The lack of corals near the mouths of the Orinoco and Amazon rivers is primarily due to:

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The massive out-flow of freshwater from the Orinoco and Amazon rivers reduces the salinity of the water near their mouths, which is not conducive to the growth of corals. Cool northern flowing currents and cool upwelling water are not major factors affecting coral growth in this region. Clear water does not necessarily prevent coral growth, and can actually be beneficial for their photosynthesis.

The Lack of Corals near the Mouths of the Orinoco