Peter, you're correct - There isnt a specific gage for the bypass reach below Shoshone HydroPlant. We rely on the upstream gage (Colorado River below Dotsero), and calculate based on whether the Xcel plant is operating at full capacity, or reduced capacity if an exchange with Denver Water is in place, for example.
Since Xcel runs two turbines at Shoshone, both ~ 700cfs, its fair to assume that flows in Barrel Springs are equal to Dotsero minus 1400cfs. Likewise, if the plant is only running one turbine, flows can be estimated as Dotsero minus ~ 700cfs. In some extreme cases, Shoshone may not be calling for any water...allowing for more storage upstream.
Before May 20th this year, Denver Water and Xcel did implement an exchange that relaxed the senior water rights at Shoshone, and reduced early season releases from headwater reservoirs on the Colorado River. This happens when snowpack and reservoir storage are below certain percentages, and allows front range water providers to capture water that would have been released/obligated to Shoshone.