Ruth Patrick's studies of freshwater ecology in the 1930s helped galvanise the later environmental movement, and her success in a profession dominated by men charted a course for other female scientists. She began focusing on ecology at a time when the dangers of pollution barely pierced the public consciousness; her husband, the entomologist Charles Hodge IV, described life with her as "like being married to the tail of a comet."
Women were so rare in the sciences when she sought a job at the Academy of Natural Sciences in 1934 that she was told she would not be paid. She was also advised not to wear lipstick. It was about seven years before she earned a salary.
"My great aim," she said, "has been to be able to diagnose the presence of pollution and develop means of cleaning things up." Her research on limnology – the study of freshwater rivers and lakes – meant wading into around 850 rivers and streams around the world, and she was one of the first scientists to speak out about global warming. Her work led to the 1972 Clean Water Act, which she helped write. It remains the US's chief federal law on water pollution, and she advised Lyndon Johnson and Ronald Reagan on environmental issues.
Her chief contribution was identifying the significance of the diatom, now a key measure of water pollution. In the 1930s, when she was completing her doctorate, she was the first scientist to focus on this single-celled organism, eaten by other underwater creatures, that is present in almost every freshwater environment.
Earlier scientists had simply measured chemical levels to describe the health of bodies of water. Patrick found that measuring the presence of the diatom, using a tool she invented called a diatometer, gave a much better picture of the health of the ecosystem's life forms. The belief that biodiversity is the chief indicator of water health is now known as the Patrick Principle.
As a child she took nature walks with her father, a lawyer who had aspired to a career in science. "I remember the feeling I got when my father would roll back the top of his big desk in the library and roll out the microscope," she recalled. "He would make slides with drops of the water samples we had collected, and I would climb up on his knee and peer in. It was miraculous, looking through a window at a whole other world."
When she was seven he gave her her own microscope with the instruction: "Don't cook. Don't sew. You can hire people to do that. Read and improve your mind."
Patrick. who published more than 200 papers and contributed to books, taught botany and limnology at the University of Pennsylvania for over 35 years. After studying the water quality near DuPont chemical plants she became an adviser on environmental issues and in 1975 the first woman on its board. Until she was 97 she worked five days a week at the Academy of Natural Sciences and at 100 she still went in to work on her multi-volume Rivers of the United States.
Ruth Myrtle Patrick, environmental campaigner: born Topeka, Kansas 26 November 1907; married firstly Charles Hodge IV (died 1985; one son), secondly Lewis H Van Dusen, Jr (died 2004; three stepchildren); died Lafayette Hill, Pennsylvania 23 September 2013.