The Agatsuma River begins its journey in Nagano Prefecture, crossing into Gunma through the Torii Pass before flowing east to join the mighty Tone River at Shibukawa. Along the way, a two-to-three-kilometer stretch of the river has spent millions of years carving through Miocene-era andesite and tuff breccia, volcanic rock laid down roughly 5 to 23 million years ago. The result is a narrow ravine with sheer walls dropping 50 meters from the old route of National Route 145 to the river surface below. The river curves and bends through this channel, creating a shifting gallery of deep pools, small waterfalls, and rock formations so strange they look deliberately sculpted. The water itself runs a vivid blue-green, a color intensified by the mineral content of the volcanic substrate.

Agatsuma Gorge reveals different faces throughout the year, but two seasons draw the heaviest crowds. In spring, wild azaleas bloom in dense natural concentrations along the rock walls, splashing the dark stone with bursts of pink and crimson. But it is autumn that has made Agatsuma famous. From late October through early November, the deciduous broad-leaved forests lining the gorge ignite in shades of amber, scarlet, and gold. The fiery canopy creates a striking contrast against the turquoise river below, a combination that photographers and painters have been chasing for over a century. A 3.5-kilometer hiking trail traces the gorge, passing through the best viewpoints and ending at Shohorai, a viewing platform that also offers a panorama of the nearby Yanba Dam and its distinctive blue reservoir.

Before the modern era, Agatsuma Gorge existed in near-total isolation. The terrain was simply too rugged and remote for casual visitors. That changed in 1895 when the first road penetrated the gorge, and later when the JR East Agatsuma Line brought rail service to the area. Access transformed the gorge from a local secret into one of Gunma Prefecture's signature landscapes. The gorge's elevation to cultural icon was cemented in 1947, when Masahiko Urano created the Jomo Karuta deck to help war-orphaned children learn about their home prefecture. Each of the 44 cards represents something of local importance, and the gorge earned the coveted 'ya' card. Today, schoolchildren across Gunma compete in karuta tournaments each winter, and the gorge's card is one that every player learns to recognize instantly.

Agatsuma Gorge sits within a broader landscape of volcanic mountains and hot springs that defines western Gunma. The Agatsuma River's headwaters rise near Mount Kusatsu-Shirane, an active volcano whose acidic runoff once made the river hostile to aquatic life. The nearby resort town of Kusatsu Onsen, one of Japan's most famous hot spring destinations, draws from the same volcanic geology that formed the gorge. The Yanba Dam, completed upstream, has altered the hydrology of the area while adding a modern landmark to the gorge trail's terminus. Together, these features form a corridor of volcanic heritage: hot water, sculpted stone, and the persistent creative force of water working against ancient rock.