Follow Up to Fall-Winter River Flows and Drought Management Planning Update

On September 19, we posted information on fall and winter management of Island Park Reservoir indicating that outflow out of Island Park Reservoir would be gradually reduced to about 60 cfs by mid-October to allow mid-winter outflows of about 120 cfs ( One of our attentive members brought it to our attention that some readers may not realize that the Buffalo River contributes a substantial amount of flow to the Henry’s Fork just 600 yards downstream of the dam, so the resulting flow in the river through Box Canyon, Last Chance, and Harriman State Park is much higher than the dam outflow itself.


For example, on September 26, outflow from the dam was 240 cfs, and flow in the Buffalo River was 192 cfs, producing a total flow of 432 cfs through Box Canyon, nearly twice what was being released from the dam and only 32% less flow than would be in the river if Henry’s Lake and Island Park dams were not even there. We project that in mid-October, outflow from the dam will be 60 cfs, and flow in the Buffalo River will be 190 cfs, for a total of 250 cfs. In the middle of the winter, we anticipate outflow from the dam to be 120 cfs, plus about 190 cfs from the Buffalo River, for a total of 310 cfs through Box Canyon, Last Chance, and Harriman. While not as high as we would all like to see, 310 cfs is over 2.5 times greater than the dam outflow alone and is a more relevant value to report than simply the dam outflow.


We thank our members for their active engagement in our work and this particular member for a great suggestion on how to present flow information. We’ll continue to add Buffalo River flow information in our Island Park flow reports.