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National Aquatic Resource Surveys

Western Mountains Ecoregion - National Lakes Assessment 2012 Results


Key Results

A total of 8,112 lakes in the Western Mountains ecoregion are represented in the National Lakes Assessment (NLA). Of these lakes, about 73% are natural and 27% are man-made.  

Trophic State and Key Stressors

The NLA uses trophic state as an important indicator of lake condition and assesses the extent of key stressors in the nation’s lakes.

  • In the Western Mountains, 2% of lakes are rated as most disturbed based on trophic state, 7% are eutrophic, 31% are mesotrophic, 59% are oligotrophic, and 1% are unassessed.
  • The most widespread stressors assessed are nitrogen (20% of lakes), phosphorus (17%) and lake drawdown exposure (10%).

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Photo of a lake in the Western Mountains ecoregion that was sampled during the National Lakes Assessment 2012Henry's Fork Lake in Utah sampled during NLA 2012. Photo: Ben Brown, Utah DEQ.

Human Use Condition

Cyanobacteria is one of three algal toxin related indicators presented in the NLA (see also chlorophyll a and microcystins). It serves as a proxy for the presence of algal toxins.

  • Based on cyanobacteria cell counts, less than 1% of lakes in the Western Mountains are in the most disturbed condition (i.e., pose a high risk of exposure to algal toxins).

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    Change from 2007 (for lakes >4 hectares)*

    • For the Western Mountains, the NLA reports a 21% decrease in the proportion of lakes in most disturbed condition between 2007 and 2012 for nitrogen.
    • There was also a decrease in the proportion of lakes in the most disturbed condition based on the following indicators: 
      • lake habitat complexity (24% decrease),
      • lakeshore disturbance (9% decrease), and
      • shallow water habitat (14% decrease). 
    • There was an increase of 2% in the proportion of lakes in the most disturbed condition for dissolved oxygen.

    To access more indicator graphics please visit the interactive NLA Dashboard.

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    * The change analysis is based on information from two points in time – 2007 and 2012.  The change in condition analysis only considers lakes 4 hectares and larger because smaller lakes were not sampled in 2007.