SR 165 begins at the western boundary of
Mount Rainier National Park on Mowich Lake Road, which continues east for to a campground at
Mowich Lake on the northwestern flank of
Mount Rainier. Mowich Lake Road is open seasonally from July to October, remaining closed to vehicular traffic during the winter and spring due to weather conditions. The southernmost section of the highway is an unpaved
gravel road—the only one to be included in Washington's state highway system. The highway travels northwest along Evans Creek through a section of the
Mount Baker–Snoqualmie National Forest, passing several
trailheads. At a junction with Fairfax Forest Reserve Road, which serves an alternate entrance to Mount Rainier National Park, SR 165 turns north to follow the
Carbon River, which it crosses on the
Fairfax Bridge, a single-lane arch bridge. The highway continues along the east bank of the Carbon River to
Carbonado, where it passes through the east side of the town and neighboring
Wilkeson. SR 165 continues north and intersects
SR 162 east of
South Prairie, turning northeast to follow the
Foothills Trail. The highway terminates at
SR 410 in southwestern
Buckley. SR 165 is maintained by the
Washington State Department of Transportation (WSDOT), which conducts an annual survey on state highways to measure traffic volume in terms of
annual average daily traffic. Average traffic volumes on the highway in 2016 ranged from a minimum of 190 vehicles near its junction with Fairfax Forest Preserve Road to a maximum of 6,900 vehicles near SR 410 in Buckley. In April of 2025, the section of the highway south of
Carbonado was closed to vehicle traffic. was the highest bridge in the state when it was built in 1921. ==History==