Authorities involved in the negotiations with dam owner PacifiCorp agreed to the deadline being put back from 30 June to September.
PacifiCorp and the authorities - DoI, and the states of Oregon and California - reached an Agreement in Principle last November, and it has been joined by 22 other stakeholders.
Negotiations cover the tandem conclusion of two agreements covering, respectively, restoration of the river basin and hydro power generation.
The Agreement in Principle opened the way for the possible removal of four dams - JC Boyle, Copco 1, Copco 2 and Iron Gate - by 2020.
However, when the deal was announced last November it was said that further studies, to 2012, were required to assess the cost-benefits. There was also a cap on state costs set at US$450M.
Following the removal of the dams, it was envisaged that approximately US$1B would have to be invested under the Klamath Basin Restoration Agreement.
