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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. 

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%).

Trophic State

National Lakes Assessment 2012 Bar Chart of the Trophic State of Lakes in the Western Mountains Ecoregion

Indicator with the Highest Percentage of Lakes in the Most Disturbed Condition: Nitrogen

National Lakes Assessment 2012 Bar Chart of the Condition of Total Nitrogen in the Western Mountains

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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.