Table 1. Summary of the five environmental flow components (EFCs) used in the Indicators of Hydrologic Alteration software, and their ecosystem influences.

     
EFC type Hydrologic parameters   Ecosystem influences
1. Monthly low flows Mean or median values of low flows during each calendar month   Provide adequate habitat for aquatic organisms
 
Maintain suitable water temperatures, dissolved oxygen, and water chemistry
 
Maintain water table levels in floodplain, soil moisture for plants
 
Provide drinking water for terrestrial animals
 
Keep fish and amphibian eggs suspended
 
Enable fish to move to feeding and spawning areas
 
Support hyporheic organisms living in saturated sediments
 
Subtotal 12 parameters
       
2. Extreme low flows Frequency of extreme low flows during each water year or season
 
Mean or median values of extreme low flow event:
 
Duration (d)
 
Magnitude (minimum flow during event)
 
Timing (Julian date of peak flow)
 
  Enable recruitment of certain floodplain plant species
 
Purge invasive, introduced species from aquatic and riparian communities
 
Concentrate prey into limited areas to benefit predators
Subtotal 4 parameters
       
3. High flow pulses Frequency of high flow pulses during each water year or season
 
Mean or median values of high flow pulse event:
 
Duration (d)
 
Peak flow (maximum flow during event)
 
Timing (Julian date of peak flow)
 
Rise and fall rates
  Shape physical character of river channel, including pools, riffles
 
Determine size of streambed substrates, e.g., sand, gravel, cobble
 
Prevent riparian vegetation from encroaching into channel
 
Restore normal water quality conditions after prolonged low flows, flushing away waste products, and pollutants
 
Aerate eggs in spawning gravels, prevent siltation
 
Maintain suitable salinity conditions in estuaries
 
Subtotal 6 parameters
       
4. Small floods, i.e., 2-10 yr events Frequency of small floods
 
Mean or median values of small flood event:
 
Duration (d)
 
Peak flow (maximum flow during event)
 
Timing (Julian date of peak flow)
 
Rise and fall rates
  May apply to small or large floods:
 
Provide migration and spawning cues for fish
 
Trigger new phase in life cycle, i.e., insects
 
Enable fish to spawn in floodplain, provide nursery area for juvenile fish
 
Provide new feeding opportunities for fish, waterfowl
 
Recharge floodplain water table
 
Maintain diversity in floodplain forest types through prolonged inundation, i.e., different plant species have different tolerances
 
Control distribution and abundance of plants on floodplain
 
Deposit nutrients on floodplain
 
Subtotal 6 parameters
       
5. Large floods, i.e., >10-yr events Frequency of large floods
 
Mean or median values of large flood event:
 
Duration (d)
 
Peak flow (maximum flow during event)
 
Timing (Julian date of peak flow)
 
Rise and fall rates
  May apply to small or large floods:
 
Maintain balance of species in aquatic and riparian communities
 
Create sites for recruitment of colonizing plants
 
Shape physical habitats of floodplain
 
Deposit gravel and cobbles in spawning areas
 
Flush organic materials such as food and woody debris such as habitat structures into channel
 
Purge invasive, introduced species from aquatic and riparian communities
 
Disburse seeds and fruits of riparian plants
 
Drive lateral movement of river channel, forming new habitats, e.g., secondary channels, oxbow lakes
 
Provide plant seedlings with prolonged access to soil moisture
 
Subtotal 6 parameters
 
Grand total 34 parameters