Estuaries are river valleys that are flooded by seawater. They form when sea level rises relative to the mouth of a river, and the shoreline retreats landward, up the river valley. This usually happens when the river is not transporting enough sediment to keep the shoreline in place. Estuaries commonly have fluctuating water chemistry. When the river flow is high and the tide is low, they have a high influx of fresh water. When the river flow is low and the tide is high, seawater extends much farther into the river valley, and the water is saltier. Due to the higher density of seawater relative to freshwater, sometimes estuaries have a layer of fresh water that overlies seawater. The waters often mix, creating brackish water. These significant changes in water salinity help shape the ecosystems in estuaries, favoring organisms that are tolerant of frequent changes in water chemistry.
Estuaries form when sea level increases relative to the mouth of the river. Rivers carve into a shoreline when sea level is low, creating valleys of various widths depending on the sediment transport capacity of the river, the ease with which the near-shore rocks or sediments can be eroded, and the duration of the low sea level. When sea level starts to rise, the river gradient is lower near the shoreline, so sediment is deposited before the river gets to the former shoreline. If the sediment supply is low or the rate of sea level rise is high, seawater will start to intrude up the former river valley, creating the estuary.
Like in all coastal environments, tides and waves can redistribute sediment within an estuary. In environments with waves, a set of barrier islands usually develops near the former shoreline, defining a bay or lagoon into which the river flows. In environments with strong tides, estuaries develop tidal islands and bars, similar to those in tide-influenced deltas.
Distribution of Depositional Environments
The landward-side of all estuaries is dominated by fluvial processes and deposits. The boundary between the river and the estuary is usually defined as the farthest upstream area influenced by seawater. Seaward of this area, the influence of marine processes increases, both in terms of water chemistry and sediment transport. Similarly, the influence of the river decreases. Environments eventually become so dominated by marine processes that the influence of the river is difficult to identify. These marine processes include all of those we've discussed before.
The lateral boundaries of estuaries usually consist of exposed land. These areas can be eroding away, developing soil, or accumulating eolian deposits. In places where the topographic gradient is low, marshes can stretch from these areas of land to a main channel that accommodates the river (and tide) flows.