The flow test at the Chester hydroelectric plant was completed successfully last week, although high levels of aquatic vegetation moving through the system made for interesting work conditions at Chester and stressed the trash rake beyond its limit. Additionally, an issue with transmission lines also caused the Island Park Dam hydroelectric facility to trip off early on the morning of the 22nd. This caused a reduction in flow for about two hours, but fortunately the effect was greatly attenuated by the time dip reached Chester. The dip also arrived at Chester during the night and thus no effect was felt in the testing process.
The flow test used 4408 acre-feet of storage waters from Island Park Reservoir and resulted in a maximum flow at IP of 2000 cfs. The originally proposed test in June would have used 9166 acre-feet and resulted in a maximum flow at IP of 2400 cfs. If you were closely watching the gage during the test you would have seen a reading of 2380 cfs, but USGS calibrated the gage during the test and found that the correct reading was indeed the 2000 cfs planned for by the involved stakeholders. Aquatic plant growth at the Island Park gage can have a drastic impact on the gage reading and large discrepancies between actual flow and the gage reading can develop in a matter of hours. Please continue to keep this in mind as you reference the USGS site. Estimated flow through the Chester hydroelectric plant during testing was between 2412 and 2743 cfs.
HFF was able to monitor the Chester turbine test flow event through our established series of water quality monitoring sondes and through partnerships with Idaho Fish and Game and the Idaho Department of Environmental Quality. Monitoring efforts took place on both the Henry's Fork and in Island Park Reservoir. Look for future blogs and reports detailing our findings from this summer.