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Tales of the Coast
Coastal Dynamics

Tides and Drowned Valley Coasts

Tide current energy is greater than wave energy at the mouth of tide-dominated estuaries, resulting in the development of elongate sandbars. The bars dissipate wave energy, helping protect the inner portions of the estuary. As in wave-dominated estuaries, riverine energy also decreases downriver from the river mouth.

A bay-head delta is usually not present in the river-dominated portion of tidally dominated estuaries. Instead, the river channel merges directly into a single or a series of tidal channels that eventually reach the sea. Sands are found along the tidal channels, while muddy sediments accumulate in the tidal flats and marshes along the sides of the estuary.

As tide energy increases relative to wave energy, the barrier system at the mouth of the estuary becomes progressively more dissected by tidal inlets, and elongate sandbars form along the margins of the tidal channels. As energy levels increase in the central, mixed-energy part of the estuary, marine sand is transported further up into the estuary, and the muddy central basin is replaced by sandy tidal channels flanked by marshes.

Chesapeake Bay
Source: National Oceanic & Atmospheric Adminstration

Tides and Drowned Valley Coasts

Effects of Tide Range: The inner end of an estuary has been defined as the limit of detectable tidal influence. Therefore, the gradient of the coastal zone and the tide range have a great influence on the length of estuaries. Estuaries become longer as gradient decreases and tide range increases.