Thomas Dolan IV, 98, of Lafayette Hill, a celebrated conservationist who identified a new category of mayfly; the former executive director of the Wissahickon Valley Watershed Association, now Wissahickon Trails; and the former president of the Eastern Pennsylvania Chapter of the Virginia-based Nature Conservancy, died Tuesday, Dec. 28, of congestive heart failure at the Hill at Whitemarsh retirement community.

Mr. Dolan was an expert in the biodiversity of streams, rivers, and lakes, and in the 1940s observed a new type of mayfly, an insect that lives in and near water, that was later named Dolania americana. He went on to pioneer early flood-plain zoning to protect watersheds in urban areas, and initiate and supervise many important local wetland preservation projects.

In the 1950s, he founded Consulting Biologists Inc., a group that evaluated and restored polluted rivers and streams. He became executive director of the Wissahickon Valley Watershed Association in 1964, and in the 1970s created the nonprofit Environmental Planning and Information Center to assist other ecological activists.

In 2017, Mr. Dolan and his family pledged funds to the Academy of Natural Sciences of Drexel University, formerly the Academy of Natural Sciences of Philadelphia, to create the Dolan Fund for Innovative Water Research. “When the family gathered together to talk about what we wanted to do for the Academy, we decided that water will be the enduring issue facing our country and the world,” Mr. Dolan said at the time.

Mr. Dolan was treasurer and secretary of the Philadelphia Conservationist Board, now Natural Lands, nonprofit, and helped create and oversee the 1,000-acre John Heinz National Wildlife Refuge at Tinicum in Delaware County near Philadelphia International Airport. He was a supporter of the Appalachian Mountain Club, an emeritus board member of Wissahickon Trails, and board member and conservation committee chief at the Philadelphia Zoo.

He won the zoo’s 2018 Conservation Impact Award, the 2013 Henry Meigs Environmental Leadership Award from the Schuylkill Center, and was named in 2000 as one of the Nature Conservancy’s 50 heroes of the environment.

“He loved science, and he loved the outdoors,” said Mr. Dolan’s son Thomas V. “So conservation was a good match for him.”

Mr. Dolan edited Wildlife Conservation by Sustainable Use, published in 2000, and Richard E. Leakey, the famed Kenyan conservationist, wrote in the book’s preface: “This volume represents important data on a critical subject. … The editors Herbert Prins, Jan Geu Grootenhuis, and Tom Dolan are to be congratulated.”

Born April 16, 1923, in Philadelphia, Mr. Dolan grew up in Devon, and attended Episcopal Academy and St. Paul’s School in Concord, N.H. He worked for the American Field Service as an ambulance driver in India and Burma, now Myanmar, during World War II, and afterward graduated from Cornell University in 1948 with a bachelor’s degree in zoology and conservation.

He told the Chestnut Hill Local in 2013 that he chose to work in conservation because, “I wanted to do something to make a difference.” He married Margaret MacLeod Knight in 1942, and they had daughter Margo, and sons Thomas V, and Brooke. After a divorce, he married Elizabeth Gubb in 1976. His wife and former wife died earlier.

After college, Mr. Dolan worked with renowned ecologist Ruth Patrick at the Academy of Natural Sciences. He later took on river survey projects for the DuPont company, and joined the Nature Conservancy in the 1980s.

Away from work, Mr. Dolan was an avid reader, fly fisherman, and birder. He liked to spend summers sailing with his family in Jamestown, R.I., and he and his second wife bought a wilderness plot at the West Boulder Reserve near Livingston, Mont.

He lived in Chestnut Hill for more than 50 years, and moved to Lafayette Hill in 2007.

“He was a great influence on me‚” said his son Thomas, who accompanied Mr. Dolan on some of his stream surveys. “He was a very calm, generous, positive person. He was a great father.”

In addition to his children, Mr. Dolan is survived by four grandchildren, two great-grandchildren, and other relatives. A sister died earlier.

Services are to be private.

Donations in his name may be made to the Academy of Natural Sciences of Drexel University, 1900 Benjamin Franklin Parkway, Philadelphia, Pa. 19103.