A fourth set of human remains has been found at the shrinking reservoir of Lake Mead as the drought gripping the western US continues to blaze and sends its water levels plunging. The lake is now at its lowest level for more than 80 years, posing a dire threat to a water source that is crucial for 25 million people. The dry spell has dried out its tributaries, threatened key hydropower production and hurt tourism at what is a popular recreation site for many hundreds of miles.