The Red River begins at the terminus of the Red Glacier on the eastern flank of Mount Iliamna, and flows southeast through Lake Clark National Park and Preserve to the western shore of Cook Inlet, approximately 126 miles (203 km) southwest of Anchorage and 48 miles (77 km) northwest of Homer, Alaska. The river was first named in 1912 by the U.S. Coast and Geodetic Survey after the red volcanic rocks covering the glacier at lower elevations and forming the beach sediments. Red Glacier starts from an elevation near 9,000 feet on Mount Iliamna and is about 10 miles (17 km) long and covers an area of 12,602 acres (5,100 ha). It terminates at an elevation of about 213 feet (65 m), and the river flows an additional 6 miles (10 km) through a broad alluvial fan to Cook Inlet.

Mount Iliamna is a glaciated stratovolcano with an elevation of 10,013 feet (3,053 m). It is one of seven volcanoes in the region that have erupted multiple times over the past 10,000 years. Lahar deposits are evident along the Cook Inlet coastline near the mouth of the Red River. A lahar is a violent mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris, and water. These deposits contain cobbles and boulders of andesite, volcanic rocks likely still hot when the lahar reached the sea. Spruce trees growing on the lahar deposits are about 300 years old, suggesting the lahar and an eruption of Iliamna Volcano occurred around this time. The lahar likely swept down Red Glacier and inundated the Red River Valley, possibly reaching the coast in as little as 30 minutes, forming an extensive delta at the shoreline.

The Red River carries large quantities of sediment to the coast, discharging it directly into the ocean. If the sediment supply rate exceeds the removal rate by waves and tidal currents, sediment buildup occurs, leading to delta formation. In Cook Inlet, however, wave erosion and strong currents rapidly disperse sediment away from the river mouth. The strong flood and ebb tidal currents rearrange river-supplied sediment into long, linear submarine ridges and islands that fan out from the river mouth. Nearshore waters adjacent to the Red River’s mouth are hazardous due to boulders, many of which become exposed at low tide, as well as extensive reefs, ledges, and adjoining mudflats. Read more here and here. Explore more of Red River and Lake Clark National Park and Preserve here: