Western Mountains Ecoregion - National Lakes Assessment Results

An estimated 14,057 lakes in the Western Mountains ecoregion are represented in the National Lakes Assessment (NLA). Of these lakes, 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 biological condition and key stressors in the nation’s lakes.

• In the Western Mountains, 4% of lakes are rated as hypereutrophic while 19% are eutrophic, 38% are mesotrophic and 39% are oligotrophic.
• The most widespread stressors assessed are nitrogen and phosphorus with 37% in poor condition for nitrogen and 30% in poor condition for phosphorus.
• Cyanobacteria are one-celled photosynthetic organisms that normally occur at low levels. Under eutrophic conditions, cyanobacteria can multiply. Not all cyanobacterial blooms are toxic, but some may release toxins, such as microcystins. For information about risks at specific locations, recreational water users should check with state, tribal or local governments.
  • Based on microcystins, detections occurred in 29% of lakes and no lakes are above the recreational benchmark.

Change from 2017*

• For the Western Mountains, the NLA reports that the percent of lakes in poor condition for atrazine detection increased by 19 percentage points.
• The percent of lakes that were in good condition for oxygen decreased by 12% between 2017 and 2022.

To access more ecoregional specific information, please visit the interactive NLA Dashboard.

* The change analysis is based on information from two points in time – 2017 and 2022.