Mallard Bay dries at low tide to expose a mudflat situated southwest and adjacent to the mouth of Portlock River on the southern shore of Kachemak Bay on the Kenai Peninsula, about 14 miles (23 km) east-northeast of Homer and 4 miles (6 km) south-southwest of Bear Cove, Alaska. The bay is presumably named for the migratory dabbling ducks observed here and that are widely distributed across the northern and southern hemispheres. Portlock River drains the outflow of Portlock Glacier and starts from a proglacial lake at the glacier terminus and flows generally west-northwest for about 4.4 miles (7 km) to Kachemak Bay. The Portlock Glacier is about 10,000 acres (4,000 ha) and starts from the Kenai Icefield at an elevation of about 4,700 feet (1,433 m) and flows northwest for about 8 miles (13 km). The glacier is named after Captain Nathaniel Portlock of the Royal Navy who was on the third voyage of Captain James Cook in 1776. In 1785, Portlock and a group of investors, including George Dixon who also served with Cook, formed a partnership called 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 that separates the Kenai Mountains to the south from the Kenai Lowlands to the north. The Kenai Mountains in the vicinity of Portlock Glacier are formed by the McHugh Complex which is a tectonic mélange consisting largely of rocks formed during the Paleozoic and Mesozoic in a deep ocean trench. The dominant rocks are weakly metamorphosed sandstone, siltstone, greywacke, and conglomerate. The McHugh Complex was intruded by igneous rocks during the Paleocene creating granitic plutons with at least one exposure in the Portlock River drainage. These rocks were 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 vast array of moraine deposits on the eastern and western slopes of the Kenai Mountains that record multiple glacier advances into coastal forests during the late Holocene time. There were three major episodes of glacial advance during the late Holocene from the Kenai Mountains. Outlet glaciers of both the Harding and Kenai icefields expanded about 3600 years ago, again around 550 AD, and during the Little Ice Age about 1500 AD. Prior to the 6th century, ice margins had retreated sufficiently to allow soil development and growth of extensive mature forests. These forests appear to have been buried by glacial outwash and overridden by advancing ice based on the occurrence of 3 to 4 feet (1 m) thick tree trunks near the present-day margin of Grewingk Glacier. The glaciation culminated with a maximum advance at around 800 AD based on radiocarbon ages from the outer rings of tree trunks correlated with lichen diameters. The Little Ice Age glacial expansion began as early as 1488 AD with glaciers again advancing into the forest. Tidewater glaciers started advancing several centuries prior to land-terminating glaciers and started retreating later. Land-terminating glaciers on the western mountain flank such as the Portlock and Dixon glaciers retreated from their Little Ice Age maxima as much as two centuries before those on the eastern mountain flank. Tree-ring ages suggest that retreat at Portlock Glacier began by 1848 AD. This may be explained by the order of magnitude differences in precipitation between the eastern and western slopes of the Kenai Mountains. The eastern tidewater glaciers receive about 10 to 33 feet (3-10 m) of annual precipitation compared to the western land terminating glaciers that receive 3 to 10 feet (1-3 m) of precipitation per year.

Since the end of the Little Ice Age, the glaciers on the Kenai Peninsula have retreated at a mean rate of about 90 feet (27 m) per year; however, this has been partially controlled by the calving of ice margins into proglacial lakes. The Portlock Glacier once partially drained directly into Mallard Bay, but glacial retreat and ice thinning exposed a low ridge between the present-day Portlock River and Mallard Bay that effectively isolated the glacial runoff to the Portlock River. The Portlock Glacier also historically drained into the Martin River before glacial thinning which has now isolated the runoff to only the Portlock River and the Martin River now mostly drains the Dixon Glacier. The low ridge now separating Mallard Bay from the Portlock River is the location of the Mallard Bay Trail in Kachemak Bay State Park, a hike of about 1 mile (1.6 km) hike with an elevation gain of 250 feet (75 m) that provides views and an open approach to the terminus of Portlock Glacier. About 0.2 miles from Mallard Bay, the trail intersects with the Emerald Lake Connector Trail which steeply climbs 500 feet over 1.3 miles with switchbacks to the Emerald Lake Loop Trail. Read more here and here. Explore more of Mallard Bay and Portlock River here: