Cartographic rule for wide rivers (double-line water features): How should contours be drawn where a river is represented by double lines on the map?

Introduction / Context:On topographic maps, water bodies affect how contours are depicted. Streams drawn as a single line are narrow enough that contours can often continue, but wide rivers drawn with double banks require a different convention to avoid implying known elevations beneath water where the bed profile is uncertain or variable.

Given Data / Assumptions:

• The river is wide enough to be shown by two parallel lines (double-line symbol).
• Bank elevations are known from surveyed ground; the bed elevation is typically not contoured.
• Standard mapping practice is followed.

Concept / Approach:Contours represent ground surfaces of known elevation. In wide water bodies, the ground surface is submerged and its elevation is not mapped as part of the land topography; hence contours terminate at the banks. This avoids misrepresenting the channel bed and preserves clarity for hydrologic interpretation and design tasks such as levee placement and floodplain analysis.

Step-by-Step Solution:

Verification / Alternative check:Inspect standard topographic sheets: contours neatly terminate at banks for wide rivers, while bridges or fords may show related spot levels but not continuous contouring under water.

Why Other Options Are Wrong:

• Drawing across the water implies known ground elevations under water—incorrect for standard contour maps.
• Parabolic arcs at the channel center are not a contouring convention.
• “Edge of water only” is ambiguous; the correct reference is the mapped banks.

Common Pitfalls:Letting contours cross into the channel; forgetting to close contours correctly at embankments or levees.

Final Answer:Contours are stopped at the banks of the river