Mud Bay is a tidal flat about 0.6 miles (1 km) wide, partially enclosed by sand spits and situated at the sheltered base of the Homer Spit on the northwest shore of Kachemak Bay, about 2.7 miles (4 km) southwest of Millers Landing and 1.5 miles (2.4 km) southeast of Homer, Alaska. This protected embayment was historically named Coal Bay, or ‘Zaliv Ugolnoy’, published in 1852 on Russian charts by Mikhail D. Tebenkov, a hydrographer and vice-admiral of the Imperial Russian Navy. In the 1940s, a road causeway was built across Coal Bay that blocked all tidal flushing and created Mariner Lagoon on the southwest side of the road. A tidal connection between Mariner Lagoon and outer Kachemak Bay is now dredged annually through the beach of Homer Spit. The estuarine circulation of inner Kachemak Bay was also affected so that two sand spits developed in Coal Bay, one that now extends south for about 0.6 miles (1 km) from the north shore of inner Kachemak Bay and one that extends north for about 0.3 miles (0.5 km) from the Homer Spit. These spits partially enclosed the head of Coal Bay creating what is locally called Mud Bay. The main part of the Mud Bay tidal flat is located in the intertidal zone across a tidal range of nearly 30 feet (9 m). Tidal flat sediment deposits are generally focused in the intertidal zone which is composed of supratidal marshes and a barren zone at lower elevations. The sedimentary layers within these tidal zones are various ratios of sand and mud and the associated accumulation rate of sediments is a function of relative sea-level change from land subsidence or emergence, climate change, and rates of sediment deposition. Although protected from the west by the Homer Spit, Mud Bay is exposed to easterly winds and waves. Despite its name, Mud Bay consists of a mixture of fine silts and clays, sand, gravel, and occasional cobbles and small boulders. The larger particles were probably transported by ice rafting during the spring breakup when much of the sediment-ladened shorefast ice from the north shore of inner Kachemak Bay drifts into Mud Bay and melts.

Tidal flats usually support a large population of wildlife and are a key habitat supporting salt marshes, marine invertebrates, fishes, and migratory birds. The general trophic structure for a typical mud flat is considerably more diverse than that for sandy beaches and is largely based on the settlement of very fine detrital material originating from marine and terrestrial ecosystems. The detritus includes associated inorganic particles, bacteria, and protozoans that are ingested by suspension and deposit feeders, but mostly the bacteria and protozoans are digested and assimilated. At Mud Bay, most of the biomass is represented by infaunal detritivores and both suspension and deposit feeders are represented. Common invertebrates include polychaetes such as lugworms and clam worms, molluscs such as Macoma clams, zooplankton such as harpacticoid copepods, mysids, and other small crustaceans. Less common are sessile epifauna such as blue mussels and barnacles that attach to the larger sediment particles where mobile snails such as periwinkles also graze the surface. Most predators in Mud Bay are transients representing other systems and predation is typically more intense in spring and summer. However, several overwintering sea duck species are heavily dependent on mud flats for food. Fishes, crabs, and ducks move onto the intertidal flats during high tides, and shorebirds move in during low tides. Mud Bay supports a dense and diverse variety of birds in early May that feed on invertebrates and fishes. More than 100 species of birds have been seen in Kachemak Bay during April-May including an extraordinary number of shorebirds. Shorebirds are a diverse group including sandpipers, plovers, avocets, oystercatchers, and phalaropes. The most numerous shorebirds in Mud Bay are the western sandpiper, and dunlin and dowitchers are also abundant. There are approximately 217 recognized species of shorebirds in the world, 81 of which occur in the Americas for all or part of their lifecycle, 52 species breed in North America, and 36 species have been sighted in Mud Bay. Most shorebirds are found near water, but several species prefer habitats far from shore. Many species are long-distance migrants, traveling from breeding grounds in the Arctic tundra to non-breeding grounds in southernmost South America every year.

Kachemak Bay was designated as a Western Hemisphere Shorebird Reserve Network site in 1995. This is part of a conservation strategy for the Americas that aims to protect the nesting, breeding, and staging habitats of migratory shorebirds. The conservation strategy includes protecting a network of sites that are a critical component of shorebird migration. Shorebirds are among a few groups of birds showing dramatic declines in population. Species that undertake hemispheric migrations rely on specific habitats and food sources to survive, but these resources are increasingly under threat from human disturbance, habitat loss and degradation, increasing predation, and climate change. Shorebird populations began to decline in the 1800s, in part due to market hunting. As humans have continued to alter the landscape, shorebird populations have further decreased, with declines increasing rapidly in recent decades. Alaska’s immense size, diverse habitats, and position at the terminus of several migratory flyways, and where over one-third of the world’s species have been recorded, make it a critical region for breeding and migrating shorebirds. Three species and seven subspecies breed nowhere else. Seven species are year-round residents of Alaska, but most shorebird species are migratory. These migratory species connect Alaska to sites in Central and South America, Asia, and locations throughout Oceania. During migration, shorebirds concentrate in huge numbers at many coastal staging and migratory stopover sites such as Kachemak Bay. The first shorebird habitat parcels designated for conservation in Kachemak Bay were Mud Bay and the adjacent Mariner Lagoon which are both owned and managed by the City of Homer. Other parcels include the Fox River Flats Critical Habitat Area at the head of Kachemak Bay and adjoining parts of the Kachemak Bay Critical Habitat Area owned by the State of Alaska and managed by the Alaska Department of Fish and Game. In 2016, several areas were added including Beluga Slough near Homer, the entirety of Kachemak Bay Critical Habitat Area, and Sixty-Foot Rock, a small island owned by the U.S. Fish and Wildlife Service as part of the Alaska Maritime National Wildlife Refuge that is an important wintering site for rock sandpiper and surfbirds. Read more here and here. Explore more of Mud Bay and Kachemak Bay here: