Downstream of a small canyon dotted with skinny ponderosa pines and incense cedar trees, rushing water cascades over large granite slabs. It flows around large boulders, pouring over round smooth rocks as it makes its way to the nearby Hetch Hetchy Reservoir. The sound is loud enough to drown out an encroaching thunderstorm.

This water, which began as snowmelt, will travel 167 miles over the next three days through a system dating back 92 years. It is this system that provides more than 2.5 million Bay Area residents with the majority of their drinking water.

Water has been a subject on a lot of people's minds lately. They've let their lush green lawns fade to brown, placed buckets in showers and opted to run the dishwasher only when completely full. But how often do people stop to think beyond their water bills and consider the long journey and feat of engineering it takes for water to reach their faucets?

"I think a lot of people probably take it for granted that when they turn on the faucet, water just appears," said Catherine Elvert, communications manager for the City of Palo Alto Utilities (CPAU) department. "A lot goes into getting that water supply and making sure that it's safe."

The journey for Palo Alto's water begins high up in the Sierra Nevadas in Yosemite National Park, where snow settles on mountains and glaciers at altitudes of 13,000 feet. In spring and summer about 12 percent of the watershed's melted snow and rainfall flows into three reservoirs: Hetch Hetchy, Cherry Lake and Lake Eleanor.

The picturesque Tuolumne River, admired for its whitewater rapids and a home for spawning Chinook salmon and steelhead trout, is the main conduit filling the Hetch Hetchy Reservoir. In fact, 60 percent of the river's flow is diverted for agriculture and urban water use in the Central Valley and the Bay Area.

Over the past four years, warm dry winters have caused rainfall and snowpack levels to drop drastically. Readings taken by the California Department of Water Resources on April 1 found that statewide peak snowpack levels were only at about 5 percent of average — a dismal 1.4 inches compared to 28.3 inches, the average since 1941.

One of the Tuolumne River's sources, the glacier at Mount Lyell, has shrunk by 6 to 8 feet in the past two years. According to the nonprofit Tuolumne River Trust, geologists anticipate the glacier could be gone in about five to 10 years if warm-weather patterns persist.

These historically low snowpack levels prompted Governor Jerry Brown to declare a drought state of emergency and issue a statewide mandate that water providers must cut their water consumption by about 25 percent from June 1, 2015, to February 28, 2016 — or face strict fines and penalties.

"The snowpack is really important; in a way, it's an additional reservoir," Peter Drekmeier, policy director of the Tuolumne River Trust, told the Palo Alto Weekly.

---

One of the factors that sets Palo Alto's drinking water apart is its quality. About 85 percent of the tap water, which is purchased and provided by the San Francisco Public Utilities Commission (SFPUC), is from the Hetch Hetchy Reservoir. Unlike water supplied from other reservoirs and the ground, Hetch Hetchy water is so pure that it doesn't need to be filtered.

On a popular trail that stretches around the Hetch Hetchy Reservoir, hikers can pose to take photos and view waterfalls like Wapama Falls, where crystal clear water plummets directly into the reservoir. This unique geology in Yosemite is what keeps the water free from sedimentation.

"It's really one of the best drinking supplies in the world," Drekmeier said.

Back in the late 19th century, engineers also saw the value of this pristine water source from the mountains. But it wasn't until the 1906 earthquake and great fire in San Francisco, which destroyed most of the city's water mains, and subsequent action by Congress that the O'Shaughnessy Dam could be built, creating the 117-billion-gallon Hetch Hetchy Reservoir.

The system can transfer more than 200 hundred million gallons of water per day to 26 different cities and water districts. It is entirely powered by gravity, a fact that allows it to create energy at two hydropower stations downstream: Kirkwood and Moccasin Powerhouse, which supply electricity to San Francisco's municipal buildings and the San Francisco International Airport.

Compare that power generation to the energy it takes to transport all other forms of water across California — nearly 20 percent of statewide energy use.

As the water flows from the Moccasin Powerhouse into the Central Valley, it travels underneath the Don Pedro Reservoir. Driving on State Route 120 to or from Yosemite, motorists can stop at a scenic viewpoint overlooking the man-made lake and stretch their legs. In wetter years, the bright blue water in the lake entices recreational boaters and water-sport enthusiasts.

This year the land is parched. Huge banks of dry land rise above the shallow water, their striations showing the heights the water used to reach. This 665-billion-gallon lake, mainly used for agriculture irrigation in Modesto and Turlock districts, is currently at only 40 percent capacity. Elsewhere in California, Lake Shasta reservoir is at 51 percent and Lake Oroville reservoir, 43 percent.

Compared to the growing number of low-level reservoirs in the state, Hetch Hetchy Reservoir's water level remains fairly stable. In early June it was at 91.8 percent of its capacity.

Steve Ritchie, assistant general manager for water for the SFPUC, said that there are many factors responsible for this comparatively good situation.

Since Yosemite reservoirs are relatively small, they tend to fill up multiple times in good years. The excess water is stored in local reservoirs closer to the Bay Area as well as in the Don Pedro Reservoir, for which the SFPUC has a water-bank agreement. This bank of water can be released in dry summer months when upstream levels are diminished.

But other factors are in play, too. In 2014 the SFPUC asked water districts to voluntarily reduce water use by 10 percent, and with the added pressure of the governor's drought mandates, residents are taking shorter showers and watering landscapes less frequently. So far in 2015, SFPUC's customers have used the least amount of water since 1977.

"Some of the high reservoir level is due to low demand, but mainly it's because we smartly manage our water and have the ability to bank water in advance with our downstream partners," Ritchie stated in an email.

Downstream, one such local source is the Calaveras Reservoir, which is currently part of the $4.6 billion Water System Improvement Program (WSIP) to seismically upgrade the entire system's infrastructure. In a valley in the Fremont hills, huge bulldozers are digging through layers of dirt and rock while crews laying cement are dwarfed by the 200-foot walls of a new earth-fill dam that's under construction. The project will restore the reservoir capacity to 31 billion gallons, providing Palo Alto with most of the 15 percent of its water supply. Today, the water in the reservoir is being held at 24 percent of total capacity due to the construction and the drought, which has dried up the local watershed that feeds into the reservoir.

---

Rising costs in Palo Altans' water bills in part are helping to cover the massive improvement project, which also includes a new tunnel underneath the bay, and upgraded Irvington Tunnel (which runs along two earthquake fault lines) and additional storage. People may grumble about higher bills, but Elvert said the security of a reliable water source is at stake.

"If you compare the cost of municipal tap water — which is incredibly pristine (and) high quality, regulated by the state and federal agencies — and if you compare that cost to bottled water, you're paying less than a tenth of a cent per gallon. It's a pretty good deal," she said.

Part of the cost also involves treating water to eliminate environmental contaminants. Hetch Hetchy water is first treated with UV light to kill any harmful bacteria or microorganisms at the Tesla Water Treatment Facility in Tracy. Then it is disinfected at a small treatment facility in Sunol. Any water that is stored in the Calaveras and San Antonio reservoirs must be treated separately from the direct Hetch Hetchy supply at the Sunol Valley Water Treatment Plant a few miles southwest of Calaveras.

Chris Nelson, water supply and treatment division manager for the plant, recently led a tour explaining how water from Calaveras and the nearby San Antonio Reservoir is treated.

He pointed to the tank with shimmering turquoise water that resembled a community swimming pool and explained that the first step is a process called flocculation, which adds a chemical to the water to force silt and sediment to fall to the bottom of the tank.

Next, the water is filtered and passes through tanks filled with minerals, sand and rocks, like a giant Brita filter, until it reaches its final destination, a large enclosed cement tub where chlorine and ammonia are added as disinfectants, along with fluoride.

Most people don't consider the measures taken every day to ensure that drinking water is safe to drink, Nelson said.

"I think everybody should backpack and learn how to treat water," he said. "They'll gain an appreciation of the energy that it requires to move water and the fact that you have to be careful about purifying your water. ... They'll learn how precious water is and how important it is to keep it pure."

After it is treated, the water is diverted to four large pipelines, where it either flows under the bay in the new Bay Tunnel or around San Jose as it heads north up the Peninsula toward San Francisco. It is here where Palo Alto intakes the SFPUC water at five locations around the city, receiving about 10 to 11 million gallons of water per day, depending on weather and the seasons.

At the Area 1 intake station off El Camino Real near University Avenue, senior water system operator Dave Cordova checked on a large blue pump in the far corner recently. A meter just to the left read "1828 gpm." The number revealed how well the pump was working to depressurize the water to a level more suitable for urban use, sending just under 2,000 gallons per minute to all the homes and businesses east of El Camino — Palo Alto's largest water service area.

---

More than 66,000 residents and tens of thousands of employees are served by Palo Alto's water utility. Last month CPAU officials announced that new restrictions must take effect to help meet the state's demand to reduce use by 24 percent of the baseline amount from 2013. This includes limiting landscape watering to twice a week, and only in early-morning or evening hours, and replacing defective plumbing or irrigation. If residents don't comply they could face fines of $100 a day following three prior complaints.

As of this past week, Palo Alto's water consumption has been reduced from the 2013 levels by a little more than 19 percent, just 5 percent shy of the state target. Though these water restrictions may be a short-term fix for a long-term problem of preserving water supplies in lean years, Elvert said it's still the best initial step for making change.

"Conservation is the first step; it's the easiest thing to do and it's the cheapest thing to do," she said. "And then you can look at alternative water sources ... which could be much more costly and involve much more."

In the future Palo Altans may consider alternative sources such as filtered waste water, diverted groundwater from the local creek watershed or desalinated water. But that may be a long ways away and for now Elvert said people should start by considering the source of the city's water and how it flows from 167 miles away straight to their taps.

"Our water supplies are a finite resource. ... The population continues to increase, yet our water supplies are finite," she said. "And so it's really important for people to understand that and do whatever they can to protect and preserve that."