Rep. David Deen (D-Westminster), the second-most senior member of the Vermont House, plans to retire this fall after 30 years in the legislature.

As chair of the House Committee on Natural Resources, Fish and Wildlife, the 73-year-old legislator has played a lead role in the ongoing debate about how — and when — to find a long-term funding source to clean up the state's polluted waterways, including Lake Champlain.

Deen, who considered retiring in 2016, told Seven Days at the time that he would stick around for another term to ensure water quality protections were firmly in place. It's not clear he succeeded in that effort. Deen acknowledged Tuesday that, even now, there's more work to be done — but he won't be the one to do it.

“Thirty years of service? Thirty is a nice round number,” Deen said when asked why he chose to retire this year. “People say I’m a slow learner. Maybe I am, but after 15 election cycles of saying, You know what? I gotta run again because there’s more to do, I finally figured it out: There’s always more to do. And I do have a personal agenda in terms of things I’d like to do, and I’d like to get to it, so now’s the time.”

A retired river steward for the Connecticut River Watershed Council, Deen served one term in the Senate, from 1987 to 1988, before losing reelection. He won a seat in the Vermont House in 1990 and has been reelected 13 times since.

Deen said two of his proudest achievements as a legislator involved expanding clean water protections. The first, he said, was the designation of groundwater as a legally protected public trust resource in Vermont. The second, he said, was the Vermont Clean Water Act of 2015, which he described as "the most far-reaching water protection bill ... that Vermont has ever passed."

"I was really proud of having been a key player in getting that passed," he added.

The Westminster Democrat, known for legislating in Birkenstocks, said he's heard from multiple prospective candidates who may run to replace him in his two-member House district, but he declined to name anyone. Dean said his only endorsement, for now, is for his seatmate, Rep. Mike Mrowicki (D-Putney).

"I want people to know that I am 100 percent behind Mike Mrowicki returning as the other representative from our district," Deen said. "He’s done an excellent job."

Deen is the fourth House chair to announce his departure: Rep. Stephen Carr, (D-Brandon), chair of the House Energy and Technology Committee; Rep. Helen Head (D-South Burlington), chair of the House General, Housing and Military Affairs Committee, and Rep. David Sharpe (D-Bristol), who chairs the House Education Committee, have also decided not to run for reelection.

Vermont Public Radio first reported news of Deen's departure.

Alicia Freese contributed reporting.