Elwha River drains a watershed of about 204,799 acres (82,880 ha) and flows generally north for 45 miles (72 km) from a perennial snowfield at an elevation of 4,763 feet (1452 m) in the Olympic Mountains to Angeles Point at Freshwater Bay on the southern shore of the Strait of Juan de Fuca, about 33 miles (53 km) east-southeast of Clallam Bay and 6 miles (10 km) west-northwest of Port Angeles, Washington. Most of the river’s course is within Olympic National Park. The Olympic Mountains are mainly accretionary wedge material consisting of sandstones, turbidites, and basaltic oceanic crust. During the Eocene or about 56-34 million years ago, vents and fissures on the ocean floor of the Farallon plate issued lava that created huge underwater mountains and ranges called seamounts. The Farallon oceanic plate was gradually subducted beneath the North America plate and some of the seafloor rocks were scraped off and accreted to the continent. This created the nascent Olympic Mountains, although all this occurred underwater, and the mountains did not emerge above sea level until about 20–10 million years ago. The mountains were sculpted by a series of glacial advances during the Pleistocene. The Last Glacial Maximum occurred during the Fraser glaciation when the vast Cordilleran Ice Sheet descended from the north. When the advancing ice sheet encountered the Olympic Mountains, the ice front split into the Juan de Fuca and Puget ice lobes carving out the current waterways and advancing as far south as present-day Olympia. Ice flowed up river valleys to an elevation of 3,800 feet (1,200 m) and when the ice retreated, sediments from Vancouver Island and the Canadian Coast Ranges were deposited and subsequently buried by alluvial sediments that originated from the Olympic Mountains.

The Klallam Tribe historically lived in villages on both sides of the Strait of Juan de Fuca. Olympic Peninsula villages included Hoko, Clallam Bay, Pysht, Deep Creek, Freshwater Bay, the Elwha valley and river mouth, the shores of Port Angeles, and creeks farther east. On the north side of the strait near Victoria is the Klallam village of Beecher Bay. The Lower Elwha Klallam historically occupied several villages along the Elwha River and on Freshwater Bay. The Elwha Klallam have a creation story centered on a rock along the river where the Creator would  scoop dirt from pits and hollows in the rock and use this to form humans. According to oral tradition, the Elwha River is home to Thunderbird, an important symbol of strength to the Klallam people. Thunderbird lived in a cave and chased the salmon upriver by sending thunder and lightning toward the mouth of the Elwha where the Klallam caught the fish. In 1855, the Lower Elwha Klallam Tribe was recognized by the United States in the 1855 Point No Point Treaty. However, the Klallam refused to move onto the Skokomish Reservation on Hood Canal and remained in their traditional villages. Settlers began arriving in the lower Elwha valley in the 1860s and pushed many Klallam from their traditional homesites. In the late 18th and 19th centuries, the tribe suffered high fatalities from infectious European diseases such as smallpox and measles. In the 20th century, the federal government persuaded the tribe to relocate from their village at Port Angeles to allow industrial development of lumber and paper mills along the waterfront. In 1968, the land at the mouth of the Elwha River was designated as the Lower Elwha Reservation which includes about 1,000 acres (405 ha) of land on and near the Elwha River. Salmon have always been an important seasonal subsistence food for the Elwha Klallam and the river provided a spawning habitat for Chinook, coho, chum, sockeye, and pink salmon, plus steelhead, coastal cutthroat, bull trout, and Dolly Varden char.

In 1910, the Elwha Dam was built about 4 miles (6 km) upstream from the river mouth under the direction of Thomas Aldwell who had previously bought up tracts of land around the river. Aldwell and his contractors cut corners on constructing the dam, including illegally not including fish passages and not securing the structure to the bedrock. In 1912, as the reservoir began filling, the lower sections of the dam gave way and a torrent of water flooded the Lower Elwha Reservation. The dam was reconstructed and completed in 1913 and the reservoir was known as Lake Aldwell. The Elwha River Hydroelectric Power Plant historic district comprised 3.5 acres (1.4 ha) including the dam, the powerhouse, five penstocks, and the surge tank. In 1926, the Glines Canyon Dam was completed about 7 miles (11 km) upstream from the Elwha Dam and impounding Lake Mills. The projects helped to fuel economic growth and development for the Olympic Peninsula and the community of Port Angeles. Before the dams were built, approximately 400,000 adult salmon returned annually to spawn in 70 miles (110 km) of river habitat. By 2014, fewer than 4,000 salmon returned each year in only 4.9 miles (7.9 km) of remaining spawning habitat. By 2010, the combined power output of both dams only provided the equivalent of 38% of the electricity needed to operate one sawmill. In 2011, the National Park Service removed the two dams as part of the $325 million Elwha Ecosystem Restoration Project. The resulting sediment release created 70 acres (28 ha) of estuary habitat on the Strait of Juan de Fuca. While the dams had a major impact on the Elwha River, the health of the lower floodplain has been significantly degraded by other human activities. Restoration activities also include removing abandoned dikes and constructing engineered logjams that will allow the river to form side channels and reconnect to its floodplain. The lower floodplain is also being reforested with native species and eradicating exotic plants. Additional actions have been identified in the middle and lower portions of the river as well as in the estuary and nearshore habitat that will further speed complete watershed recovery. Read more here and here. Explore more of the Elwha River here: