Bighorn River Spring Flush: March 24th, 2015. Here at Dunoir Fishing Adventures we have been receiving calls inquiring about if/when there will be a spring flush on the Bighorn River. We talked to the Bureau of Reclamation Office today, and they are tentatively planning on flushing the Bighorn River in Thermopolis starting on March 24th. Fishing can be great following the flush, but can be a challenge during the actual flush. Fish do get scattered, but if you know where to look for them your odds of catching a few big fish are good. Should this date change on the flush we will let you know immediately, so keep posted.