The little minnow is a threatened species of fish indigenous only to the Rolling River basin. Little minnow populations have declined (in magnitude and range) as dams and diversions limited access to upstream river stretches and as floodplain areas, critical to little minnow spawning, were transitioned to agricultural lands. Little minnow spawn in shallow (no deeper than 3 ft), vegetated floodplain areas between February and May. Eggs require sustained high flows for approximately 21 to 28 days before hatching. Little minnow reach sexual maturity in their first or second year and have a lifespan of approximately 6 years. Scientists suggest using the conditions within each spawning season that generate the largest extent of effective spawning habitat as an indicator of success for each year’s spawn. Further, scientists suggest that good spawning conditions do not need to occur every year – it would be sufficient if there were good conditions in 25% of years, so that, on average, each little minnow would have a chance to spawn in their lifespan.
HEC-EFM Relationship:
Season: 2/1 to 5/31
Duration: 24 days, Minimums (sustained highs) and then Maximum (largest extent)
Rate of change: Not applied
Percent exceedance: 25% (4 yr) - Flow frequency
Hypothesis tracking: Increased flow will improve (+) floodplain spawning
Geographical queries: Depth (0 to 3 ft) and vegetation (aquatic plants)
Comments. For the little minnow, successful spawning depends on having a sustained inundation long enough for the eggs to incubate. These types of scenarios, where any interruption will affect the ecosystem dynamic being modeled, typically use an initial statistical setting of Minimums or Maximums for the Duration query. Minimums return a flow that will be that high or higher for the length of the duration. Maximums return a flow that will be that low or lower. This relationship also thinks in terms of percentage of years. The Percent exceedance - Flow frequency option returns a flow and stage that are equaled or exceeded in the percent of years specified by the user. The Percent exceedance - Flow duration option returns a flow and stage that are equaled or exceeded in the percent of time specified by the user.