What causes stream pollution?

Stream pollution primarily occurs when harmful substances enter water bodies, which negatively impact water quality and aquatic ecosystems. These harmful substances can include industrial waste, agricultural runoff, sewage, heavy metals, plastics, and nutrients like nitrogen and phosphorus that can cause algal blooms. Such pollutants disrupt the natural balance of the water environment, harm aquatic life, and pose risks to human health if the water is used for drinking or recreation.

Natural sedimentation processes, seasonal changes in water temperature, and overpopulation of aquatic species can influence the overall health of a stream but do not directly cause pollution. Sedimentation is part of a natural process that can actually contribute to a stream's ecological balance. Seasonal temperature changes can affect aquatic life cycles but are not considered pollution. Overpopulation of aquatic species can lead to competition for resources and may influence water quality, but it doesn’t introduce harmful contaminants into the water. Hence, the introduction of harmful substances is the direct cause of stream pollution, making it the correct answer.