1. Salinity: Coral reefs require a certain level of salinity to thrive. The salinity of water at the mouths of rivers is often lower than that of the open ocean due to the influx of freshwater from the river. This can make it difficult for corals to survive.
2. Sedimentation: Rivers carry a large amount of sediment, which can smother and damage coral reefs. The high level of sedimentation at river mouths can make it difficult for coral larvae to settle and grow.
3. Pollution: Rivers often carry pollutants from land, such as fertilizers, pesticides, and sewage. These pollutants can be harmful to corals and can contribute to coral bleaching and disease.
4. Water temperature: The water near the mouths of rivers can be warmer than the open ocean due to the influx of freshwater. This can stress corals and make them more susceptible to disease.
5. Turbidity: The water at river mouths is often turbid, meaning that it contains a lot of suspended particles. This can reduce the amount of light that reaches the corals, which can inhibit their growth and photosynthesis.
As a result of these factors, corals are not typically found at the mouths of rivers. They prefer to live in clear, warm, and salty waters with low levels of sedimentation and pollution.