Reservoirs tend to reduce high flows and increase low flows, which creates a more stable flow regime. In these regulated systems, communities of benthic macro invertebrates often have reduced biodiversity because the few species that thrive in the more stable flow conditions out compete all of the others. Flooding initiates a return to more natural conditions which encourages the community to rebound to its original biodiversity. Scientists maintain that the timing is not important, but the high flows should occur once every two years, on average.
HEC-EFM Relationship:
Season: 10/1 to 9/30
Duration: 1 day, Means (average) and then Maximum (high)
Rate of change: Not applied
Percent exceedance: 50% (2 yr) - Flow frequency
Hypothesis tracking: Increased flow will improve (+) benthic biodiversity
Geographical queries: Not applied
Comments. HEC-EFM relationships do not need to be complicated. The background basically says that benthic biodiversity is a function of high flows. So to build this relationship, use statistical queries that focus on high flows of short duration at any time in the water year. Choosing a duration is related to the flashiness of the river system being studied. For the demonstration project, a value of 1 day was chosen, though any duration between 1 and 7 days would likely be a good indicator.