Why are coral reefs absent at the mouth of a river?

Coral reefs are absent at the mouth of a river primarily due to the high levels of sediment and nutrients that river runoff introduces into coastal waters. These conditions reduce water clarity and light penetration, which are essential for coral photosynthesis. Additionally, increased nutrient levels can lead to algal blooms that outcompete corals for space and resources. The lower salinity and fluctuating water conditions near river mouths further inhibit coral growth and survival.