Map 1 - Topographic location map

Map 2. Watershed Priority Map


Map 3. Land Ownership Map



Map 4. Slopes Greater than 40 Percent Map



Map 5. Wilderness/ Roadless Map


Map 6. Vegetation Map


Map 7. Lodgepole Pine Map


Map 8. Lodgepole Pine and Wilderness/Roadless Map


Map 9. Lodgepole Pine, Wilderness/Roadless and Slopes >40% Map


Map 9. Lodgepole Pine, Wilderness/Roadless, Slopes >40%, and Ownership Map

The Final Priority map was made by increasing the priorities on the Composite Hazard map for those watersheds that have a water supply feature (diversion, reservoir or other). Then those results are recategorized into the five hazard categories.

Watershed steepness or ruggedness is an indicator of the relative sensitivity to debris flows following wildfires (Cannon and Reneau 2000). The more rugged the watershed, the higher its sensitivity to generating debris flows following wildfire. The ruggedness of a watershed is calculated here using a formula from Melton (1957) that uses the watershed area and height, or change in elevation.





The map above is the categorized ruggedness for the South Platte Headwaters Watershed. It generally shows that the western portion of the watershed is steep and the middle portion is flat.

Roads can convert subsurface runoff to surface runoff and then route the surface runoff to stream channels, increasing peakflows (Megan and Kidd 1972, Ice 1985, and Swanson et al. 1987). Therefore, watersheds with higher road densities have a higher sensitivity to increases in peak flows following wildfires. Road density in miles of road per square mile of watershed area were used as an indicator of flooding risk. Roads data need to be consistent within the fifth-level watershed to allow for appropriate comparisons during prioritization. Therefore, the U.S. Census Bureau’s Tiger database was used as a consistent roads layer.





The map above is the categorized road density for the South Platte Watershed.

This ranking map has been adjusted. The largest density of roads in this watershed are in towns and other areas that have roaded for development. We are interested in roads that would increase the risk of flooding following wildfires in forested areas. We adjusted the roads analysis to reduce the influence on roads outside of forested areas. The adjustment method that was used involved a comparison of the Tiger roads data with conditions on the ground. This was done by looking at some vegetation type mapping to eliminate roads in developed areas. Then digital images including Google Earth were used to look for roads that were not in the roads data and evaluate roads that were in subdivisions and towns. Adjustments were then made to the roads density calculations based upon those evaluations. The adjusted road density ranking is shown in the map above.

The Flooding and Debris Flow Hazard is the combination of ruggedness and road density. The procedure from the Front Range Watershed Work Group determined that slope should have a higher value than road density in this ranking. The effect of road density on post-wildfire effects was determined to be more variable than slope. For example, an area with a shallow slope and high road density would have little influence on post-wildfire erosion. The determination that slope would have a higher value than road density was based on professional judgment, experience and the results of the Upper South Platte Watershed Assessment Test Case. This assumption was used in the initial calculation of Flooding/Debris Flow Hazard for the South Platte Headwaters watershed.




The map above is the categorized Flooding/Debris Flow Hazard for the South Platte Headwaters Watershed. It generally shows that areas of the watershed with high road densities and high ruggedness rank high in this combined factor. The best way to look at this map is to look at a single watershed on the ruggedness and road density maps, noting the rankings on each. Then look at this map and see how they resulted in the final ranking for this component.