At Monday's meeting, Mayor Mike Livengood thanked Erwin for her years of service, saying, "You always spoke your piece."
Her constituents are what she will miss most of all, Erwin said.
"I think some constituents don't understand what the role of an alderman is and what people need to remember is that aldermen are there to represent their constituents. I always welcomed calls from them and made a point to talk to people, like with the pseudoephedrine ordinance," she said.
Erwin said over the years, she has noticed the aldermen taking a more active role in getting input from their wards.
"Karen (Erwin) was very dedicated to her alderman job," Livengood said. "She worked with her constituents, followed up with them and cared for their needs. She was a very open-minded alderman, who looked at all sides of an issue. She is going to be missed."
In an emotional announcement, Erwin said she enjoyed being a part of the board of aldermen.
"I worked very hard toward our parks system and the infrastructure and our roadways," she told The Missourian. "Those were the biggest things we as a board helped accomplish."
Erwin has served as an alderwoman for 5 1/2 years, first being elected in 2004. She was re-elected three times, most recently in 2008 when she ran unopposed.
The alderwoman also was the chair of the parks, buildings, development and public service committee.
"We do most of our work in the committee meetings, so we have all of our questions answered by the board meetings," she said.
Erwin said she hoped to finish her term, which would end in April, "but I had no other choice."
Livengood said since it is too late to file for a general election, he would be responsible for appointing someone to fill Erwin's seat until April 2010.
"I will have to come up with some people and get the board of aldermen's approval," Livengood said.
The mayor also will appoint another alderman to replace Erwin as chair of the parks, buildings, development and public service committee. That appointment also will need to be approved by the board.
