Up to a thousand people marched in the Swiss village of Goumois on the French border in canton Jura on Saturday calling for the authorities to save the River Doubs.
Two marches, one from each side of the border, joined together at the bridge over the river in response to an appeal by environmental organisations.
There have been serious problems with the health of the river since 2009 with fish regularly found dead. Traces of an unknown fungal infection were found on the fish. Most of the 450-kilometre waterway flows through French territory.
The Swiss environmental protection organisation Pro Natura, one of the instigators of the demonstration said no one source was the course of the problem in the River Doubs.
In a joint statement issued with the WWF and the Swiss Fishing Association, Pro Natura said a combination of factors, including the use of fertiliser and untimely water releases from hydro-electric dams was to blame for damage to the river.
swissinfo.ch and agencies