The forest conditions that are of concern are the wildfire hazard based on existing forest conditions.

The wildfire hazard was determined using the FlamMap model (http://www.firemodels.org/content/view/14/28/). The basic data was derived from the LANDFIRE (http://www.landfire.gov/) database updated for bark beetle mortality conditions using USDA Forest Service, Rocky Mountain Region Aerial Detection Survey Data (http://www.fs.fed.us/r2/resources/fhm/aerialsurvey/). The FlamMap model results are shown on the map below.



The FlamMap analysis results in four categories of wildfire hazard ranging from lowest (Category 1) to highest (Category 4). Sixth-level watersheds were then rated for wildfire hazard based on the percentage of their area in the highest two categories. The results were categorized by sixth-level watershed into five categories that are used throughout the analysis.

The Wildfire Hazard Map for the Big Thompson Watershed is shown below.





The highest ranked watersheds based on wildfire hazard are;

Miller Fork
Upper Buckhorn Creek
Headwaters Little Thompson River
Wind River
West Fork Little Thompson River

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 Big Thompson watershed. Black Canyon Creek is the highest rated watershed based on ruggedness. The patterns displayed on this map are confirmed by the slope map.

The highest ranked watershed based on ruggedness are:

Black Canyon Creek
Wind River
Fall River
Headwaters North Fork Thompson River
Glacier Creek
West Creek
Miller Fork

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 Big Thompson watershed.



The map above is the categorized Flooding/Debris Flow Hazard for the Big Thompson 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;

Wind River
Black Canyon Creek

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 Big Thompson Watershed is shown below.



The highest ranked watersheds based on composite hazard are;

Wind River
Fall River
Miller Fork
Black Canyon Creek
Glacier Creek

Similar to the Saint Vrain Watershed, the Big Thompson Watershed runs from high mountain peaks out onto the plains. The plains watersheds tend to skew the results of the assessment because they are so different. The following watersheds were eliminated from the assessment area:


The map showing the 5th level watersheds is shown below. The list of watersheds and areas is available on the project files page.