The Dirty Glacier flows generally north for about 1.7 miles (2.7 km), from an elevation of 3,800 feet (1,158 m) in the Chugach Mountains of western Prince William Sound to its terminus at an outwash plain about 1 mile (1.6 km) from the head of Harriman Fjord, about 71 miles (114 km) west-southwest of Valdez and 16 miles (26 km) northeast of Whittier, Alaska. The glacier was first observed by the Harriman Expedition in 1899 and named in 1905 by Ulysses S. Grant and Daniel F. Higgins of the US Geological Survey, for the rock rubble and debris of a medial moraine. Medial moraines form where lateral moraines from two tributary glaciers merge. Lateral moraines are piles of glacially transported rocks and debris scraped from the sides of the ice flow. They form ridges of unconsolidated rock that run parallel to the direction of flow and remain after the glacier retreats. The rocks transported by the Dirty Glacier originated from the northeast flank of an unnamed peak on a ridgeline of the peninsula separating Harriman Fjord from Port Wells. This peninsula is part of the Chugach Mountains of Southcentral Alaska—one of the northernmost among several mountain ranges that make up the Pacific Coast Ranges along the western edge of North America. These mountains are part of an ancient tectonic accretionary complex called the Chugach terrane, which is exposed for about 1,367 miles (2,200 km) along the southern Alaska coast. Most of the Chugach terrane consists of turbidites deposited in a submarine trench between 75 million and 52 million years ago. In Prince William Sound, the turbidites of the Chugach terrane are known as the Valdez group, comprising thick sequences of deformed greywacke and partially metamorphosed sandstone, siltstone, mudstone, and slate. The weathering of these rocks creates a highly fractured, friable substrate that becomes rubble when degraded by weather or scraped by glaciers.

The name ‘Chugach’ comes from the Chugach Sugpiat, the Alaska Natives who inhabit the Kenai Peninsula and Prince William Sound. The archaeological record and oral histories suggest that the Chugach people have occupied Prince William Sound since shortly after the glaciers vacated the fjords. Uqciuvit is a former village at the northern end of Esther Passage on Port Wells, where cultural items dating to the 18th century were found. This roughly coincides with the first European contact in 1741, when Vitus Bering made landfall on Kayak Island. In 1793, Russian fur traders, expanding east from Kodiak, established a trading post in Prince William Sound at Nuchek. In 1867, the Alaska Purchase transferred the territory from Russia to the United States, and within a few years miners and fishermen arrived to exploit resources in a largely unpopulated and unregulated land. In 1899, Edward H. Harriman sponsored an expedition that included some of the most renowned scientists and naturalists of the time, to explore and document the Alaskan coast. Harriman was the director of the Union Pacific Railroad, which owned and operated the SS George W. Elder—a 250-foot (76 m) luxuriously outfitted steamer used to support the expedition. They were the first to enter and explore Harriman Fjord, following the retreat of the Barry Glacier, which had blocked access to earlier explorers. In 1905, Dirty Glacier was a small ice stream reaching nearly to tidewater near the terminus of Harriman Glacier. In 1935, the glacier’s debris-covered terminus lay about 1,968 feet (600 m) from the beach, with bare ice a further 656 feet (200 m) inland. By 1961, the glacier had retreated 3,368 feet (1,026 m) from the beach. Between 1961 and 2000, the terminus retreated to about 4,200 feet (1,276 m), continued thinning, and was on the verge of splitting into two distinct ice tongues. The most recent observations indicate that the tributary glaciers have now separated at an elevation of about 650 feet (198 m) and continue to retreat toward their respective cirque basins.

The Dirty Glacier is exceptional for having a thick rock debris cover that blankets much of its terminus. This debris derives mostly from ablated moraines and rock avalanches—a type of large, fast-moving landslide with a long runout that often flows far over the glacier surface. Glacier retreat or deglaciation is often assumed to trigger alpine slope failures resulting in rock avalanches. However, recent studies suggest that temperate glaciers do not serve as effective natural slope buttresses. In fact, glacial retreat is likely less significant than three factors: the loss of groundwater, which dewaters surrounding slopes as glaciers withdraw; climatic changes that accelerate the melting of ice-filled rock joints and enhance freeze–thaw expansion; and post-glacial seismicity (isostatic earthquakes). Massive ice sheets covered much of the northern hemisphere during the Last Glacial Maximum, and their weight caused the continental crust to sink. Now many of those ice sheets have melted, and alpine glaciers continue to melt. The removal of this ice weight creates stress imbalances in Earth’s crust, with the primary response being an isostatic rebound during which earthquakes help relieve stress and strain. Small-scale field studies have shown that a thin debris cover accelerates ice ablation by increasing solar energy absorption and transmitting heat rapidly to the ice surface, while a thick cover significantly reduces ablation by acting as a barrier to heat transfer. The thick rock debris cover on Dirty Glacier has been proposed to explain its relatively slow retreat compared with other small glaciers in Harriman Fjord. Read more here and here. Explore more of Dirty Glacier and Harriman Fjord here: