There are 4 major rivers which arise in the Olympic Mountains and flow through valleys into the Pacific Ocean. One of these rivers is the Hoh. The Hoh River, flows into the Pacific in the southern coastal section of the park. The valley through which the Hoh flows, was cut by glaciers. It is U-shaped, unlike stream-cut valley which are more V-shaped. The rivers of the park, including the Hoh, are fairly young geologically speaking. They have also been subject to floods as well as the effects of large landslides. The rivers of the western section of the park are prime habitat for steelhead, which swim up those waters. The rivers in the valleys, where the rain-forests grow, serve as the major arteries of the forest community. The temperate Hoh rain-forest, (pictured above), is one of the most spectacular of the various varieties of forests found in the northern hemisphere. It is also the most productive, in terms of amount of biomass per acre, of any on earth. The rain-forest features Sitka spruce, western hemlock, western red cedar, and Douglas fir.

The signature characteristics of the temperate rain-forest are the very large sizes of trees, and the presence of a lot of mosses, lichens, liverworts, and multilayered canopies. A necessary component of the lush vegetation is moisture. This is produced in great abundance by precipitation on the west side of the Olympic Range. Moisture laden air flowing in from the Pacific Ocean meeting the mountains produces more than 4 meters of precipitation annually in these valleys.

The Hoh River Valley receives 305 – 457 cm of rain per year, and the amount of rainfall may reach as much as 457 cm. The lush rain-forest also requires a year round supply of moisture, and although there is not much rain in the summer, summer fog supplies the necessary moisture. This temperate rain forest requires a mild coastal climate in addition to rain and fog. The valleys which contain the rain-forests
happen to be aligned with the prevailing, southwest flowing winter storm-track. This allows sizable amounts of moisture to be funneled into the upper levels of the valleys.