DENVER -- Oil companies have gained control over billions of gallons of water from Western rivers in preparation for future efforts to extract oil from shale deposits under the Rocky Mountains, according to a new report by an environmental group that opposes such projects.

The group, Western Resource Advocates, used public records to conclude that energy companies are collectively entitled to divert more than 6.5 billion gallons of water a day during peak river flows. The companies also hold rights to store, in dozens of reservoirs,...