The river's length, including its longest source tributary, the Middle Fork, is 18 miles (29 km).

The main stem Salmon River, below the North Fork, is about 13 miles (21 km) long.

[4] The North, Middle, and South Fork watersheds are composed of shale, and sandstone, both sedimentary rocks, while the watershed of the lower Salmon River is composed of "coastal piedmont consisting of porous, unconsolidated deposits of Olympic alpine glaciers, including gravels, sands, silts and clays.

"[2] More than 80% of the watershed is covered by coniferous forests (including Sitka spruce, western hemlock, and Douglas-fir), with the remainder being hardwood stands and unforested wetlands.

Some number of hatchery Winter steelhead smolts are planted in the river every year.