The states of the Lower Colorado River Basin have traditionally played an oversized role in tapping the lifeline that supplies 40 million people in the West. California, Nevada and Arizona were quicker to build major canals and dams and negotiated a landmark deal that requires the Upper Basin to send predictable flows through the Grand Canyon, even during dry years. But with the federal government threatening unprecedented water cuts amid decades of drought and declining reservoirs, the Upper Basin states of Wyoming, Utah, Colorado and New Mexico are muscling up to protect their shares of an overallocated river whose average flows have already dropped 20% over the last century.