The archetypal estuary is a fertile meeting ground of freshwater and ocean currents..
Most estuaries are fertile frontiers of saltwater and freshwater, usually situated along coastal river mouths or sounds where tidal lurches and inland drainage ensure a roiling, brackish, nutrient-rich environment. Estuaries also rarely form in interior lakes or other large water bodies at river or stream inlets. Whatever their form or geological profile, estuaries are a mosaic of habitats.
River Mouth
Most types of estuaries involve the mouth of a river -- that is, its outlet into a larger body of water, usually an ocean. In many cases an estuary constitutes a delta, a specific alluvial landform that develops at the mouth of a river where the deposits build up more rapidly than coastal currents can dismantle them.
In glacially affected areas, an estuary might form along a drowned river valley, where rising sea levels associated with retreating glaciers infiltrate the ultimate lower portion of river channel, creating a sound. Fjords, which are brackish arms of the ocean cupped in a glacially carved coastal valley, also harbor estuaries. The estuary zone in these cases encompasses the tidally influenced portion of the lower river, where marine and freshwater mix.
Spits and Bars
Bar-built estuaries are those hemmed in from the open ocean by a spit, bar or barrier island of sand that parallels the coastline, shielding them from rough coastal waves. Such formations derive from a process called littoral drift, wherein sediments are transported along a coastline by so-called longshore currents. When the sediment-laden currents encounter a concavity in the coastline, as at the mouth of a river or a bay, they lose some energy and deposit sand, building up a spit from the wave-ward edge of the contour. A spit fully spanning the mouth of a bay becomes a baymouth bar, and the estuary to the landward of it may convert to a lagoon.
Tidal Zones
While part of an estuary may always be waterlogged by the channel or channels of river outflow, portions are exposed with the recession of the tides, large-scale movements of water driven by the gravitational pull of the moon and to a lesser extent the sun. The mudflats revealed by an ebb tide provide rich habitat for burrowing invertebrates and a variety of larger animals, from shorebirds to raccoons, that forage on the burrowers and also on deposited ocean flotsam. With the next onrush of tidal waters, these mudflats are flooded again.
Habitat Mosaic
The different sections of an estuary function together as a unified ecosystem mosaic utilized by different organisms. Many types of fish spend their developing stage in estuaries, where they are somewhat protected from open-ocean predators and currents. In a tropical estuary, mangroves might form stilt-legged forests, providing roosting ground for birds and accumulating nutrient-rich muck around their roots.
Along a Pacific Coast estuary in the American Northwest, bald eagles might forage for fish, waterfowl and carrion in the shallows and mudflats, then retreat to groves of shore pine or Sitka spruce on headlands or seastacks bordering or encompassed within the estuary.
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