Since Dwight Eisenhower was president, tiny Middle Park Water Conservancy District has hoarded a precious gem: 20,000 acre-feet of water rights on Troublesome Creek, near Kremmling, and the authority to build a dam for it.  In October, Middle Park gave its treasure to a private rancher. For $10.   The Middle Park district, which primarily serves ranchers and hay growers in Grand and Summit counties, has only a few hundred thousand dollars of revenue each year, and no ability to raise potentially tens of millions of dollars for environmental permitting and hundreds of millions for construction, the district’s attorney said.  The private buyer, Circle C Ranch Kremmling LLC, owns the property on Colorado River tributary fork East Troublesome Creek northeast of Kremmling, where Middle Park had been planning a dam for decades.