Case No. #1508
The Backflowing River at Shimanto
Summary
During the rainy seasons of July 2022 and June 2023, local fishermen and tourists at a specific stretch of the Shimanto River in Kōchi Prefecture — near a guardrail-less submersible bridge — observed the river surface appearing to flow upstream for periods of several to more than ten minutes before resuming normal direction. A hydraulic engineer identified a large riverbed rock formation capable of generating a recirculation wake at certain flood volumes, but field-scale confirmation without flow measurement data was not possible. A local tradition holds that the river reverses when its deity is angered.
Details
The Shimanto River in Kōchi Prefecture is known as Japan's last pristine river. Its middle reaches are crossed by several low-clearance "submersible bridges" with no guardrails, which are popular tourist destinations. The riverbed in the relevant stretch is relatively uniform with no rapids or falls.
On July 14, 2022, a local fisherman in his sixties watching from atop a submersible bridge near Ekawasaki observed fallen leaves and surface foam moving upstream on the downstream side of the bridge. The phenomenon lasted approximately eight minutes before normal downstream flow resumed. Rainfall had been heavy the previous day and water levels were elevated.
Three similar reports followed from the same stretch in June 2023 — two from fishermen, one from a tourist crossing the bridge. The tourist noted: "The bridge has no guardrail so the water surface is right there. It was flowing upstream. I was confused because rivers are supposed to flow downstream. I photographed it."
A hydraulic engineer conducting a site survey identified a large submerged rock formation at the relevant location. At certain flow volumes during flooding, a recirculation wake (downstream-side counter-current) could theoretically form behind the rock. Whether this would produce visible surface backflow at the reported scale "is theoretically possible but cannot be confirmed without flow measurement data."
Local oral tradition names the phenomenon as something that has been known for generations and attributes it to the river god's anger during heavy rain.
Timeline
- July 14, 2022Local fisherman observes upstream-moving leaves and foam at a submersible bridge near Ekawasaki. Lasts ~8 minutes. High water after heavy rain.
- June 2023Three similar reports from the same stretch during the rainy season flood period.
- Summer 2023Hydraulic engineer surveys site; identifies rock formation as potential recirculation wake source. Scale unconfirmed without measurement.
- Autumn 2023Flow gauge installation under consideration. Continuous observation framework established.
- PresentRainy-season water level and backflow correlation monitoring planned.
Witness Testimony
Local fisherman (male, 60s)"Leaves were moving upstream below the bridge. Foam too. The river was flowing backwards. I watched for about eight minutes and then it suddenly went normal again. It happens sometimes after heavy rain. People have known about it for a long time."
Tourist (female, 40s)"With no guardrail you could see the water surface clearly. It was flowing upstream. Rivers are supposed to go downstream. I was confused. I photographed it. You can see it — the river is flowing backwards."
Hydraulic engineer (male, 50s)"A large submerged rock can produce a recirculation wake at certain flood volumes — theoretically a counter-current on the downstream face. Whether that would produce visible surface backflow at that scale requires flow measurement data to confirm."
Kyu's Analysis
The river flowed backwards. Fisherman and tourist both witnessed it. Theoretical explanation exists but is unconfirmed. Shimanto's deity is angry in the rain.
