The Hoh River valley, on the west side of the Olympic Peninsula, is about 90 km long and is one of the major valleys originating in the interior Olympic Mountains. It was glaciated to its present-day mouth during what is believed to have been the Stuck Glaciation, radiocarbon dated at more than 47,000 yr ago, and later by less extensive ice tongues of Salmon Springs, Evans Creek, and younger ages. Alpine glaciers of Neoglacial age are active today on the slopes of Mount Olympus (2,428 m).
Modern vegetation in the valley is predominantly of the rain forest type, divisible into Pacific coastal and subalpine communities. Coastal forest is established at elevations below 500 to 600 m, subalpine forest continues to the tree limit near 1,700 m, and alpine tundra occurs only on the heights in the interior. A winter-wet, cool, temperate climate prevails in the rain forest. Annual precipitation is greater than 3,000 mm, and the average July temperature lies between 12° and 15°C. In the alpine zone, temperatures are a few degrees lower, and precipitation reaches an estimated 5,000 mm or more.
Unglaciated ground along the coast adjacent to the Hoh valley was a refugium for Quaternary plant communities of tundra and park tundra during times of glaciation. Vegetation during the Puyallup Interglaciation resembled modern coastal forest and was presumably extensive; later, during Salmon Springs and Fraser Glaciations, tundra spread over the lowland, except during the nonglacial interval between the early and late Salmon Springs Glaciations. At this time and during the Olympia Interglaciation, subalpine forest was in evidence. The pollen stratigraphy and radiocarbon chronology of three bog sections from the lower Hoh valley show tundra at low elevations from 18,800 ± 800 yr B.P. until about 10,000 yr B.P.
Holocene vegetation is portrayed by three pollen assemblages: Pinus-Alnus-Picea-Pseudotsuga-Pteridium (10,000 to 8,000 yr B.P.), Picea-Tsuga-Alnus-Pseudotsuga-Pteridium (8,000 to 3,000 yr B.P.), and Tsuga-Thuja-Abies (3,000 to 0 yr B.P.). The sequence implies a climatic trend from a cool and relatively humid climate in the beginning, to increasing warmth, then maximum warmth and lower humidity, and finally to a cooler and quite humid climate at the close. The sequence also reflects the gradual replacement of open, successional forest communities by a late Holocene-age closed rain forest. Pollen influx is low, generally less than 1,500 grains cm−2 yr−1, except around 8,000 yr. B.P., when values exceeded 5,000 cm−2 yr−1. Traces of tephra from the eruption of Mount Mazama, found in one bog section, mark an extreme Pacific continental limit for this ejectamenta. Refugia on the Olympic Peninsula contained the stock from which major migrations of plants northwestward along the north Pacific coast took place toward the close of Fraser Glaciation and during Holocene time.