Contour lines indicate stream direction by forming "V" shapes that point upstream (towards higher elevation), with water flowing in the opposite direction, downhill and perpendicular to the lines.
Understanding Contour Lines and Flow
Contour lines are lines on a map that connect points of equal elevation. When these lines cross a stream or river, they exhibit a distinctive pattern that reveals the water's flow direction. The fundamental principle is that water always flows downhill perpendicular to the contour lines.
The "V" Rule Explained
The most defining characteristic of contour lines crossing a stream is the formation of a "V" shape.
- The "V" points upstream: When contour lines cross a valley or stream, they bend sharply upstream, forming a "V" shape. The point of this "V" always points towards the higher elevation, which is the upstream direction of the water flow.
- Water flows out of the "V": Consequently, the water flows downhill, out of the open end of the "V," representing the downstream direction.
- Rising Valley Floor: This pattern occurs because, as one proceeds upstream along a river valley, the floor of the valley naturally rises. Therefore, successively higher and higher contour lines first parallel and then cross the stream further upstream, creating the "V" shape that points uphill.
Practical Implications and Examples
Understanding the "V" rule is crucial for various applications, from hiking and navigation to hydrological studies.
Identifying Upstream vs. Downstream
To determine stream direction on a topographic map:
- Locate a stream indicated by a blue line.
- Observe the contour lines as they cross the stream.
- Note the "V" shapes formed by the contour lines. The point of the "V" indicates upstream (the direction from which the water is coming).
- The open end of the "V" indicates downstream (the direction in which the water is flowing).
Using Perpendicular Flow
While the "V" rule shows the overall direction, remember that water's immediate path is perpendicular to the contour lines. This means that if you were to draw a line from any point on a contour line straight down to the next lower contour line, crossing them at a 90-degree angle, that would represent the steepest downhill path, which is how water generally flows.
The Rising Valley Floor
The geological reason for the "V" shape pointing upstream is directly related to the topography of a river valley. As the ground elevation increases upstream, the contour lines, which represent specific elevations, must bend and wrap around the higher ground of the valley sides before crossing the lower elevation of the stream channel itself. This creates the characteristic upstream-pointing "V."
Summary Table for Quick Reference
| Feature of Contour Lines Near a Stream | Implication for Stream Direction |
|---|---|
| Form "V" shapes | Indicates presence of a stream/valley |
| Point of the "V" | Always points upstream (to higher elevation) |
| Open end of the "V" | Indicates downstream (to lower elevation) |
| Water flow relative to contours | Always flows perpendicular to contour lines, downhill |
| Elevation along stream bed | Rises as you move upstream |
By applying these principles, anyone can accurately interpret stream direction from a topographic map, an essential skill for outdoor navigation and environmental understanding.
