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 above map shows the ruggedness analysis for the Cache La Poudre watershed. The patterns displayed on this map are confirmed by the slope map.

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.

The US Forest Service roads data was used on National Forest System (NFS) lands. On all other lands the U.S. Census Bureau’s Tiger database was used.




The map above is the categorized road density for the Cache La Poudre Watershed. The results were adjusted because the road densities in four watersheds were skewing the results because they contained developments or towns that were outside of the forest. The road density in those watersheds was adjusted to remove their influence over the distribution of the results. The road density in the following sixth-level watersheds were adjusted to reduce their influence over the distribution of the results.

Gordon Creek
North Fork Cache La Poudre River-Panhandle Creek
North Fork Lone Pine Creek
Horsetooth Reservoir

The highest ranked watersheds based on road density are;

North Fork Cache La Poudre River-Panhandle Creek
Gordon Creek
North Fork Lone Pine Creek
Horsetooth Reservoir

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 calculation of Flooding/Debris Flow Hazard for the Cache La Poudre watershed.



The map above is the categorized Flooding/Debris Flow Hazard for the Cache La Poudre 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.

The highest ranked watersheds based on the Flooding/Debris Flow analysis are;

Bennett Creek
Headwaters South Fork Cache La Poudre River
Gordon Creek
Pennock Creek
South Fork Lone Pine Creek
Sevenmile Creek-Cache La Poudre River
Skin Gulch-Cache La Poudre River

The Composite Hazard Ranking combines the first three components (Wildfire Hazard, Flooding/Debris Flow Hazard and Soil Erodibility) by numerically combining their rankings for each sixth-level watershed and then re-categorizing the results. The Composite Hazard Ranking map is useful in comparing relative watershed hazards based solely on environmental factors.

The Composite Hazard Ranking for the Cache La Poudre Watershed is shown below.

High-severity fires can cause changes in watershed components that can dramatically change runoff and erosion processes in watersheds. Water and sediment yields may increase as more of the forest floor is consumed (Wells et al. 1979, Robichaud and Waldrop 1994, Soto et al. 1994, Neary et al. 2005, and Moody et al. 2008) and soil properties are altered by soil heating (Hungerford et al. 1991).

The data used in this analysis is the U.S. Department of Agriculture - Natural Resources Conservation Service (NRCS)SSURGO soils data combined with the U.S. Forest Service soils data. SSURGO data does not cover all the watershed but is available at a scale that generally ranges from 1:12,000 to 1:63,360. The U.S. Forest Service soils data is comparable with the SSURGO data in resolution. Areas without SSURGO data were filled in with U.S. Forest Service data.

The soil analysis used a combination of two standard erodibility indicators: the inherent susceptibility of soil to erosion (K factor) and land slope derived from USGS 30m digital elevation models. The K factor data (kwfact or Kw) from the SSURGO and U.S. Forest Service soils data spatial databases were combined with a slope grid using Natural Resources Conservation Service (USDA NRCS 1997) slope-soil relationships to create a classification grid divided into slight, moderate, severe and very severe erosion hazard ratings.

The map below is the categorized soil erodibility map for the Cache La Poudre Watershed.





The highest ranked watersheds based on soil erodibility are;

Black Hollow-Cache La Poudre River
Hague Creek
Hill Gulch-Cache La Poudre River