Montana Natural Heritage Program
 
KEY TO AQUATIC ECOLOGICAL SYSTEMS
 
 
 
 
 
2 
 Stream 1st-2nd order or <30 miles long  
2 
 Stream 3rd-5th order or 30 - 100 miles long  
2 
 River >5th order or >100 miles long   Medium Prairie River
2 
 River >5th order and >200 river miles long and a direct tributary to the Yellowstone or Missouri River (Little Missouri, Tongue, Bighorn and Powder Rivers)  
2 
Yellowstone River downstream from Billings or the Missouri River downstream from Great Falls
  
 
3 
Perennial 1st or 2nd order stream above 1000m elevation in the Wolf Mountains, Powder River Basin or Missouri Breaks
  
3 
Small (< 100m2 in area), low gradient intermittent pools separated by long breaks of dry stream bed, typically lacks downstream connectivity to a large mainstem river
  
.
4 
Stream 1st-2nd order or <30 miles long
  
4 
Stream 3rd-5th order or 30 - 100 miles long
  
4 
River >5th order or >100 miles long
  
4 
River >5th order and >200 miles long and a direct tributary to the Missouri River (Milk or Marias Rivers)
  
4 
The Missouri River downstream from Great Falls
  
.
5 
1st order, origin from Valley Floor groundwater output
  
5 
1st-2nd order or <30 miles long
  
5 
3rd-4th order or >30 and <60 miles long
  
5 
4th-5th order, >60 miles long and a direct tributary to the Yellowstone or Missouri River
  
 
6 
Cool-cold water tributaries that originate in the foothills and flow out to prairie rivers that are too warm to support coldwater fisheries
  
6 
Coldwater tributaries that flow to foothills and valley intermountain transitional rivers that maintain coldwater fisheries
  
 
7 
1st - 2nd order or <15 miles long
  
7 
3rd - 4th order or 15 - 60 miles long
  
7 
5th order or >60 river miles and a direct tributary of the Intermountain Transitional River
 
 
8 
1st or 2nd order, elevation >2400m, hydrology driven by alpine snowmelt
  
8 
1st order, elevation from 1500 to 2400m, hydrology fed by ground water discharge
 
-- BROWSE AQUATIC ECOSYSTEM LIST AND MAPS --
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