Nathaniel P. Reed, an environmentalist and onetime Interior Department official who was a key architect of the Clean Water Act and Endangered Species Act and who spearheaded efforts to preserve wildlife and open spaces from Alaska to his longtime home state of Florida, died July 11 at a hospital in Quebec City. He was 84.

He was on a fishing expedition to the Grand Cascapedia River in Quebec on July 3, his son Adrian Reed said, when he caught and released a 16-pound salmon. Shortly afterward, Mr. Reed slipped and struck his head on a rock. He never regained consciousness and died eight days later.

Mr. Reed was a courtly developer and investment banker born into wealth. His deep-seated appreciation for the environment had its roots in his mother’s efforts to block the development of an early Florida theme park.

He went on to fight the state’s environmentally damaging sugar industry and led efforts to block the building of a barge canal across Florida and an airport that would have paved over much of the Everglades.

“He was a transformational figure in Florida,” former senator Bob Graham (D-Fla.) said in an interview. “Florida’s a different place today than it would have been without him.”

As one of the few Republicans prominent in the environmental movement, Mr. Reed served as an assistant Interior secretary from 1971 to 1977 under presidents Richard M. Nixon and Gerald R. Ford. In that role, he helped preserve millions of acres of wilderness in Alaska, banned dangerous pesticides and endured death threats from Western ranchers after he sent federal agents to stop the widespread killing of federally protected eagles. (In 1971 alone, hundreds of golden and bald eagles were found poisoned or shot to death in Wyoming and Colorado.)

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In 1972, Mr. Reed accompanied the president’s daughter Julie Nixon Eisenhower on a tour of the Everglades. Two years later, Florida’s Big Cypress National Preserve was established as one of the country’s first two national preserves.

Mr. Reed also had roles in the banning of DDT and other chemical agents dangerous to wildlife and humans. He took steps to preserve California redwood forests, blocked construction of a jet airport near Jackson Hole, Wyo., and called for a treaty protecting polar bears from hunting.

He also sought to ban what he called “the whole shady business of trafficking in captured wild animals in the guise of providing ‘pets.’ ”

He had a hand in writing two of the country’s most far-reaching environmental laws, the Clean Water Act of 1972 and the Endangered Species Act of 1973.

“I suggest to you that the American dream, based as it is on the concept of unlimited space and resources, has run aground on the natural limits of the earth,” he wrote in a 1974 essay. “It has foundered on the shoals of the steadily emerging environmental crisis, a crisis broadly defined to include not only physical and biographical factors, but the social consequences that flow from them.”

When opponents scoffed at Mr. Reed’s desire to save little-known but endangered animals and habitats, he said: “The president appointed me to defend the critters that couldn’t come to Capitol Hill to defend themselves.”

After leaving Washington in 1977, Mr. Reed returned to Florida, where he launched an advocacy organization, 1000 Friends of Florida , and was appointed to numerous commissions by Republican and Democratic governors to help guide the state’s environmental future.

“People move to Florida for the ‘quality of life,’ ” he told the St. Petersburg Times (now Tampa Bay Times) in 2004, “then destroy the ‘quality of life’ by overbuilding and overcrowding.”

Above all, he sought to preserve and restore the Everglades, the unique Floridian environment of freshwater wetlands that had been highlighted since the 1940s by writer and activist Marjory Stoneman Douglas. Mr. Reed battled construction projects of the Army Corps of Engineers and Florida’s sugar industry, a heavy polluter of waters flowing through the Everglades.

He was so reviled among Florida’s politically powerful sugar barons that he was shown in advertisements that likened him to Cuban dictator Fidel Castro.

“One thing I can promise you,” Mr. Reed told The Washington Post in 1996, “we’re not going away.”

Beginning in the 1990s, thousands of acres of sugar plantations were bought by the state of Florida and converted back to wetlands.

Nathaniel Pryor Reed was born July 22, 1933, in New York City and grew up mostly in Greenwich, Conn., and Hobe Sound, Fla. His ancestry reached back to the Mayflower, and the family became wealthy in the 19th and early 20th centuries from oil and mining, as well as from manufacturing armaments.

Mr. Reed spent much of his childhood in Florida, wading in rivers and surf, canoeing across Lake Okeechobee and casting a line in the water for fish. Over time, he saw how the state’s growing population was damaging the state’s land, water and wildlife.

He also learned from the example of his mother, who mobilized support to block the building of a “holy land” theme park near the family’s home on a barrier island north of Palm Beach. She led an effort to buy the land, which is now preserved as a state park.

Mr. Reed attended the private Deerfield Academy in Massachusetts and was a 1955 graduate of Trinity College in Hartford, Conn. He spent four years as an intelligence officer in the Air Force before joining the family-run Hobe Sound Co. in Florida, managing real estate, orange groves and a hotel.

In 1966, Mr. Reed joined the campaign of renegade Florida Republican gubernatorial candidate Claude R. Kirk Jr., who unexpectedly won the election and became Florida’s first Republican governor in almost 100 years.

“If you want to change the things that you have been hollering about for the last 15 years in Florida,” Kirk told Mr. Reed, “there’s a desk.”

At a salary of $1 a year, Mr. Reed became the state’s first environmental adviser and one of the first in the nation.

He led efforts to clean up a once-pristine lake north of Orlando that had become so choked with weeds that it was “too thick to navigate and too thin to cultivate.” He helped block construction of a cross-Florida barge canal that would have dug a permanent trench across the peninsula.

In the late 1960s, he joined other environmental advocates, including Douglas and Joe Browder, to oppose the building west of Miami of what would have been the world’s largest airport.

Browder and Mr. Reed were denounced by a local mayor as “white militants,” and the project was eventually halted — but not before a two-mile-long runway was built through the Everglades.

Survivors include his wife of 53 years, the former Alita Weaver of Hobe Sound; three children, Adrian Reed and Alita “Lia” Bohannon, both of Hobe Sound, and Nathaniel P. Reed Jr. of Earlysville, Va.; and five grandchildren.

A brother, Joseph Verner Reed Jr., who was chief of protocol under President George H.W. Bush and a longtime top official with the United Nations, died in 2016.

In the 1980s, Mr. Reed began to turn against what he saw as increasingly extremist and mean-spirited elements of a Republican Party veering to the right. He denounced James G. Watt, President Ronald Reagan’s Interior secretary, as “an extremist” who “can’t tell the difference between national parks and industrial parks.”

Increasingly disaffected with the GOP, he eventually joined forces with Graham, a former Democratic governor and U.S. senator, to launch the Florida Conservation Coalition, a consortium of environmental groups. Mr. Reed was the organization’s vice chairman at the time of his death and had served on the boards of many other environmental groups, including the National Geographic Society and the Everglades Foundation.

“It’s not just good enough to be against everything,” Mr. Reed said in 1988, stating his activist credo. “We should be judged on our innovative ideas and pragmatic problem-solving.”