Mud Bay is a tidal flat about 0.6 miles (1 km) wide, partially enclosed by sand spits 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. Historically, this protected embayment was named Coal Bay, or “Zaliv Ugolnoy,” as 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, blocking all tidal flushing and creating 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, leading to the development of two sand spits. One extends south for about 0.6 miles (1 km) from the north shore of inner Kachemak Bay, and the other extends north for about 0.3 miles (0.5 km) from the Homer Spit. These spits partially enclose the head of Coal Bay, creating what locals call Mud Bay. The main part of the Mud Bay tidal flat is located in the intertidal zone, which spans nearly 30 feet (9 m) of tidal range. Tidal flat sediment deposits are generally concentrated in the intertidal zone, composed of supratidal marshes and a barren zone at lower elevations. The sedimentary layers in these zones consist of varying ratios of sand and mud, and the sediment accumulation rate depends on factors such as relative sea-level changes due to land subsidence or emergence, climate change, and sediment deposition rates. Although protected from the west by the Homer Spit, Mud Bay is exposed to easterly winds and waves. Despite its name, Mud Bay contains a mixture of fine silts, clays, sand, gravel, and occasional cobbles and small boulders. The larger particles were likely transported by ice rafting during the spring breakup, when sediment-laden shorefast ice from the north shore of inner Kachemak Bay drifts into Mud Bay and melts.

Tidal flats support diverse wildlife populations and are crucial habitats for salt marshes, marine invertebrates, fish, and migratory birds. The trophic structure of a typical mud flat is more diverse than that of sandy beaches, largely based on the accumulation of fine detrital material from marine and terrestrial ecosystems. This detritus includes inorganic particles, bacteria, and protozoans, which are ingested by ssuspension and deposit feeders, with bacteria and protozoans primarily digested and assimilated. At Mud Bay, infaunal detritivores and both types of feeders dominate the biomass. Common invertebrates include polychaetes such as lugworms and clam worms, molluscs like Macoma clams, and zooplankton such as harpacticoid copepods, mysids, and other small crustaceans. Less common are sessile epifauna like blue mussels and barnacles, which attach to larger sediment particles, where mobile snails such as periwinkles graze. Most predators in Mud Bay are transients from other systems, with predation more intense in spring and summer. Several overwintering sea duck species depend heavily on mudflats for food. Fish, crabs, and ducks move onto the intertidal flats during high tides, while shorebirds arrive at low tides. In early May, Mud Bay hosts a dense and diverse array of birds feeding on invertebrates and fish. More than 100 bird species are seen in Kachemak Bay during April and May, including many shorebirds. This diverse group includes ssandpipers, plovers, avocets, oystercatchers, and phalaropes. The most numerous shorebirds in Mud Bay are western sandpiper; dunlins and dowitchers are also abundant. Of the 217 recognized shorebird species worldwide, 81 occur in the Americas for part of their life cycle. Fifty-two species breed in North America, and 36 have been sighted in Mud Bay. Most shorebirds are found near water, but some prefer habitats far from shore. Many are long-distance migrants, traveling annually from Arctic breeding grounds to non-breeding areas in southernmost South America.

Kachemak Bay was designated a Western Hemisphere Shorebird Reserve Network site in 1995 as part of a conservation strategy for the Americas. This initiative aims to protect the nesting, breeding, and staging habitats of migratory shorebirds. Shorebirds are one of the few bird groups experiencing dramatic population declines. Species undertaking hemispheric migrations rely on specific habitats and food sources, now increasingly threatened by human disturbance, habitat loss and degradation, predation, and climate change. Shorebird populations began declining in the 1800s due to market hunting. As humans have continued to alter landscapes, these populations have decreased further, with declines accelerating in recent decades. Alaska’s vast size, diverse habitats, and position at the terminus of several migratory flyways—where over one-third of the world’s shorebird species have been recorded—make it crucial for breeding and migrating shorebirds. Three species and seven subspecies of shorebirds breed exclusively in Alaska. While seven species reside in Alaska year-round, most shorebird species are migratory, connecting the state to locations in Central and South America, Asia, and Oceania. During migration, vast numbers of shorebirds gather at coastal staging and stopover sites like Kachemak Bay. The first shorebird habitat parcels designated for conservation in Kachemak Bay were Mud Bay and Mariner Lagoon, both owned and managed by the City of Homer. Additional parcels include the Fox River Flats Critical Habitat Area at the head of the bay and parts of the Kachemak Bay Critical Habitat Area, managed by the Alaska Department of Fish and Game. In 2016, more areas were added, such as Beluga Slough near Homer, the entire Kachemak Bay Critical Habitat Area, and Sixty-Foot Rock. Sixty-Foot Rock, a small island managed by the US Fish and Wildlife Service as part of the Alaska Maritime National Wildlife Refuge, is a vital wintering site for rock sandpiper and surfbirds. Read more here and here. Explore more of Mud Bay and Kachemak Bay here: