County commissioners have appointed Republican Rep. Ann Rivers to fill an empty state Senate seat.

Rivers was sworn in Monday to replace Sen. Joseph Zarelli, who resigned from the 18th District seat on March 31. Rivers is also campaigning for the seat in the November election.

Rivers was elected to one of the district's state House positions in 2012. Commissioners in Clark and Cowlitz counties now must choose someone to replace her in that position.

Zarelli was the top Republican budget writer in the state Senate. He says it was time for him to move on and focus on what he's going to do next.