Funnel-shaped estuaries
Typical estuaries are formed under the influence of tidal currents, whereby the river mouth is widened in a funnel shape (funnel estuary): In these tidally influenced transition zones between marine and fluvial areas, the tides cause a pendulum motion of often greater volume than the seaward discharge of the river water. The high flow velocities associated with the pendulum motion ensure that erosion is greater than sedimentation. The heavy salt water advances wedge-shaped into the river from the sea at high tide and is overlain by the fresh water of the river. In the process, the strong tidal flow erodes the riverbed and banks, and much material is transported upstream.
This material cannot be removed by the usually weaker ebb current. For this reason, delta formation can occur at the upper end of estuaries. A remnant of such a delta is the branching of the Elbe into the Norderelbe and Süderelbe in Hamburg. Shipping has used estuaries as natural harbours for centuries. Here, ships were protected from storms and pirates. Before motorisation, vessels were driven by the tidal current to the harbour towns, which were usually founded at the inner end of the funnel and, being located in the middle of the country, had a large catchment area. Although not quite as confusing as deltas, estuaries also mostly require pilots.
Due to the Coriolis force, seawater is deflected clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. Depending on the geographical conditions, the riverbanks can be affected differently by erosion and sedimentation.
In the Amazon, the flood flow can be traced up to about 870 km from the coast.
| Characteristics of four German estuaries flowing into the German Bight |
| Eider | Elbe | Weser | Ems |
| Catchment area above the dam [10³-km²]. | 02 | 135 | 038 | 013 |
| Mean water discharge rate [m³/s] | 23 | 725 | 323 | 125 |
| Mean tidal range in the area of the weir [m]. | 02 | 002,4 | 004,1 | 002,8 |
| Length of inner estuary [km]
(excluding funnel-shaped estuary) | 21 | 120 | 070 | 050 |
Lagoon estuaries
On coasts without a significant tidal range, the current acceleration caused by the tides is missing. The defining feature "exposed to the tide" is therefore missing. Here, the slowing of the current dominates at river mouths. Dunes on the coast form an additional flow obstacle. On uplift coasts, sedimentation in the accumulation area leads to rapid siltation and thus to delta formation. On subsidence coasts, sedimentation is compensated for by the continuous rise in sea level, especially in rivers with a low bedload. This results in microtidal estuaries, not funnel-shaped but lagoon-shaped, such as the Lower Warnow and the Szczecin Lagoon at the mouth of the Oder. Bays formed during the ice age can also become estuaries, such as Traveförde in the Trave estuary.
Water quality
Estuaries, like deltas, are characterized by the transition of freshwater to saltwater (brackish water), a material load resulting from the movement of water. This depends on the flow velocity of the outflowing freshwater of the river in relation to the tide and the amount of saltwater flowing up with it. The tide in turn depends on the position of the moon (see spring tide, neap tide) and the respective weather conditions (possible storm surge) and thus also results in a very individual change of flora and fauna from the river area to the sea due to the resulting average water temperature.
