The Work Group identified an important risk factor for water uses related to transport of debris and sediment from upstream source water areas. The source water areas (i.e. watershed areas) above important surface water intakes, upstream diversion points and drinking water supply reservoirs that have a higher potential for contributing significant sediment or debris are called Zones of Concern. These zones also can be used by stakeholders to further define project areas that focus on watershed protection actions.

There were several methods suggested by the Front Range Watershed Protection Data Refinement Work Group (2009) to define Zones of Concern. The Saint Vrain Watershed Stakeholders agreed to use the five-mile upstream distance. This approach is based on Colorado State Statute 31-15-707 which allows municipal water providers to enact an ordinance to protect their water intakes within five miles upstream of their intakes. This municipal statute that has been in place since the late 1800's and has been tested in court several times and upheld.

During the third stakeholder meeting, the stakeholder group suggested that some Zones of Concern be extended to 11 miles upstream. An analysis of the Zones of Concern following that meeting found that if all the Zones of Concern were extended to 11 miles upstream, then most of the upper Saint Vrain Watershed would be covered with Zones of Concern. The result of using the 11 miles upstream criteria to determine Zones of Concern in this watershed would not be helpful to the stakeholders because the analysis would not discriminate critical areas. Therefore, the five mile upstream criteria for Zones of Concern was retained.

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.

Updates following Stakeholder Meeting #3

Changes to the Final Priority map were made due to the large percentage of red and orange watersheds. The distribution of the results for each analysis category were examined. While some of the hazard rankings were not evenly distributed, many of those analyses were reviewed and some were adjusted through the stakeholder review process. Therefore, the only adjustments to hazard ranking distributions were made to the Final Priority ranking.

The adjustments to the final ranking were to increase the size of the lowest category. This was done because the watersheds that are on the plains, east of the foothills, have very different hydrologic function and rank lower than most of the watersheds in the foothills and above. The adjustment resulted in a better distribution of hazard ranking within the foothills areas. It also reduced the hazard rankings of some of the highest elevation watersheds. The adjusted rankings are shown on the map below.

The forest conditions that are of concern for the watershed assessment are the wildfire hazard based on existing forest conditions. The wildfire hazard was modeled using the FlamMap model. You can read up more on FlamMap at http://www.firemodels.org/content/view/14/28/





The following map shows the Wildfire Hazard ranking for the Saint Vrain Watershed.





Pasted Graphic

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 was the U.S. Department of Agriculture - Natural Resources Conservation Service (NRCS) STATSGO soils data. STATSGO soils data is available for the entire watershed and is relatively coarse soils data, created at a scale of 1:250,000. SSURGO soils 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 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 STATSGO or SSURGO spatial databases was 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 above is the categorized soil erodibility map for the Saint Vrain Watershed. The results show that the area of greatest concern in regard to soil erosion is in the first set of watersheds in the foothills. The plains watersheds have lower slopes and the higher watersheds have less erodible soils.

The above map shows the watersheds in blue that have one or more drinking water uses due to diversions or reservoirs, as indicated in the SWAP database. The green watersheds have no identified drinking water uses.

The map was modified following the second stakeholder meeting to include the Middle Left Hand Creek watershed. There is a water intake just downstream of the watershed boundary. The stakeholders agreed to this change.