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Joshin’etsukogen NP, Nagano
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A local historical account says “zako” here meant iwana—char—not just small or ordinary fish. Mountain fishers carried these substantial catches over steep passes to sell at hot-spring towns such as Kusatsu and Happō.
Zako is managed without hatchery-fish releases: most tributaries are protected as natural spawning streams, and fish under 20 cm must be released. In a 2017 survey, char density was 0.89 per m² in protected reaches versus 0.40 in fishing reaches.
Lodge wastewater once made char disappear from the smaller Kozako River in Zako’s upper watershed. Around 1974, Shiga Kogen stream water turned yellow and warmed in winter from 1–2°C to 12°C; a shared treatment plant opened in 1976, and char gradually reappeared.
In 1863, 12 local sponsors helped build the 25 km Yokokura canal, carrying highland water to fields below. The project reportedly took 25,000 person-days and cost 10,000 ryō—a reminder that Zako’s watershed helped sustain farming far downstream.
Some of the surprisingly level ground beside upper Zako, at 1,400–1,600 m, is sediment from an ancient lake. Volcanic welded tuff from Kayano-daira dammed the river, allowing sand, mud and gravel to settle before the lake disappeared.
After 379 mm of rain fell near Zako in 3 days during Typhoon Hagibis (October 11–13, 2019), a monitored reach saw char over 20 cm drop to 0.01 per m² in 2020. Their density returned to its pre-storm range by 2022.
An invasive freshwater diatom was found in lower Zako in 2021; efforts to stop its upstream spread did not prevent it reaching upper waters and a tributary. Follow-up surveys found no decrease in char density at monitored sites, though the alga’s spread was confirmed.
