As the severe drought in the US southwest continues for a third straight year, water levels in the area's lakes and reservoirs are reaching historic lows.

Lake Mead, the largest reservoir in the United States, is at its lowest level since the dam was complete and the lake first was filled in the 1930s. 

california drought before and after
Top: Boats are moored at the Lake Mead Marina on 25 July, 2007. Bottom: The area where the Lake Mead Marina was once located is seen on 17 July, 2014. North America's largest man-made reservoir has dropped below 1,082 feet above sea level, the lowest it's been since the Hoover Dam was built in the 1930s(Ethan Miller/Getty Images)

The effect of increased demand and diminished supply is visible on Lake Mead's canyon walls. A white mineral band often compared to a bathtub ring marks the depleted water level.

california drought before and after
The Desert Princess paddle wheeler sails past mineral-stained rocks on the upstream side of the Hoover Dam in the Lake Mead National Recreation Area, Arizona. A white "bathtub ring" of mineral deposits has been left by higher water levels on the rocks around the lake as high as 130 feet(Ethan Miller/Getty Images)

California's Folsom Lake is currently at 40% of its total capacity, while Lake Oroville is at just 32%.

Large areas of the US have been subjected to drought and above average temperatures since February 2012, resulting in weather patterns not seen since 1988, 1954, and the Dust Bowl and Great Depression years of the 1930s.

These before and after photos show the effects of drought on  Lake Oroville, the second largest reservoir in California.

Top: The Green Bridge passes over full water levels at a section of Lake Oroville near the Bidwell Marina on 20 July, 2011. Bottom: The Enterprise Bridge passes over a section of Lake Oroville that is nearly dry on 19 August, 2014. Lake Oroville is currently at 32% of its total capacity
Top: The Green Bridge passes over full water levels at a section of Lake Oroville near the Bidwell Marina on 20 July, 2011. Bottom: The Enterprise Bridge passes over a section of Lake Oroville that is nearly dry on 19 August, 2014. Lake Oroville is currently at 32% of its total capacity(Paul Hames/California Department of Water Resources via Getty Images) (Justin Sullican/Getty Images)
Top: The Enterprise Bridge passes over full water levels at a section of Lake Oroville on 20 July, 2011. Bottom: The Enterprise Bridge passes over a section of Lake Oroville that is nearly dry on 19 August, 2014
Top: The Enterprise Bridge passes over full water levels at a section of Lake Oroville on 20 July, 2011. Bottom: The Enterprise Bridge passes over a section of Lake Oroville that is nearly dry on 19 August, 2014(Paul Hames/California Department of Water Resources via Getty Images) (Justin Sullican/Getty Images)
Top: The Bidwell Marina at Lake Oroville on on 20 July, 2011. Bottom: The Bidwell Marina at Lake Oroville on 19 August, 2014
Top: The Bidwell Marina at Lake Oroville on on 20 July, 2011. Bottom: The Bidwell Marina at Lake Oroville on 19 August, 2014(Paul Hames/California Department of Water Resources via Getty Images) (Justin Sullican/Getty Images)

The drought has resulted in agriculture-related losses of about $2.2bn this year in California. It also has fuelled wildfires in several parts of the state, destroying homes and swathes of land.

California's popular lake and river tourism industry is suffering from the effects of the drought. Boat ramps at marinas that were once teeming are now hundreds of metres from the water.

Rings showing the dropping water levels are visible on the banks of Lake Oroville
Rings showing the dropping water levels are visible on the banks of Lake Oroville(Justin Sullivan/Getty Images)
Boaters launch their boats hundreds of yards away from designated boat ramps at Folsom Lake
Boaters launch their boats hundreds of yards away from designated boat ramps at Folsom Lake(Justin Sullivan/Getty Images)

Lake Mead is expected to drop to 1,080 feet above sea level this year, down from a high of 1,225 feet in 1983. A projected level of 1,075 feet in January 2016 would trigger cuts in water deliveries to Arizona and Nevada. At 1,000 feet, drinking water intakes would go dry to Las Vegas, a city of two million residents that is almost completely dependent on the reservoir.

The region is stressing water conservation, prohibiting grass lawns for new homes and fountains at businesses. Officials say the overall effort has reduced consumption 33% in recent years even while the Las Vegas area added 400,000 residents.

If cuts do come, they'll test the agreements forged in recent years between big, growing cities and farmers. In California, home to 38 million residents, farmers in the sparsely populated Imperial Valley in the southeast have water rights ensuring that they get water regardless of the condition.

In Arizona, reduced deliveries of Colorado River water would largely affect the Central Arizona Project, which manages canals supplying 80% of the state's population. A tiered system means farmers would face cuts first, shielding Native American tribes and big cities.