by Joe Kozfkay, Regional Fisheries Manager
On Tuesday evening and Wednesday morning, February 3rd and 4th 2015, a series of unfortunate occurrences at east Boise’s Barber Dam (owned by Ada County and operated by Enel Green Power) caused Boise River flows to drop to zero for a period of seven hours.
Maintenance activities are being performed on the dam’s spillway, and to ensure construction crew safety, Barber Pool is being held at less than full pool levels, with all river flows being routed through one turbine. Unfortunately, the gate that allowed water to reach this turbine closed Tuesday night and alarm systems failed. Upstream flows were then caught in Barber Pool, until the point where the pool was refilled and additional water began cresting the dam’s spillway.
During the seven-hour period of zero cubic feet per second river flow over or through Barber Dam, several miles of the Boise River were de-watered or did not receive the “normal” wintertime minimum flow of 240 cubic feet of water per second. Very few people saw the river in this condition as it occurred in the middle of the night. From a fish habitat standpoint, riffles and run habitats were likely severely de-watered, whereas pool habitat likely retained water. Farther downstream areas were less impacted as infiltration of groundwater re-wetted the river channel.
Winter is a critical period for many aquatic species. Dramatic river flow reductions can have negative impacts on aquatic organisms such as fish and invertebrates. Fish survive the winter by reducing activity levels and seeking habitat where they can avoid expending energy while at the same time avoiding predators. This is especially true for young trout. The Boise River possesses wild, spring-spawning rainbow trout and wild, fall-spawning brown trout. Young rainbow trout reside on the river’ edges, usually near downed wood or other cover. A rapid drop in river level may force young rainbow trout to seek alternative cover and become susceptible to predators. It may also cause stranding and death.
For the most part, brown trout spawn throughout the month of November. Boise River water temperatures result in an approximate 60-day brown trout egg incubation period. In early February, young brown trout are just beginning to hatch or have just recently done so. At this critical life stage, water level drops are known to cause brown trout mortality. Other fish species also reside in riffles and near shore areas and may have been affected by the dramatic drop in river flow, including sculpin, dace, and whitefish.
Idaho Fish and Game crews conducted visual surveys of this river segment on Wednesday afternoon, February 4th. No dead adult fish were observed, which was a positive sign. Measurement of potential impacts to young trout and other species are only beginning and will be much more difficult to determine. Crews are at the river today, sampling riffle and river margin areas in the de-watered area and in downstream reaches to make a relative comparison. This should provide some information on possible impacts of the de-watering event.
Other types of suspected impacts may not be readily measured, especially for young trout, non-game fish, or invertebrate numbers and species. Based on the results of these and subsequent surveys, Fish and Game staff will determine whether appropriate mitigation should be pursued from responsible parties.