To the 42-year-old woman dubbed "Madame Loire" by the French press, this is a sacred duty. The Loire is France's longest river--630 miles from its source in south-central France to its estuary on the Atlantic--and one of the most historic. Generations of French kings built their most beautiful chateaus in the temperate Loire Valley. It is home to some of France's most prestigious vineyards. The wetlands around the relatively shallow, meandering river and its tributaries provide a rich habitat for hundreds of species of birds and other animals; eel, trout and Atlantic salmon ply the waters.

An ambitious construction scheme was hatched in the early '80s by local officials and organizations determined to tame the river. The plans included two major dams, at Serre-de-la-Fare and Chambonchard, and two smaller ones. The stated aim was to prevent flooding, expand irrigation and boost water flow during dry years. Opponents suspected other motives: increasing the water supply to cool four nuclear reactors along the river and boosting development in areas now subject to flooding.

The one obstacle the dam builders never anticipated was the feisty Jean. A native of Nantes, she had been fascinated by nature since childhood, studied biology in college and got a master's degree in ecology. In 1985 she ran into a former teacher who was trying to organize opposition to the dams. "I went to some meetings with him and was soon gripped by the same passion to save the Loire," she explains.

In 1986 the antidam groups got funding from the World Wildlife Fund-France and formed Loire Vivante. Jean, then an unemployed single mother, was named its first coordinator in early 1987. Among her first acts was to organize environmental-impact studies showing that the dams would harm water quality, threaten biodiversity, destroy several villages, displace hundreds of people--and still fail to provide good protection against flooding. In 1989 the group launched its most spectacular and effective action: the occupation of the Serre-de-la-Fare site by several hundred ecologists who camped in tents, cooked over open fires, strummed guitars--and blocked the bulldozers for five years.

The Serre-de-la-Fare dam was finally canceled in 1994, and two others are in jeopardy. One smaller dam was built, but Jean believes the ecological impact will be limited. She is now turning her attention to fighting a plan to enlarge the port of Nantes-St. Nazaire, which would destroy wetlands in the Loire estuary.

It hasn't been easy for Jean to raise her two daughters, now 16 and 13, while devoting herself to a more than full-time job. But the payoff has been making a difference to a cause she deeply believes in. "I could never have spent as much time and energy," she says, her large green eyes shining, "on something that was less important to me."