Why are coral reefs absent at the mouth of rivers?

Answer 1

Corals need access to sunlight (the algae living in their cells need light for photosynthesis) so the water where they grow needs to be shallow and clear. Too much sediment in the water can smother the coral polyps and too much fresh water kills them.

Answer 2

Corals seem to grow best where the water is warm and shallow. They also like clear, sunny and unsettled water in order to bring in the nutrients they require to grow. The process of a water course flowing involves erosion that takes rocks, stones, sand and particles downstream. When rivers come into contact with the sea, the rivers are generally flowing fairly slowly and this is when the smaller particles are in suspension. River Deltas commonly form as a result. Corals do flourish where there is a high concentration of particles, because they are not very nutritious (mainly composed of rock particles) and they also tend to block out the light. So the mouth of rivers may represent warm, shallow and unsettled water (3 out of the 5 criteria), but they don't represent clear or sunny environments.