The researchers studied water quality in the Three Gorges Reservoir area under different water flow conditions. They used data from six years after the reservoir was filled to calculate how long water stays in different parts of the reservoir. They found that water quality varies in different areas and seasons. During the wet season, the whole river should be evaluated by river standards. In the dry season, the upstream area should follow river standards, while the downstream area should follow lake standards. Tributary bays should be evaluated by lake standards. This method of evaluation matches the actual conditions of the reservoir, helping to protect the water environment better.