For years, Colorado River states have been negotiating a plan to avoid the worst – a shortage in Lake Mead so bad it could trigger unprecedented cutbacks. With the region experiencing drought conditions since 2000, even California, which has senior rights, came to the negotiating table. State officials said they were willing to voluntarily reduce Colorado River allocations to keep water levels in Lake Mead – the reservoir that holds water behind Hoover Dam for Arizona, California, Nevada and Mexico – from slipping below a critical threshold.