Where is light penetration in a river or stream likely to be lowest

The light penetration in a river or stream is likely to be lowest AT THE MOUTH.

Streams and rivers are bodies of water that flow in one direction only; they travel from the source to the mouth. A river mouth refers to the part of a river, where the river merge with another river, stream or other forms of water bodies.

Around the mouth of the river, the water is usually murky as a result of all the sediments that the river has picked up during its travel upstream, this significantly reduce the amount of light that can penetrate the water at this point.  

Due to decrease in the amount of light that penetrate the water at the mouth region, there is decrease in the level of oxygen in the water and a corresponding decrease in diversity of plant and animals that can be found in the mouth region of the river.

from my research i have to the conclusion that decomposers, such as fungi, bacteria and invertebrates, play a critical role in nutrient cycling and without them the earth's ecosystem may not support life as we know it. elements such as carbon, nitrogen and phosphorus exist in limited amounts on earth, although huge amounts of carbon are being released into the atmosphere from underground sources now by humans. it is still extremely important that other nutrients be retained by the process of re-use. decomposers are organisms that are able to chemically break down dead material and make these elements available for use by other organisms. without this breakdown of dead material, dead bodies would pile up. so decomposers also play a role in clean up also. without them, life might not continue as we know it and dead material would overwhelm the earth.    

the man is bald so he has no hair to get wet