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.