Weather can affect the physical, biological and chemical conditions in streams. Some examples:

• Flooding - Flooding increases water levels. Fast flowing surface water entering waterbodies often adds pollution and sediment, but it can also dilute any pollution present in the water.
• Drought - A long period without any or very little rain will lower the water levels, possibly making some stream stretches dry up. This affects numerous water chemistry components and stresses animals living in the streams.
• Cloudy weather - Plants produce food for themselves from sunlight and give off oxygen as a byproduct through the process known as photosynthesis. Prolonged cloudy weather may affect the amount of dissolved oxygen due to plants, because of the slower rate of photosynthesis caused by decreased levels of sunlight.
• Rain - Rainstorms and showers tend to dilute pollution, increase turbidity, and add nitrates and other non-point source pollution to waterbodies.
• Wind - Strong wind creates surface waves that will increase dissolved oxygen levels.
• Temperature - As water temperature decreases, its ability to hold oxygen increases.