Impacts of Summer and Fall Storm Events on Fish Populations in the Western United States

The influence of summer and fall storm events on fish populations in the western United States can be highly variable and depends on a number of factors, including storm hydrology, habitat, and sediment and toxicity input characteristics.  Long-term monitoring of three streams in South Dakota, Colorado, and New Mexico provided the opportunity to evaluate impacts to fish populations from summer and fall storm events.  Fish sampling in 2010 in a small stream in the Black Hills, SD, revealed an almost complete removal of brook trout and brown trout from an approximate 3 km reach, following an intense summer storm event.  The storm event resulted in an approximate 140-fold increase in flow (from 3 to 425 cfs) in 35 minutes.  The confined nature of the stream channel in this reach likely amplified the effects of the increased flow, resulting in few refuge areas for fish.  A similar summer storm event near Victor, CO on August 2, 2010 resulted in a 120-fold increase in flow in Fourmile Creek (from approximately 30 to 3,620 cfs) in 50 minutes.  While brown trout biomass and density values in 2010 were less than in fall 2009, substantial numbers of brown trout were still present following this storm event.  Fourmile Creek has a much broader floodplain than the stream sampled in the Black Hills, which likely dampened the impacts of the storm event, despite the rapid increase in flow.  In summer 2007, an intense storm event occurred over a hydrothermal scar in the Red River, NM drainage.  Hydrothermal scars lack vegetation and consist of highly erosive and acidic material.  The summer 2007 storm event resulted in a landslide that deposited a large amount of sediment directly into the Red River, reducing habitat and water quality conditions.  In fall 2007, 13 resident trout were collected from 11 sites in an approximate 23 river km reach, compared to 691 from the same 11 sites in fall 2006.  The resident trout population in the Red River has since recovered to levels that are similar to those observed in years prior to the 2007 mudslide.  These case studies illustrate the variability in the impact of summer storm events on fish populations in the western United States, based on different storm hydrology, habitat, and external input characteristics.  Furthermore, these examples illustrate the importance of evaluating antecedent flow events when assessing the health of stream fish communities.