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 it was named in 1905 by Ulysses S. Grant and Daniel F. Higgins of the U.S. 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 formed from piles of glacially-transported rocks and debris scraped from the sides of the ice flow. They form ridges of unconsolidated rock that trend parallel to the direction of flow and remain when 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 Mountain of Southcentral Alaska, one of the northernmost of the several mountain ranges that make up the Pacific Coast Ranges along the western edge of North America. The Chugach Mountains are about 250 miles (402 km) long and 60 miles (97 km) wide, and the highest point is Mount Marcus Baker with an elevation of 13,094 feet (3,991 m). The mountains are part of an ancient tectonic accretionary complex called the Chugach terrane that is exposed for about 1,367 miles (2,200 km) along the southern Alaska coast. Most of the Chugach terrane is comprised of turbidites deposited in a submarine trench between 75-52 million years ago. In Prince William Sound, the turbidites of the Chugach terranes are known as the Valdez group consisting of thick sequences of deformed greywacke and partially metamorphosed sandstone, siltstone, mudstone, and slate. The weathering of these rocks creates a highly fractured and friable substrate that quickly becomes rubble when degraded by weather and scraped by glaciers. The name ‘Chugach’ comes from Chugach Sugpiaq or ‘Cuungaaciiq’, the Alaska Natives inhabiting 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 situated at the northern end of Esther Passage on Port Wells where cultural items were found dating to the 18th century. This roughly coincides with the first European contact in 1741 when Vitus Bering made landfall on Kayak Island. In 1793, the Russians were expanding east from Kodiak and established a trading post in Prince William Sound at Nuchek. Spanish, English, and American explorers and merchants engaged in the maritime fur trade soon followed. 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 the resources of a largely unpopulated and unregulated land. In 1899, Edward H. Harriman sponsored an expedition that included some of the most well-known scientists and naturalists of the time to explore and document the coast of Alaska. Harriman was the director of the Union Pacific Railroad which owned and operated the SS George W. Elder, a luxuriously outfitted steamer of 250 feet (76 m) that was used to support the expedition. They were the first to enter and explore Harriman Fjord following the retreat of the Barry Glacier that had blocked access to earlier explorers. In 1905 and 1909, Grant and Higgins observed this area and made measurements and photographs that are still used as a baseline for glacier studies. In 1905, Dirty Glacier was a small ice stream reaching nearly to tidewater near the terminus of Harriman Glacier. By 1909, the terminus was about 1,300 feet (400 m) up the valley from the shoreline, and in 1935, the glacier’s debris-covered terminus was about 1,968 feet (600 m) from the beach with bare ice about 656 feet (200 m) further up the glacier. In 1961, the glacier had retreated 3,368 feet (1,026 m) from the beach, and between 1961 and 2000 the terminus had retreated to about 4,200 feet (1,276 m) from the beach, and it was continuing to thin and on the verge of separating into two distinct ice tongues. The most recent observations indicate that the tributary glaciers have separated at an elevation of about 650 feet (198 m) and are continuing to retreat toward their respective cirque basins.
The Dirty Glacier is exceptional in having a thick layer of rock debris covering a large proportion of the terminus. The rock cover comes mostly from ablated moraines and rock avalanches. A rock avalanche is a large and fast-moving landslide that typically exhibits a long runout, flowing very far over the glacier surface. Glacier retreat or deglaciation is often assumed to be the cause of alpine slope failures that result in rock avalanches but recent studies have suggested that temperate glaciers do not make a good natural slope buttress. Glacial retreat is probably not as important as the loss of groundwater when surrounding slopes are dewatered as glaciers withdraw, climatic changes that accelerate the melting of ice-filled rock joints and enhance freeze-thaw thermal expansion, and post-glacial seismicity or 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 are still melting. Removal of the ice weight creates stress imbalances in the earth’s crust, and the primary response is an isostatic rebound where earthquakes are part of the process of relieving stresses and strains. The role of rock debris cover in modifying glacier behavior has been observed in small-scale field studies where a thin cover of rock debris will enhance ice ablation and a thick cover will significantly reduce ablation. A thin cover of rock debris accelerates the melting rate of the underlying ice through increased absorption of solar energy and rapid transmission of this heat to the ice surface. Increasing debris thickness causes a corresponding decrease in the ablation rate by increasing the solar insulation. Debris cover thicker than a particular depth acts as a barrier to heat transfer and decreases ice-melting rates. The thick layer of rock covering Dirty Glacier has been proposed as an explanation for the relatively slow rate of retreat compared to other small glaciers in Harriman Fjord. Read more here and here. Explore more of Dirty Glacier and Harriman Fjord here: