1.1 Assessing Slope of the Land
Forested ecosystems occupy a rich array of varied landforms. In the Pacific Northwest, this diversity is readily apparent as one surveys the landscape– volcanic peaks contrast with wide valleys; steep, forested hillsides surround gently rolling savannas; and rapidly cascading mountain streams transition to meandering river floodplains. This varied topography is an integral part of the forest, influencing climate, soils, water, vegetation and aquatic life (Figure 1.1). Natural resource technicians are often called upon to assess the topography, and one of the common elements measured is the slope of the land. How steep is a hillside? Does it drain to a stream? Are there cliffs or bluffs present? Topographical field data collected by technicians are used to inform decisions about land management activities such as providing shade for streams, building roads or trails, and prescribing timber harvesting operations.
Defining Slope
Slope is essentially the gradient or incline of the land. A steep slope refers to a sharp incline; a gentle slope to a slight incline. The steep, forested slopes in Figure 1.1 contrast with the gentler slope of the river’s path as it flows between them.
Driving down a highway you may see a road sign that reads “6% Grade” or “Steep Grade.” The grade of the road is, essentially, its slope. The sign in Figure 1.2 indicates that the road descends at a six percent grade or a six percent slope.
A six percent slope means that the road elevation changes 6 feet for every 100 feet of horizontal distance (Figure 1.3).
Mathematically, slope is defined as “the rise over the run ” (or the rise divided by the run), in which rise equals change in elevation and run equals horizontal distance:
[latex]\displaystyle slope=\frac{{rise}}{{run}}[/latex] or [latex]\frac{{\text{elevation change}}}{{\text{horizontal distance}}}[/latex] or in this case: [latex]\frac{{\text{6 ft}\text{.}}}{{\text{100 ft}\text{.}}}=.06[/latex]
To express slope as percent slope, simply multiply the slope fraction by 100. So, .06 = 6%.
[latex]\displaystyle \left( {\frac{{rise}}{{run}}} \right)\left( {100} \right)=[/latex]%slope
[latex]\left( {\frac{{6ft}}{{100ft}}} \right)\left( {100} \right)=[/latex]6%
In the road example below, the six-foot change in elevation is the rise, and the 100-foot horizontal distance of the road is the run. Driving uphill means climbing a “positive” six percent slope (Figure A). Driving downhill, the “rise” is actually a drop, so there is a “negative,” or downhill, slope (Figure B). When dealing with slope, a positive slope simply means uphill and a negative slope means downhill. A negative number does not mean “minus” as in algebraic expressions.
Note that the actual road distance is the hypotenuse of the illustrated slope triangle. This length is called slope distance. Slope distance is always longer than the horizontal distance, or run. Applying the Pythagorean theorem (a2 + b2 = c2) to this triangle, the slop