Primary source for large public water-supply schemes: choose the most important one among the following.

Introduction / Context:
Public water-supply projects require dependable yield at scale, feasible intake works, and treatability. Among natural sources, rivers typically provide the required volume and replenishment for cities.

Given Data / Assumptions:

• Inland setting with traditional freshwater sources.
• Large population requiring significant, renewable supply.

Concept / Approach:
Rivers, fed by watershed runoff and baseflow, offer continuous flow and the potential for impoundment (reservoirs) to smooth seasonal variability. Lakes and ponds may be insufficient in capacity or not proximate; small streams are limited in dependable yield; seawater requires energy-intensive desalination (not typically the first choice unless geography dictates).

Step-by-Step Solution:

Verification / Alternative check:
Most metropolitan systems worldwide draw from rivers/reservoirs as primary sources.

Why Other Options Are Wrong:

• Lakes/ponds may lack sufficient dependable yield.
• Streams are typically too small for major cities.
• Sea water requires desalination—costly and energy-intensive.

Common Pitfalls:
Ignoring the importance of dependable yield (not just volume at a single time) when selecting a source.

Final Answer:
Rivers