Still water and contours: The boundary (shoreline) of a still lake on a map represents which of the following?

Introduction / Context:
A still water surface is an equipotential surface of gravity, hence level. The shoreline plotted on a map is the intersection of this level surface with the ground. Understanding what that line represents helps with contour interpretation and setting reference elevations in the field.

Given Data / Assumptions:

• The lake water is still (no wind setup or waves considered).
• Map shows the boundary between water and land at the same instant.
• Contours represent lines of equal elevation on land.

Concept / Approach:
Because the water surface is level, every point along the shoreline has the same elevation. Therefore the lake boundary is itself a contour line at the lake’s water level. While “level” and “horizontal” have technical distinctions, on a small area they are practically similar; however, the shoreline specifically corresponds to a contour (constant elevation) rather than a generic “horizontal surface.”

Step-by-Step Solution:

Verification / Alternative check:
If lake level changes seasonally, new shorelines trace different contours; historic maps may show variants tied to their survey dates.

Why Other Options Are Wrong:

• Level surface/horizontal surface: Describe the water plane itself, not the boundary line on land.
• Concave surface: Not applicable; large-scale curvature of Earth is negligible at map scale.

Common Pitfalls:
Confusing the concept of a level plane with the mapped boundary; remember the contour is the line of intersection at a single elevation.

Final Answer:
Contour line (single elevation)