While boating and fishing are permitted, contact with the water is prohibited because Lake Hennessey is part of a municipal water supply. It is a reservoir where some drinking water for the city of Napa comes from spring through fall, especially in drought (low rainfall) years. Unlike Lake Berryessa, which is a reservoir that doubles as a recreation lake and municipal water supply for the cities of Vacaville, Fairfield, Suisun, and Vallejo, Lake Hennessey is not large enough to dilute the impact of contaminants. This is why people and pets are told to stay out of the water.
Much of the time, the water that comes from the tap in the cities of Napa, American Canyon, and Calistoga is from the Sacramento River/San Joaquin River Delta. Like many areas, Napa receives an annual allocation (measured amount) of water from the State Water Project. This water comes from snowmelt in the Sierra Nevada Mountains and rainfall in the Sacramento River watershed. The water for the city of Napa arrives at the Jamieson Canyon Water Treatment Plant via the North Bay Aqueduct, a 27-mile underground pipeline that brings water from the Delta into Napa. The State Water Project stores water in reservoirs all over California, releasing water as needed to flow downriver for people to use. Much of the water we receive is stored up north in Lake Oroville and released to flow down the Sacramento River into the Sacramento/San Joaquin River Delta. When the water reaches the Delta, it is pumped from the Baker Slough tributary in Solano County to the North Bay Aqueduct for delivery to the Jamieson Canyon Water Treatment Plant.
When the water arrives in Jamieson Canyon for treatment, it can be very murky and full of organic particles from the delta. The water can be dark brown and during storms might even have an unpleasant smell from all the dirt, grit, and other particles it picks up along the way. To make the water drinkable, it goes through a multistage treatment process to make it clear and clean.
Elements are added to the water to help remove contaminants, using the power of science to separate larger particles out of the clean water. Flocculants, like alum, bond to particles to make them heavy enough to sink to the bottom. Coagulants neutralize the charges on particles so they can settle to the bottom instead of bouncing around in the water.
Science also neutralizes waterborne pathogens (harmful viruses and bacteria) so they are no longer harmful. At the Jamieson Canyon Treatment Plant, ozone is used to neutralize pathogens. Ozone, or O3, is made up of three oxygen atoms weakly bound together. The third oxygen atom is a free radical- it easily detaches from its weak bond and bounces around in the water, re-attaching to contaminants, viruses, and bacteria to oxidize and render them harmless. Oxidation is important because it breaks down the cell walls and functions of pathogens so they can no longer duplicate or cause harm. Ozone is a common way to treat water because oxygen is plentiful in supply and it does not add any extra chemicals to the water; once the free radical atom attaches to and oxidizes the other particles, the two remaining oxygen atoms become O2, the oxygen we breathe.
Not all water treatment plants use ozone; UV light also has an oxidizing effect and is used to neutralize pathogens in some facilities. In an important polishing step, fine filters are used to trap any remaining contaminants before the water heads off to businesses and homes. The final stage of treatment involves the addition of certain chemicals, like chlorine, to keep the water safe for drinking as it travels through miles of pipeline before coming out the tap.
In drought years, the State Water Project reduces the amount of water it allocates (gives) to cities because there is less water to be released from its reservoirs. The City of Napa maintains two open water reservoirs, Lake Hennessey and Milliken Reservoir, to ensure that the residents always have enough water. When the snowpack is low in the mountains, less water flows in the Sacramento and San Joaquin Rivers. Low flow in these rivers means less water for communities who receive water from the Delta. When water allocations are reduced due to statewide water shortages, the City of Napa can draw upon their own reservoirs to make up for the reduction in water supply received from the Delta. These reservoirs provide several years of backup water supply for the residents of Napa, and make it a very water secure community. Even in severe drought years, the residents are able to access the water they need, thanks to these reservoirs.
Although we have access to the water we need piped into our homes, not all of the reservoirs in the Napa River Watershed are open to the public. Most of the reservoirs in Napa County are on private property and inaccessible. This includes Calistoga's Kimball Reservoir, St. Helena's Bell Canyon Reservoir, Yountville's Rector Reservoir, and Napa's Milliken Reservoir. Keeping these reservoirs safe and secure helps maintain reliable water sources, especially for cities who use their reservoirs as primary water supplies. Lake Hennessey is the only reservoir in the Napa River Watershed that is open to the public for hiking, fishing, and boating. You can kayak, canoe, or operate a small fishing boat from the boat launch ramp off Sage Canyon Road, east of Rutherford. Some of the fish species commonly caught include Bass, Trout, and Bluegill. Miles of hiking and mountain bike trails are accessible from Moore Creek Park, off of Chiles Pope Valley Road. Mountain bikers enjoy specially designed trails such as a Whiskey Ridge, Conn Peak, Sam the Eagle, and Old Man’s Beard. Or you can hike up Moore Creek Trail to the “Secret Swimming Holes” to cool off on a warm day. The park's Shoreline Trail features bird signage provided by the Audubon Society and is popular among birders. As a regular stop along the Pacific Flyway, you may spot a Canadian Goose, Mallard, Grebe, Kinglet, or Sandpiper taking a break from their migrations in the winter months.
Reservoirs play an important role in the health of our watershed and its inhabitants by providing recreation and water security in times of changing climate. If you get a chance to visit Lake Hennessey, take a moment to marvel at our water supply and appreciate the beautiful space it creates for people to enjoy our watershed!
Check us out! Books and resources available from the Napa County Library:
1). Drip! Drop! How Water Gets to Your Tap by Barbara Seuling
2). Water Isn't Wasted! How Does Water Become Safe to Drink by Riley Flynn
3). Water for All by Sally Morgan
4). The Magic School Bus: At the Waterworks by Joanna Cole
5). How Long is the Water Cycle? by Emily Hudd
Go deeper! Explore these great resources online:
City of American Canyon Source to Tap Video
Scientific American Flocculants At Home Science Experiment
Moore Creek Park Map & Information
Drone Footage of Lake Hennessey
CalFish Species of Fish in Lake Hennessey
ebird Bird Species Sighting Graph