What does a transition refer to in the context of water bodies?

In the context of water bodies, a transition refers to a location where features transition from different states. This concept often involves the areas where different ecosystems meet, such as where freshwater rivers meet saltwater oceans (estuarine environments) or the differences between upland and lowland river systems. These transitional zones are critical because they often support a unique diversity of species and facilitate various ecological processes. Understanding these transitions is key in studies related to hydrology, ecology, and environmental science as they highlight the dynamic nature of aquatic ecosystems and the interactions between different environments.

The other choices, while potentially relevant to water bodies, do not encapsulate the broader ecological context of a "transition" as effectively. They focus on specific types of changes or conditions rather than the broader concept of feature change within ecosystems.