinctive features. For example, the ancestral-spirit basket, which contains ancestors’ clothing and ornaments, must be worshipped in life-cycle rituals. During the Japanese colonial period, “pestle music” performances became a tourism highlight, but they also brought pressure from touristification. During recent droughts, the lakeshore saw the reappearance of the “swing festival,” a Thao tradition documented by anthropologists in 1958, rebuilt on exposed dry ground to recreate the old lakeside scene. It is worth noting that during the Japanese colonial period there was once a claim that the “Thao were a branch of the Tsou,” but differences in language and ritual are clear. Only in 2001 were the Thao officially recognized as Taiwan’s tenth Indigenous people. ^{18 19}

Water Level and the Nine Frogs Stack: Natural Pulse and Human-Made Indicator

The Nine Frogs Stack is located on the Shuiwatou Trail, between Dazhuhu and Ita Thao. The heads of the nine frogs correspond to different elevation levels, with the lowest frog at 745.90 meters. When all nine frogs are exposed, this does not mean Sun Moon Lake is in severe water shortage; at that point the storage rate is still around 60%. It mainly reflects the daily 1- to 2-meter fluctuation from pumped-storage hydropower and seasonal water conditions. In recent years, 2015, 2020, 2021, 2023, and 2026 all saw multiple instances when all nine frogs were exposed and the muddy base appeared. In late March 2026, because spring rains were limited and power-generation water use was high, the water level fell to about 741.6 to 742 meters, more than 6 meters below full pool, and Taipower took the opportunity to conduct dredging and cleaning. ^{12 20}

The Mass Swim and the Tourism Environment: A Struggle over Sustainability

Every year around the eighth lunar month, Sun Moon Lake reaches its noisiest moment: the mass swim across the lake. This event began in 1983 with only 557 participants, and has now evolved into an international event attracting tens of thousands of people each year. In 1995, the International Olympic Committee recognized it as the world’s largest long-distance swimming event. ^{21 22} The 43rd event in 2025 drew 24,736 registered participants, including 361 international swimmers from 35 countries and 130 swimmers with disabilities, setting a recent high. The 2026 event is expected to be held in September, with registration capped at about 20,000 people. ²³

Yet this major event also brings environmental challenges. Although local businesses welcome the tourism crowds, environmental groups and some residents worry about the burden on water quality from tens of thousands of people entering the area in a short period of time. ²⁴ This tug-of-war between “tourism value” and “ecological sustainability” is precisely a microcosm of what Sun Moon Lake faces today. In addition, urban land readjustment in 1983 led to the division of Thao lands, while commercial development, such as hotels and BOT projects, has conflicted with ancestral-spirit worship. Some development environmental impact assessments have been criticized for insufficiently evaluating effects on Thao culture, such as interference with rituals. ²⁵

Aftertone: Between Sun and Moon

“The water to the south of the mountain is round like the sun; the water to the north of the mountain bends like a half-moon.” This is how Qing-period writer Cao Shigui defined the lake’s name in Diary of Officialdom. ²⁶ Sun Moon Lake was historically known as Shuishalian Lake, Shuishe Lake, Dragon Lake, and by other names. Wenwu Temple has a “Year Steps” staircase with 366 steps, one for each day of the year, engraved with solar terms and notable figures, symbolizing blessings through the passage of time. ²⁷

Today’s Sun Moon Lake still changes color between sunrise and sunset, but the isolated mountain lake that once existed sank into history as early as 1934. It is an energy artery, an ethnic scar, and a tourism showcase. It is constantly balancing energy demand, ecological sustainability, and the traditional territory of the Thao people. The next time we stand at Shuishe Pier and look at Lalu Island wrapped in morning mist, perhaps we can try to listen to the sounds beneath that 18.18-meter rise in water level: there are the whispers of Thao ancestral spirits, and also the ceaseless electric pulse of this island. When droughts bring ruins or rituals back into view, they remind us that submerged settlements and memories still remain on the lakebed.

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