Which factor most directly influences stream flow?

• A. Precipitation runoff in watersheds
• B. Ocean salinity
• C. Moon phase
• D. Tidal forces

Answer: (A)

Stream flow is governed most directly by the amount of water entering the stream from precipitation that runs off the land within its watershed. When rain falls in a watershed, some water infiltrates the soil, but a portion becomes surface runoff that immediately feeds the stream, increasing its discharge. The more rainfall across the basin and the less water that soaks away or evaporates, the higher the stream flow will be. Snowmelt also adds to this runoff, contributing to higher discharge as the season warms.

Ocean salinity is about salt concentration in seawater and doesn’t determine how much water streams carry. Moon phase and tidal forces mainly affect water levels near coastlines and can cause tides to push into rivers near their mouths, but they don’t set the general, direct flow of inland streams.