[2] The Rapid River originates at 47°49′19″N 121°7′28″W / 47.82194°N 121.12444°W / 47.82194; -121.12444,[3] at Grouse Lake and at 47°49′40″N 121°04′24″W / 47.82778°N 121.07333°W / 47.82778; -121.07333 on the west slopes of Jove Peak on the crest of the Cascades, 6 and 9 miles (10 and 14 km) east of the Beckler River.
The latter source flows through Lake Janus—at 4,146 feet (1,264 m)—before meeting the Grouse Lake source creek, whereafter the river continues southwest.
Both Jove and Union peaks are on the high crest of the Cascade Range, separating the Rapid River's drainage basin from the Little Wenatchee River drainage to the east.
[4] From its source near the crest of the Cascades the Rapid River flows southwest and west, collecting numerous headwater tributaries.
Soon the North Fork Rapid River joins as well, followed by Meadow Creek.