Which set of factors is listed as most important for the self-purification of a stream?

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Self-purification in a stream comes from processes that dilute pollutants, transfer and maintain oxygen, and biologically break down contaminants, all shaped by several interacting factors. Temperature changes how fast microbes and chemical reactions proceed, so warmer water can speed degradation but may lower dissolved oxygen availability. Sunlight drives photosynthesis by aquatic plants and algae, boosting oxygen production during the day and supporting greater biological activity. The buffering capacity of the water, provided by carbonates, helps keep pH in a range that supports microbial communities essential for pollutant breakdown and nitrification. Flow conditions promote mixing and dilution with cleaner water and enhance oxygen transfer from the atmosphere, which aids both aerobic decomposition and overall water quality. When these elements—temperature, alkalinity (carbonates), sunlight, and flow-driven mixing—are all considered, they best explain the stream’s capacity to purify itself. Relying on dissolved oxygen alone misses how temperature, buffering, light-driven production of oxygen, and hydraulic mixing collectively influence purification rates.