To calculate the stream gradient, you need to determine the change in elevation (vertical distance) of a stream over a specific horizontal distance. The stream gradient is typically expressed as a ratio of vertical drop to horizontal distance, often in units like feet per mile or meters per kilometer.
Here’s how you can calculate the stream gradient:
1. Identify two points along the stream: Choose two locations along the stream, preferably separated by a known distance. You will need the elevation at each point to calculate the vertical drop.
2. Determine the change in elevation: Find the elevation (height above sea level) at both points. Subtract the lower elevation from the higher elevation to get the vertical drop (also called the “elevation difference”).
Vertical Drop = Elevation of Point 1 – Elevation of Point 2
3. Measure the horizontal distance: Measure the horizontal distance between the two points along the stream. This is the straight-line distance along the streambed, not the path length of the stream. You can use a map or GPS tool to measure this distance.
4. Calculate the stream gradient: Divide the vertical drop by the horizontal distance to calculate the stream gradient.
Stream Gradient = Vertical Drop / Horizontal Distance
Example:
– Elevation at Point 1 = 500 meters
– Elevation at Point 2 = 300 meters
– Horizontal Distance between the points = 2 kilometers
Vertical Drop = 500m – 300m = 200 meters
Stream Gradient = 200 meters / 2 kilometers = 100 meters per kilometer
This means the stream has a gradient of 100 meters of vertical drop for every 1 kilometer of horizontal distance.
