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A total of 8,112 lakes in the Western Mountains ecoregion are represented in the 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%).

• Trophic State

  

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

  

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.

• Henry's Fork Lake in Idaho sampled during NLA 2012. Photo: Ben Brown.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).

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.