As some of you have heard, flow in the Henry’s Fork at St. Anthony dropped suddenly at around 4 p.m. on Thursday, October 23rd. Flow dropped from 1050 cfs to 622 cfs over a one-hour-period and then increased to 1080 cfs over the next 45 minutes. The river bottom was briefly exposed in shallow areas along the banks and in side channels. We received a report that some small fish were stranded and that brown trout redds were exposed. The event was short enough that gravel in the redds most likely remained saturated.
Although we are still analyzing data, it appears that the flow disruption resulted from re-inflation the rubber dam at the Chester hydroelectric plant. The dam had been deflated to lower the level in the backwater about one foot to allow maintenance. We are communicating with Fall River Rural Electric Cooperative and visited the plant yesterday to get a firsthand view of the control and data-recording instruments. Fall River is extremely concerned about the potential for operations at the plant to affect river flows downstream and is working with HFF and agencies to review data recorded at the plant—as well as operations procedures—to understand exactly what happened and to prevent future flow disruptions.