What Is Lake Okushima (Oku-Shima)? The Shima Blue Reservoir Created by Shimagawa Dam
Lake Okushima (Oku-Shima) is a reservoir with a shoreline loop of about 4 km, created by the Shimagawa Dam and located further into the mountains beyond Shima Onsen (hot spring resort).
It sits in Shima, Nakanojō Town, Agatsuma District, Gunma Prefecture, at the innermost end of the Shima Onsen area.
Because the natural scenery and the dam facilities blend into one, the lake can look like a quiet mountain waterside or a powerful piece of civil engineering, depending on where you view it from.
The Mountain Lake Created by Shimagawa Dam
The Shimagawa Dam stands on the upper reaches of the Shima River, and its reservoir is Lake Okushima.
The Shimagawa Dam is a gravity concrete dam with a height of 89.5 m and a crest length of 330.0 m, completed in 1999 (Heisei 11).
It is the tallest of the dams managed by Gunma Prefecture, serving roles such as flood control and water supply.
Roads and scenic viewpoints line the lake, so you can enjoy changing views as you drive around it.
The dam is not a tourist structure, so stay out of restricted areas and management facilities, and follow the signs and fences as you look around.
Shima Blue: A Landscape Shaped by Light and Water
The lake water ranges in color from blue to blue-green, and its impression shifts with the direction of the sunlight and the brightness of the sky.
The Shima Blue is easiest to observe from April to May, when snowmelt flows in, and on clear mornings when sunlight strikes the surface.
Even on cloudy or rainy days the water takes on a calm shade, so rather than chasing the exact color you saw in a photo, it is better to enjoy the lake as it looks that day.
Seeing Nature and Dam Architecture Together
The 89.5 m-high body of the Shimagawa Dam presents a powerful form that contrasts with the green of the mountains and the blue of the lake.
By focusing on the spread of the water on the lake side and the height and shape of the dam body on the dam side, you can enjoy different scenery from the same spot.
Shima Blue at Lake Okushima: How It Looks Each Season and the Best Time to Visit
The Shima Blue is influenced by the water and the surrounding plants in each season, and its combination with the background colors changes too.
When planning your itinerary, it is easier to choose a time based on the seasonal scenery you want to see than to try to predict a day when the water will look especially blue.
The table below organizes the combinations of season and scenery.
| Season | Impression of the Lake Surface | Surrounding Scenery |
|---|---|---|
| Spring | Bright blue | Budding leaves and lingering snow |
| Summer | Shifting blue-green | Deep mountain green |
| Autumn | Changes with the light | Contrast with autumn leaves |
| Winter | Quiet tones | Snow and bare trees |
Spring (April–May): The Easiest Time to Watch the Blue Change
In spring, snowmelt flows into the lake, and from April to May the Shima Blue tends to look especially beautiful.
Fresh green spreads across the mountainsides, and you can enjoy the contrast between the blue lake and the soft new colors.
Temperatures can differ greatly between morning, evening, and midday, so layered clothing you can put on and take off is handy.
Summer: Wrapped in Deep Green
In summer the surrounding mountains are covered in deep green, and the blue and blue-green of the lake appear between the trees.
On days with strong sunlight the reflection off the water also becomes intense, so check the scene with your own eyes while adjusting your shooting position.
Weather can change in the mountains, so bring rain gear and be ready not to stay outdoors during thunder or heavy rain.
Autumn (Usually Mid-October to Early November): The Contrast of Fall Foliage and the Lake Takes Center Stage
Around Lake Okushima, the mountain trees usually change color from mid-October to early November.
How the autumn leaves progress varies with the temperature, so it is reassuring to check local updates and photos before your visit.
During the foliage season it is easy to be distracted by the scenery, so do not stop on the roadway; enjoy the view from a place where you can park.
Winter: Watch Out for the Loop Road Closure
In winter part of the lake surface may freeze, and you can gaze at snow-clad mountains and a quiet lake.
On the other hand, the lakeside loop road is closed in winter, though you can still go as far as the embankment (entei) side.
Check road information before you set out, and do not force your way forward in a car without winter equipment.
Top Things to See at Lake Okushima: The Dam Body, the Observation Deck, and Seseragi Park
At Lake Okushima, the basic way to enjoy it is not to rely on a single viewpoint but to compare places that differ in how close the water is, how the mountains overlap, and how the dam is structured.
Seseragi Park and an observation deck are laid out along the shore, and because the view changes as you move, making brief stops at several viewpoints lets you appreciate the scenery more fully.
Compare the Lake and the Valley from the Dam Body
Near the 89.5 m-high dam body, you can compare the calm water on the lake side with the steep drop into the valley on the downstream side.
Do not lean out beyond the fences or enter maintenance passages; enjoy the view from the areas open to the public.
On windy days, take care that hats and light belongings are not blown away.
At the Observation Deck, Notice How the Mountains Overlap
From the open observation deck, taking in not only the lake but also the mountain ridgelines stretching beyond it creates a composition that captures the essence of Lake Okushima.
Looking in turn at the trees in the foreground, the lake in the middle ground, and the mountains in the background conveys how the water is held within a narrow mountain valley.
At Seseragi Park and the Waterside, Look for Reflections and Small Changes
When the wind is light, mountains and clouds are mirrored on the lake, and when the wind picks up, fine ripples scatter the light.
By paying attention not only to the depth of the blue but also to reflections, ripples, and the plants along the shore, you can enjoy the lake in a way that is less affected by the weather.
How to Drive the Lakeside Road: One-Way Traffic and Safe Stops
The route around Lake Okushima is signposted as a one-way road with a loop of about 4 km, designed so you can circle the lake by car.
Avoid driving against the flow or stopping suddenly, and if you pass a spot you wanted to see, come back around by a safe route.
You can check the right action for each situation in the table below.
| Situation | Action | Point to Note |
|---|---|---|
| Starting the loop | Check the signs | Do not go the wrong way |
| Spotting a view | Head to a parking spot | Do not stop on the road |
| Walking to look around | Walk along the edge | Watch for cars |
| Bad weather | Leave early | Do not push it |
Check the Direction of Travel at the Entrance
Before entering the lakeside, check the direction of the one-way traffic and any restriction signs posted that day.
Give priority to on-site road signs and staff instructions over the guidance of a map app.
Stop Only in Designated Places
Even if a view you want to photograph suddenly appears, do not stop on the roadway or near curves.
Pull into a place where parking is allowed, and check the movements of pedestrians and other cars before getting out.
In crowded spots, do not occupy a space for a long time; sharing with others lets everyone look around calmly.
When Walking, Mind the Roadway and Your Footing
The lakeside road is not a pedestrian-only space used solely by tourists.
Limit where you walk to places you can confirm are safe on-site, and do not force yourself to move around after dark or in poor visibility.
Wet fallen leaves, snow, and frozen road surfaces are slippery, so easy-to-walk-in shoes are best.
Put Restrictions and Natural Conditions First
Around the dam lake, the areas open to traffic may change due to construction, inspections, snowfall, or falling rocks.
If fences or gates are closed, do not cross them; change your plans instead.
Photographing Lake Okushima: Tips for Capturing the Shima Blue Naturally
When photographing the Shima Blue, recording the day's light together with the surrounding scenery, rather than heavily editing the color, conveys the true character of Lake Okushima.
Taking some shots that fill the frame with the lake and others that include the mountains or the dam adds variety to your travel record.
The table below summarizes the differences in lighting conditions and how to shoot.
| Condition | How to Shoot | How It Looks |
|---|---|---|
| Clear morning | Look for front lighting | Bright blue |
| Cloudy | Include the mountains | Softer colors |
| Light wind | Aim for reflections | Nearly mirror-like |
| Foliage season | Include the shore | Contrasting colors |
Change Your Position to Match the Direction of the Sun
The morning of a clear day is a good time to see the Shima Blue, but even on the same stretch of water, changing your viewing direction slightly alters the strength of the reflection and how the blue appears.
If your screen glares white, avoid putting the sun directly in front of you and look for a position where you can see the boundary between the lake and the mountains.
That said, do not step onto the roadway or move beyond the fences just to find a shooting spot.
Shoot Without Blocking the Flow of People and Cars
If you use a tripod, do not block the path; leave enough width for other visitors to pass.
When photographing people as well, choose a place where no cars are coming from behind, and do not occupy the same spot for a long time.
Drone flights and commercial shoots are subject to different conditions than ordinary handheld photography, so you must check the manager's rules and the relevant laws in advance.
Access and Getting Around: From Shima Onsen Town to Lake Okushima
Lake Okushima lies even further beyond the Shima Onsen town, so you need to decide in advance how you will get there from the hot spring town.
The starting point for public transport is Nakanojō Station on the JR Agatsuma Line, from which the basic route is to transfer to a local bus bound for Shima Onsen.
Travelers heading to Shima Onsen by public transport can feel more at ease by checking with their accommodation or the tourist information center about how to get from the town to Lake Okushima.
The table below organizes the approach for each travel style.
| Travel Style | Planning Focus | What to Check |
|---|---|---|
| Rental car | Loop around the lake | Road restrictions |
| Public transport | Base yourself in the onsen town | Onward transport |
| Canoe experience | Use a tour operator | Reservations and conditions |
| Winter visit | Focus on the embankment side | Snow and closures |
By Car: Check the Lakeside Road Direction and the Weather
If you come by rental car, keep your speed down, bearing in mind that passengers unaccustomed to mountain-road driving may be with you.
It takes about 10 minutes by car from the Shima Onsen town to Lake Okushima, but since the return trip passes through the same mountainous area, it is wise to plan with enough time to get back to the town before sunset.
By Public Transport: Use Nakanojō Station and the Onsen Town as Your Base
If you use the train and local buses, the basic route is to head toward Shima Onsen from Nakanojō Station on the JR Agatsuma Line.
Routes and service conditions to Lake Okushima may change, so check with the transport operators and tourist information on the day.
If you go on foot, judge according to your fitness, taking into account the slopes, the weather, and how much daylight is left for the return.
Enjoy Canoeing Through a Local Operator
Canoe tours run by local operators are held on Lake Okushima.
The lake, with a loop of about 4 km, is said to be an easy size for beginners to paddle around, but rather than venturing onto the lake on your own, check the season, participation requirements, equipment, and booking method with the operator before taking part.
Because tour details may change depending on the weather and water level, allow some flexibility, including possible changes to your plans.
Summary: Enjoy the Shima Blue of the Day and Explore Lake Okushima Safely
Lake Okushima is a place where you can take in the Shima Blue water surface, the seasonal mountain scenery, and the form of the 89.5 m-high Shimagawa Dam all at once.
The bright blue of spring (April–May), the green of summer, the autumn leaves of fall (mid-October to early November), and the stillness of winter each have their own appeal, and not insisting on the exact color you saw in a photo leads to a more satisfying visit.
On the lakeside road, follow the one-way loop of about 4 km and use the designated parking spots, and after checking for winter closures and same-day restrictions, make a comfortable plan that includes getting there from the Shima Onsen town.
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