The Tappan Zee Bay and Haverstraw Bay are truly special bodies of water within a majestic river. These brackish embayments, or bays, are integral to the Hudson River ecosystem that function as fountains of life and cradles for a rich diversity of juvenile species that help sustain the Atlantic Ocean’s coastal marine ecosystem.

The Hudson is a coastal, tidal river that flows in a dynamic continuum, receiving synergistic contribution from its tributaries on its journey from its headwaters to its confluence with the Atlantic Ocean. However, the Hudson is not really a single river, but a main stem arterial system that connects a network of flowing rivers that collectively drain the watershed.

The broad bays that form in the Hudson’s lower reaches integrate freshwater, estuarine, and marine ecosystems, and provide a pathway for aquatic organisms as well as a transport system linking the terrestrial and ocean ecosystem. Sediments sourced from the watershed are the main commodity exported by the Hudson and its tributary system that continually provision the estuary’s ecosystems. These sediments and nutrients provide ingredients necessary for the synthesis of carbon-based lifeforms and are foundational to multiple food webs.

Migratory marine fishes interdependently transfer precious marine-derived nutrients back to the watershed during their spawning runs. Scientists have demonstrated the inland migration of those marine-derived elements into forests and shoreline ecosystems. In turn, enhanced vegetative growth improves habitat complexity and niche space that benefits aquatic and terrestrial invertebrates, as well as the respective tributary’s ecosystems. While the primary function of a river is conveyance, a complementary, bidirectional and intimate relationship exists between rivers, oceans, watersheds, and their respective ecosystems.

An estuary is defined as a semi-enclosed water body where a coastal river meets and merges with the sea. Also, in estuaries, terrestrial landforms and flowing water physically and ecologically interact with one another. This land-water communication is seasonally synchronized between both biomes, with leaf litter and other forms of organic material exported to the lower reaches of the estuary as a form of carbon. This resupply of carbon replenishes food webs, revitalizes the life-supporting potential of the estuary, and that makes these some of the richest ecosystems on Earth.

The Hudson River estuary is prominent among the coastal estuaries of the eastern U.S. owing to the additive influence of its geography, geology, geomorphology, and watershed productivity. These characteristics synergistically combine to cement the Hudson’s reproductive capacity, which results in an immense production of aquatic life.

The sum total of unique physical and biological characteristics coinciding within the Hudson River Estuary have created a multidimensional transitional zone, known as an ecotone, where steep environmental gradients, boundaries of ecological communities or ecosystems, or zoogeographic borders occur. Ecotones typically facilitate higher species richness, diversity, and abundances. For migratory species, ecotones provide a means of exploiting rich bioenergetic transformations and movement into multiple habitats within short spatial spans.

Where coastal rivers merge with the ocean, the two water bodies become sinuously intertwined with each other as constituent parts of the Earth’s water cycle. Rivers drain the landscape and deliver the surfeit of falling freshwater and material from the watershed to the embracing ocean. Oscillating ocean tides and the falling river water driven by countervailing gravitational forces collide in the Hudson. Marine waters laden with dissolved salt ions have a greater density than freshwater, causing them to dive under the lighter layers of the freshwater sheath. As these layers slide past each other, they form a countercurrent exchange system not dissimilar to a fish’s circulatory system, causing hydrodynamic interaction that blends fresh and salt waters into brackish regions. The ocean and river mix within the broad embayments of the lower estuary creating an ecological ecotone known as Tappan Zee Bay or Haverstraw Bay.

Tappan Zee Bay or Haverstraw Bay are shallow river regions within the Hudson River estuary where the river’s velocity begins to slow down and spread out as the valley floor levels upon approaching the sea. These roiled, wind-swept, leavened, brackish waters become the mixing bowls of the estuary where kinetic energy and solar energy are focused upon the basic biological ingredients. These bays represent ecological chalices where physical and biological conditions erupt into a profusion of life that is conceived, sustained, and nurtured.

Respiration and photosynthesis are complementary biochemical processes that maintain a reciprocating renewal of life. Cyanobacteria and a wide array of other photosynthetic organisms are the primary producers and the foundations upon which aquatic ecosystems are balanced. Carbon in the form of sediment is transported from the Hudson’s headwaters along with other essential elements to supply the chemical precursors of these processes.

This combination of various forms of free energy and a near limitless supply of recycled nutrients become mixed within the protective waters of the Hudson River estuary. Within the estuary, the shallow bays are both the fountain and the cradle of aquatic life. Consequently, many marine species have evolved adaptive evolutionary strategies to cross the salt barrier to exploit the vast primary production and protective waters of the estuary at various stages of their life cycles to renew their populations.

Within the Hudson’s waters, one of the premiere animal migrations found anywhere on Earth occurs each year. The basic ingredients for life are delivered from the watershed and that erupts into a biotic richness within the Hudson’s waters. To be sure, estuaries are sources of renewal, resurgence, rebirth, recovery, resilience, and recurrence. In fact, 80 percent of coastal marine fisheries are supported by estuaries, and because the Hudson River forms the second largest estuary on the East Coast, it represents the storehouse and supply chain for the Mid-Atlantic Bight and the coastal ocean ecosystems.

The iconic suite of fishes that migrate between fresh and salt water, to and from the Hudson each year, are highly evolved organisms that have the unique ability to cross the salt boundary as part of their life-history patterns and renewal of their populations. The odds of surviving to adulthood for any wild fish population is extremely remote, and each species develops an evolutionary response to this dilemma.

Diadromy, which refers to species that migrate between saltwater and freshwater, likely arose as an evolutionary strategy to ensure greater survivability within the relatively protected freshwaters while juvenile fishes remain highly vulnerable to predation or strong currents. As the juveniles mature and become more physically robust, they direct themselves towards the ocean in the hopes of exploiting increased forage opportunities.

While diadromy was an extraordinary evolutionary success, its dual habitat usage became its weakness as it placed their most vulnerable life stage within close proximity of the dense human settlements along coastal rivers. Nevertheless, the importance of Tappan Zee and Haverstraw Bay once again rise to the forefront to this ecological exposition, because within these richly productive brackish waters, fish can adjust their osmotic balance — cellular salt concentrations — with respect to their ambient environment. Moving from highly saline marine waters to freshwater requires a highly refined, complex, cascading series of physiological changes that each fish must undergo before moving from one realm to another. It is the brackish bays that accommodate the internal readjustment to occur.

Many iconic and regionally important species call the Hudson River estuary home: from striped bass, menhaden, American eel, and American shad, to Atlantic sturgeon, shortnose sturgeon, river herring, bluefish, white perch, tomcod, and many others.

Thus, the Hudson River estuary and places like Haverstraw Bay and Tappan Zee Bay will always be a critical piece of this unique annual fish migration — the Greatest Show on Earth.