Mallard Bay is on the southern shore of Kachemak Bay on the Kenai Peninsula and adjacent to the mouth of the Portlock River, approximately 14 miles (23 km) east-northeast of Homer and 4 miles (6 km) south-southwest of Bear Cove, Alaska. The bay dries completely at low tide to expose a mudflat and is presumably named after the migratory dabbling ducks observed here, which are geographically widely distributed across the northern and southern hemispheres. The Portlock River originates from a proglacial lake at the terminus of the Portlock Glacier and flows west-northwest for about 4.4 miles (7 km) to Kachemak Bay. The glacier, covering roughly 10,000 acres (4,000 ha), starts from the Kenai Icefield at an elevation of about 4,700 feet (1,433 m) and extends northwest for approximately 8 miles (13 km). It is named after Captain Nathaniel Portlock of the Royal Navy, who participated in Captain James Cook‘s third voyage in 1776. In 1785, Portlock, along with investors including George Dixon, who also served with Cook, formed the King George’s Sound Company to explore the commercial fur trade and visited Kachemak Bay. The axis of the bay is roughly aligned with the Border Ranges Fault, which separates the Kenai Mountains to the south from the Kenai Lowlands to the north. The Kenai Mountains near Portlock Glacier are formed by the McHugh Complex, a tectonic mélange consisting largely of rocks formed during the Paleozoic and Mesozoic eras in a eep ocean trench. The dominant rocks are weakly metamorphosed sandstone, siltstone, graywacke, and conglomerate. During the Paleocene epoch, the McHugh Complex was intruded by igneous rocks, creating granitic plutons with at least one exposure in the Portlock River drainage. These rocks have been severely deformed by seismic activity and sculpted by repeated glaciations. The southern Kenai Mountains are dominated by two icefields, which are drained by 38 distinct outlet glaciers, including seven that reach tidewater in fjords on the Pacific coast.

The recent retreat of outlet glaciers from the Harding and Kenai icefields has exposed a variety of moraine deposits on the eastern and western slopes of the Kenai Mountains. These deposits record multiple glacier advances into coastal forests during the late Holocene. There were three major episodes of glacial advance during this period. Outlet glaciers from both icefields expanded about 3,600 years ago, again around 550 AD, and during the Little Ice Age around 1,500 AD. Prior to the 6th century, ice margins had retreated sufficiently to allow soil development and the growth of mature forests. These forests appear to have been buried by glacial outwash and overridden by advancing ice, as evidenced by 3 to 4 feet (1 m)  thick tree trunks near the present-day margin of Grewingk Glacier. The glaciation culminated in a maximum advance around 800 AD, based on radiocarbon dating of tree rings correlated with lichen diameters. The expansion of glaciers during the Little Ice Age began as early as 1488, with glaciers advancing into forests. Tidewater glaciers started advancing several centuries before land-terminating glaciers and began retreating later. On the western flank of the Kenai Mountains, land-terminating glaciers like the Portlock and Dixon glaciers retreated from their Little Ice Age maxima up to two centuries earlier than those on the eastern flank. Tree-ring data indicate that the retreat of the Portlock Glacier began by 1848. This discrepancy can be attributed to significant differences in precipitation between the eastern and western slopes of the Kenai Mountains. Eastern tidewater glaciers receive about 10 to 33 feet (3-10 m) of annual precipitation, compared to 3 to 10 feet (1-3 m) for the western land-terminating glaciers.

Since the end of the Little Ice Age, glaciers on the Kenai Peninsula have retreated at an average rate of about 90 feet (27 m) per year. This retreat has been influenced by the calving of ice margins into proglacial lakes. The Portlock Glacier once partially drained directly into Mallard Bay. However, glacial retreat and ice thinning have exposed a low ridge between the present-day Portlock River and Mallard Bay, effectively isolating the glacial runoff to the Portlock River. Historically, the Portlock Glacier also drained into the Martin River. Due to glacial thinning, runoff is now isolated to the Portlock River, and the Martin River primarily drains the Dixon Glacier. The low ridge separating Mallard Bay from the Portlock River hosts the Mallard Bay Trail in Kachemak Bay State Park. This trail is about 1 mile (1.6 km) long, with an elevation gain of 250 feet (75 m). It offers views and an open approach to the terminus of Portlock Glacier. Approximately 0.2 miles (0.3 km) from Mallard Bay, the trail intersects with the Emerald Lake Connector Trail. This connector trail climbs steeply, gaining 500 feet (150 m) in elevation over 1.3 miles (2 km) using switchbacks to reach the Emerald Lake Loop Trail. Read more here and here. Explore more of Mallard Bay and Portlock River here: