溺湾（Ria，又称为谷湾）是海岸线地形的一种，属沈水海岸的一种型态。溺湾是一种狭长的楔形入海口，是由非冰河挖蚀、由河流自然侵蚀而成、原本与海相连的开放性河谷，因陆地下沉或海平面上升之故，在局部沉没至海中之后所形成的崎岖海岸线。溺湾地形通常会保留冲积平原的部分地理特征，例如树干状或甚至枝晶状的边缘，但有时也会见到呈直线状、没有明显分歧的外观。

溺湾海岸（ria coast）通常是由一系列方向平行的溺湾接连排列而成，两两相邻的溺湾之间有比较明显、向内陆延伸的山脊相隔。当陆地沉没至海水中时，原本的河口沉入水中，山脊则成为伸入海中的岬状地形，在海湾附近连成一片非常不规则的锯齿状海岸线。有部分地势较高的山峰，会在离海岸一段距离之处冒出水面形成岛屿，因此在海岬外罗列数个带状分布的小岛，是溺湾海岸常见的一种地理特征。

A ria is a coastal inlet formed by the partial submergence of an unglaciated river valley. It is a drowned river valley that remains open to the sea. Typically, rias have a dendritic, treelike outline although they can be straight and without significant branches. This pattern is inherited from the dendritic drainage pattern of the flooded river valley. The drowning of river valleys along a stretch of coast and formation of rias results in an extremely irregular and indented coastline. Often, there are islands, which are summits of partly submerged, pre-existing hill peaks. A ria coast is a coastline having several parallel rias separated by prominent ridges, extending a distance inland. The sea level change that caused the submergence of a river valley may be either eustatic (where global sea levels rise), or isostatic (where the local land sinks). The result is often a very large estuary at the mouth of a relatively insignificant river (or else sediments would quickly fill the ria). The Kingsbridge Estuary in Devon, England, is an extreme example of a ria forming an estuary disproportionate to the size of its river; no significant river flows into it at all, only a number of small streams.