Gaining vs losing streams and groundwater interaction Consider a channel–aquifer system. Identify the correct statements about effluent (gaining) and influent (losing) conditions and their relation to relative levels of the water table and stream stage.

Introduction / Context:
Surface water and groundwater are hydraulically connected in many basins. Whether a stream gains or loses water depends on the gradient between the stream stage and adjacent water table, which has major implications for baseflow, ecology, and contaminant transport.

Given Data / Assumptions:

• Unconfined conditions near the stream bank.
• Hydraulic communication between streambed and aquifer (no impermeable barrier).
• Steady conceptual description (no transient bank storage considered).

Concept / Approach:

If the water table stands above stream stage, hydraulic gradient points toward the stream and the stream is effluent. If the stream stage exceeds the water table level, water infiltrates downward and the stream is influent. These relationships are standard hydrologic definitions.

Step-by-Step Solution:

Verification / Alternative check:

Hydrographs showing sustained baseflow in dry periods indicate effluent behavior; abrupt post-storm declines can imply losing reaches.

Why Other Options Are Wrong:

• All statements (a)–(d) correctly reflect the standard definitions; choosing fewer than all is incomplete.

Common Pitfalls:

• Confusing “effluent” with “influent” terminology.
• Ignoring spatial variability: different reaches can alternate between gaining and losing behavior.

Final Answer:

All the above.