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Streamflow on the North Fork Cache La Poudre River, a tributary of the South Platte River in north-central Colorado, has been modified by impoundments for a century. A proposed expansion of the largest reservoir on the North Fork, Halligan Reservoir, presents an opportunity to modify dam operation to achieve environmental flows that sustain the river ecosystem while augmenting municipal water supplies. Over the past century, decreases in flood-related disturbances have resulted in reduced bed scouring through sediment transport and significant shifts in community composition and population structure of the dominant woody species present along the North Fork. We propose a four-step method to characterize environmental flows that maintain sediment mobility and riparian vegetation composition and structure. Our environmental flow standards explicitly address the fundamental role of sediment in creating and maintaining riparian habitat. Environmental flows to transport bedload are lower magnitude, higher frequency events (2 yr recurrence interval) that serve many in-channel functions, whereas environmental flows directed at riparian vegetation respond over longer time scales to high magnitude, lower frequency events (10 yr and 25 yr floods). Field evidence suggests the need for a 10 yr flood to saturate over-bank areas and exclude xeric species near the channel. Managed 25 yr floods serve as a target flow for generating canopy gaps, creating regenerative habitat, enhancing biogeochemical processes, maintaining habitat heterogeneity, and possibly disrupting the coarse bed-surface layer and scouring pools to maintain fish overwinter habitat within the North Fork. Where field evidence is lacking, selection of target flows can be guided by daily discharge exceedence values.

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