psh 9 was created with the primary and secondary state highways in 1937, including a branch that connects the main highway and us 410 in montesano to us 101 and psh 14 near artic. the current truss bridge over the chehalis river between south montesano and montesano was built in the 1950s alongside two other bridges in the area by the department of highways. sr 107 was established during the 1964 highway renumbering to replace the psh 9 branch and was codified in 1970. the new highway was truncated to its current terminus, a diamond interchange with us 12, after montesano was bypassed by us 12 in 1967. the highway is closed occasionally by seasonal flooding, most recently in 1994 and 2008.