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Mt. Bonoreyama (Bonomine) Day Hike | The Shirayazawa Gorge Is the Whole Point — Plus an Honest Test of Heart-Rate Hiking with the Suunto Vertical 2

2026.04.17

We took the Shirayazawa route up Mt. Bonoreyama (also known as Bonomine, 969 m) in Hanno, Saitama Prefecture, as a day hike. Two things drew me here. One is that you get a taste of stream walking. The other is the reputation of the gorge — everyone says it's spectacular.

With the hot season coming, this is a good mountain for finding some cool air beside water. This time I also had a slightly different theme in mind: using a Suunto Vertical 2 GPS watch to watch my heart rate and see whether it really makes climbing less tiring. Here's an honest report, from the stream walking through to what the heart-rate data changed.

Route overview

• Mountain: Bonoreyama (Bonomine, 969 m), Okumusashi area, on the border of Saitama and Tokyo
• Route: Shirayazawa trailhead — the gorge (along the stream) — Iwatakeishi — Gonjiri Pass — Bonoreyama summit (out and back)
• Distance / elevation gain: About 9 km total, roughly 700 m of cumulative gain
• Character: The first half is varied, with rock beside the stream and one waterfall after another. The second half is a steep climb through forest. The summit is broad, with a shelter and benches, so it's easy to rest
• Access and parking: There's parking for a few cars at the Shirayazawa trailhead

The Shirayazawa gorge | Cool water and a corridor of rock

The highlight starts almost as soon as you leave the trailhead: the gorge on the Shirayazawa stream. A gorge, in hiking terms, is a narrow deep ravine walled in by rock on both sides. Two of them come one after the other, and the scale only registers once you see a person standing in one. Watching the stream run through the gap between those rock walls stopped me in my tracks repeatedly. It really is what people say — remarkable that somewhere this far from ordinary life is this close to central Tokyo.

Along the way you pass Fujikake Falls and Tengu Falls, and the fact that you climb with the sound of water the whole way is genuinely soothing. Being beside the stream keeps it cool, which is exactly why I'd recommend this route as the weather warms up. Wet rock is slippery, so we watched our footing carefully at the rope sections and the stream crossings. There are kajika frogs in the clear water too — their call, which you hear in June and July, is one of the sounds of summer along a stream like this.

Iwatakeishi, Gonjiri Pass, and a summit in white

Past the stream you cross a forest road once, and from there the real climbing starts. You go by the large boulder called Iwatakeishi and keep gaining height to Gonjiri Pass. It's a complete change from the cool of the gorge — a sweaty, steep climb — but the mesh fabric from Montbell (Japan's best-known outdoor brand) that I was wearing stayed comfortable and dried quickly.

Near the top we were in solid white cloud. "There must be a spectacular view out there, and it is entirely white" was about all I could say. By the time we reached the summit, though, blue sky was showing through, the light came in and the ranges appeared. The summit of Bonoreyama is wide and open, with a shelter, benches and tables, and there were small children up there too, which gave it a friendly atmosphere. After about three hours of climbing, the drink at the top went down very well.

Trying heart-rate hiking with the Suunto Vertical 2

The thing I most wanted to test on this trip was climbing with a Suunto Vertical 2 GPS watch (this video was made in collaboration with Suunto).

The idea was simple: keep your heart rate around 130 and you'll tire less. In practice, the moment the trail steepens your heart rate jumps to around 150, so you ease off and let it settle, over and over. Doing that, I was surprised by how little fatigue built up in my legs. It's the same logic as alternating work and rest at the gym — keeping your body steady using data rather than guesswork makes sense.

The other thing I appreciated was near the summit, when the cloud closed in. Being able to see where I was on the map on my wrist meant no anxiety about getting lost with no visibility. Knowing exactly how much further you have to go is also very welcome late in a descent when you're tired. A mountain I'd previously climbed purely on feel became interesting in a different way once heart rate and pace were being recorded. If you've been wondering whether a smart watch is really necessary, I think it's worth trying once.

Final thoughts

Bonoreyama is a mountain that packs stream walking and satisfying rock into one day, and it's a welcome one in hot weather. The 969 m on paper is modest, but with a gorge, waterfalls and a steep climb it's varied enough to feel like a proper outing. If you enjoy moving over rock, you'll love it.

To summarise the points that matter:

• Best season: Cool beside the stream, so ideal as the weather heats up. Watch your footing though — the rock is slippery when wet
• How to climb it: Keeping your heart rate steady noticeably reduces leg fatigue. On a cloudy day, having a map (GPS) is reassuring
• What to wear: There's a sweaty steep section, so quick-drying mesh plus a hat for the sun is the comfortable choice

Bonoreyama gives you the beauty of a ravine and the sound of water without asking much in return. If it appeals, go and see it.

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