Frequently Asked Questions

A backpacker admires Lake Mead at a distance

Healthy Lakes – Healthy Lives

This Healthy Lakes – Healthy Lives synthesis is presented as a collection of common questions and concerns about the health of the Lakes - both as water sources and as ecosystems. It considers a number of resources and data to provide information about the condition of the ecosystem within a set of measures that represent, or indicate, its overall state. As managers continue to be challenged by a growing community, a changing climate and a dynamic ecosystem, it is more important than ever to make sure citizens understand the lakes and how their health is so closely linked to their lives.


Common Questions

  • Is it safe to swim in lakes Mead and Mohave?

    Is it safe to swim in lakes Mead and Mohave?

    Yes. Lakes Mead and Mohave are extensively monitored for indicators related to human health and the water is consistently found to be within the good range. Occasionally, following a heavy rainstorm or when someone creates an unsanitary condition on the shoreline (such as a dirty diaper or pet waste), unhealthy levels of bacteria may be present in localized waters for a few days. Like many areas in the country, lakes Mead and Mohave have seen increasing amounts of blue-green algae, which under certain conditions can produce algal toxins and be a health hazard. The National Park Service maintains a program to assess algae conditions and the presence of algal toxins, and to test for bacteria, so they can inform the public should unsafe conditions exist.

    A wake boarder in Lake Mead

    Swimming in Lake Mead can be an enjoyable experience for the whole family.

  • What is the concern with cyanobacteria blooms?

    What is the concern with cyanobacteria blooms?

    Algae growth is a healthy part of a lake’s ecosystem and provides food for many of the smaller fish in the lake. Cyanobacteria, also known as blue-green algae, are a particular type of algae that can grow quickly when temperatures and nutrients are just right and create an algal bloom. This particular type of algae, under certain conditions, can produce toxins that are harmful to humans and pets. Drinking untreated water can lead to stomach illnesses; swimming in water containing these toxins can cause skin irritations. To be safe, you should avoid areas with high levels of algae, avoid green algae growth in shallow coves and watch for any public warnings released about algae in the area. Pets or small children, who may drink the water as they play, are more susceptible to algal toxins; do not allow your pets or small children to play where algae is noticeable.

    Algal Bloom

    Algal blooms can occur anywhere on the lake if conditions are right.

  • Can I eat the fish from lakes Mead and Mohave?

    Can I eat the fish from lakes Mead and Mohave?

    Yes. Fish from lakes Mead and Mohave have been sampled for heavy metal and mercury concentrations since 2002. During a Lake Mead sampling conducted in 2008, researchers found unsatisfactory mercury concentrations in only 10 fish out of 221 sampled. Acting conservatively to protect human health, the State of Nevada has issued fish consumption advisories for the major sportfish species in lakes Mead and Mohave. Keeping with standard advisories for fish, such as tuna for example, these advisories set recommended limits on the number of meals and ounces that should be consumed per month. After obtaining a fishing license, check advisories from the Nevada Department of Wildlife before heading out for a day of fishing.

    Fishing for large mouth bass

    Fishing on Lake Mead is a great way to enjoy the lake and bring home dinner.

  • Why is the lake so blue?

    Why is the lake so blue?

    Many factors affect a lake’s color such as water depth, clarity and chemical makeup. Lake Mead is unique for many reasons – one of which is that it is very deep. The unusual depth is a result of the series of desert canyons that dotted the landscape prior to being filled with water. When Hoover Dam was built, the canyons began to fill water from the Colorado River, creating Lake Mead. This depth, coupled with very clear water, sunny skies and lots of dissolved minerals, gives the lake its signature blue color.

    Lake Mead is an extremely clear lake. The clarity of water, widely considered a simple indicator of its level of productivity, is based on the amount of algae that can grow given the nutrients present. Key nutrients, such as phosphorus and nitrogen resulting from organic materials (plants and animals) within the watershed, are needed to produce algae for healthy fish and wildlife. An overabundance of nutrients, usually from treated wastewater or agricultural fertilizers within the lake's watershed, can result in the growth of nuisance algae. Lakes Mead and Mohave are considered to have algae productivity within the lower to middle-range of lakes overall, resulting in noticeable water clarity that can be appreciated by many visitors.

    Lake Mead

    Lake Mead is known for its signature blue color.

  • Is Lake Mead a safe drinking water source?

    Is Lake Mead a safe drinking water source?

    Yes. Lake Mead has provided safe drinking water for Southern Nevada residents since the 1930’s. Lake Mead provides an outstanding water source consisting of over 97% Colorado River water which has flowed over 700 miles throughout the undeveloped lands of seven national parks, including 277 miles of the Grand Canyon National Park. The Southern Nevada Water Authority utilizes state of the art water treatment technology that employs both chlorine and ozone for disinfection. While chlorine has been a common component in water treatment for years, ozone is a more recent addition to the disinfection lineup. With the power to kill off microscopic organisms more effectively – this chemical superhero has added a level of safety to our water supply that many other cities don’t have.

    In addition to routinely monitoring for human pathogens, Lake Mead has also been widely monitored over the past twenty years for the presence of contaminants such as pesticides, heavy metals, and pharmaceuticals. While these contaminants can be detected at very low levels due to highly specialized testing equipment, Lake Mead has an extremely low level of such contaminants compared to other bodies of water located adjacent to an urban area. Treated drinking water from Lake Mead meets or surpasses all local, state and national standards for drinking water. Southern Nevada’s water management practices are known, observed and studied by other municipalities throughout the world.

    Safe Drinking Water

    Drinking water from Lake Mead meets or exceeds all health standards.

  • What fish or wildlife might I see in or along the lakes?

    What fish or wildlife might I see in or along the lakes?

    Lakes Mead and Mohave are home to at least 15 different fish species. Most of the species now present in the lakes were introduced and not native to the Colorado River ecosystem, although a few native species still persist in small numbers. The population estimate for the native and federally listed endangered razorback sucker in Lake Mohave is approximately 3,000 adult fish. In Lake Mead, the population is approximately 400. Fish more likely to be encountered by visitors include those fish primarily sought by anglers, including striped bass, largemouth bass, smallmouth bass and channel catfish. Visitors strolling the marinas and docks will see large numbers of common carp congregated near the docks.

    The most common aquatic bird on either lake is the American coot. Clark's/Western grebes, with their white necks, are frequently seen on Lake Mead; double-crested cormorants are frequently seen in Black Canyon, below Hoover Dam. During spring migration, vast rafts of eared grebes, estimated at over 16,000 individuals, have been seen in upper Lake Mead. Mud-flats near tributaries, such as where Las Vegas Wash enters Lake Mead, contain shorebirds such as ibis, avocets and sandpipers. Visitors to high use areas near marina and beaches will see numbers of ring-billed and California gulls. Great blue herons are year‐round residents and a fairly common sighting along the shores of both lakes. Peregrine falcons may be seen along steep-walled canyons overlooking lake waters, and bald eagles use both lakes for winter habitat. Over 109 species of aquatic and aquatic-dependent birds have been documented on lakes Mead and Mohave.

    The two mammals most frequently seen along the shoreline are desert bighorn sheep and coyotes. Bighorn sheep are frequently seen in areas within the Black Canyon below Hoover Dam.

    A Bighorn Sheep
  • What contaminants are in the lakes?

    What contaminants are in the lakes?

    Water in lakes Mead and Mohave is highly suitable for recreation, and is not uniquely contaminated compared to other water bodies influenced by urban watersheds, but such contaminants pose risk to fish and wildlife populations. Las Vegas Wash is the most significant contributor of contaminants to Lake Mead; he largest concentrations of contaminants are generally found near where the Las Vegas Wash flows into Lake Mead. Contaminants from the wash typically include inorganic chemicals such as perchlorate, selenium, and other metals. Also included are legacy organic chemicals such as DDT that was once manufactured in Las Vegas Valley and currently used organic chemicals that include pesticides and PAHs. Further emerging contaminants of concern include personal care products and endocrine disrupting compounds.

    In Lake Mead, the concentrations of the inorganic contaminants, perchlorate and selenium, are within current guidelines established for the protection of human health. Perchlorate was formerly produced in Henderson as a component of rocket fuel. It leached into groundwater from the plant site and from there it seeps into Las Vegas Wash. Selenium naturally occurs in soils within areas of Las Vegas Valley. Selenium concentrations are highest in the Las Vegas Wash, and historically have not been of concern in the open waters of Lake Mead. More study is needed to assess potential for selenium to effect fish and birds in Lake Mead.

    Organic contaminants, including legacy chemicals such as DDT and other pesticides, have been well studied both within the Las Vegas Wash and the Boulder Basin. While these compounds are present in low amounts that pose no threat to human health, they may represent a threat to ecosystem health. Overall, concentrations of legacy contaminants, such as pesticides, are declining due to regulations and mitigation. Volatile organic compounds (VOCs) related to gasoline and boating traffic at Lake Mead NRA are not found to be present in the open water column at levels of concern.

    A group of contaminants considered to be emerging contaminants of concern include personal care products, and endocrine disrupting compounds. Endocrine disrupting compounds are those which trigger hormonal responses in wildlife and people. They include pharmaceuticals that find their way into wastewater which returns back to the lakes, and many commonly used synthetic compounds, such as plasticizers in packaging (such as BPA). Emerging contaminants, including endocrine disrupting compounds, have been well studied in Lake Mead. They are present in low concentrations, especially near the Las Vegas Wash, but not seen at levels currently known to pose a problem to human health for drinking water or the recreating public. They have been, however, documented to cause a number of health effects to individual fish. Several studies have documented biomarkers of effects in common carp.

  • Do the lakes have healthy ecosystems?

    Do the lakes have healthy ecosystems?

    Yes. A healthy lake ecosystem is defined by a variety of biological, physical and aesthetic factors. To evaluate an ecosystem’s health, indicators such as water quality, nutrients, contaminants, ecosystem diversity and species abundance are measured. Determining health can be tricky because the lakes are dynamic, constantly changing systems, but the elements that need to be functioning or in good working order for lake ecosystems are fairly well known.

    One important measure in lake ecosystems is productivity. This is the general balance reached in the growth of the primary producers (plankton, zooplankton, and algae) that support the food web. Such productivity is influenced by the amount of nutrients, climate changes, contaminants and infestation by non-native aquatic invasive species. By standard measures of productivity and trends in the primary producers such as plankton, zooplankton and algae, both lakes Mead and Mohave are well within ranges that are considered healthy.

    The water quality of the lakes, including levels of contaminants, is considered overall healthy based upon standards set by the U.S. Environmental Protection Agency and the states of Nevada and Arizona. Water quality in the lakes is considered well within good ranges for water that supports fish, wildlife, and a high quality recreational setting.

    The fish populations are dynamic, with a variety of native Colorado River fish and other species introduced for sport fishing. These species must constantly adapt to changing conditions, and they often compete with each other. Sport fish populations are stable, as are catch rates of anglers. While native fish species numbers have declined since the dams were built 60 to 80 years ago, with boosts from active management programs they seem to be at a point of relative stability.

    Over 93 species of aquatic birds have been documented on lakes Mead and Mohave. As Lake Mead drops in elevation, removing some bird habitat, differing habitats are created in newly exposed mud flats and river banks. The lake ecosystems support healthy populations of fish and wildlife.

    A bald eagle catches a silver salmon

    Lake Mead provides a healthy environment for a variety of plant and animal species.

  • How are the lakes doing overall?

    How are the lakes doing overall?

    Water quality is in good condition as a source for community drinking water, in support of a variety of high quality recreational uses, and to support healthy populations of fish and wildlife. Lake ecosystems appear to be functioning within a healthy range. There is concern, however, that overall conditions may be declining in the face of continued drops in lake levels. These conclusions are based in part by a recent review by Lake Mead National Recreation Area and University of Nevada, Reno of over a decade of interagency monitoring data to assess overall lake health and conditions. The review looked at four lake ecosystem attributes: lake and riverine ecosystem health, biotic condition, ecological processes, and stressors and disturbances. Lake and riverine ecosystem health was found overall to be in good, but declining condition. Basic water quality parameters are considered well within good ranges. Potential problems with nutrient balance and dissolved oxygen can occur at times and in areas of the lake. The overall condition assessment of declining is based upon the stress placed on the ecosystems and lake health given the dramatic and continuing decline in water levels.

    Biotic (or living elements of ecosystem) condition was found overall healthy, with some groups of animals and plants doing better than others. Sport fish populations are large enough and the body condition of individual fish support appears stable enough to support the important recreational fishery. Native fish populations within Lake Mohave, however, are declining, even with intensive management. Lake Mead native fish populations are important as a unique self-sustaining population but exist in low numbers. Zooplankton composition is influenced by the invasion of the non-native quagga mussels but appear overall stable to date. The lakes invertebrate groups have shifted from a mostly native oligochaeate-chironomid (worms and flies) assemblage to a non-native quagga mussel-mudsnail-oligochaeate assemblage dominated by non-native invertebrates. Bird populations appear stable, but changes in lake levels alter available habitats, favoring some species and stressing others.

    Ecological processes were found to give significant reason for concern, primarily due to declining lake levels driven by the persistent drought. Colorado River and tributary inflows are forecasted to decrease due to persistent drought conditions and greater municipal water demands associated with population growth. Climate models project overall declines in water availability in the Colorado River. Climate models also predict changes in rainfall patterns with greater probabilities for higher intensities and frequencies of summer rainfall, with attendant flashfloods.

    The main ecosystem stressor is the non-native quagga mussel, which have become the dominant bottom organism in vast areas of the lakes, in particular the areas with rocky bottoms. Quagga mussels are reproducing in the lakes year around, with juvenile veliger larvae comprising a significant percentage of the overall zooplankton population.

  • What are the biggest threats to water quality and water-dependent ecosystems?

    What are the biggest threats to water quality and water-dependent ecosystems?

    The greatest threat is the continuing decline of lake surface levels. More shallow areas within the lakes may lead to an overall warming in lake temperatures, posing an increased risk for algal blooms and increased potential for pathogens that may impact people or wildlife. Climate change models predict less water availability overall in the Colorado River basin into the future. Warmer lake waters, and warmer tributary waters, bring changes in the water mixing patterns in the lakes, potentially creating conditions where nutrients are more available to develop nuisance algae and contaminants are more likely to reach wildlife.

    Climate change models also predict increasing numbers of violent thunderstorms and flash floods which deliver contaminants from the Las Vegas Wash, Muddy River and Virgin River. Increasing urbanization within the watersheds of the Las Vegas Wash, Muddy River and Virgin River increase potential for contaminant runoff to the lakes. While not present in levels believed to affect human health, contaminants related to the urban runoff of Las Vegas Valley and emerging contaminants of concern such as pharmaceuticals and endocrine disrupting compounds are present in Lake Mead in levels that have been noted to produce affects within individual fish. Additional study is needed on potential for impact to wildlife populations.

    The non-native invasive quagga mussel has potential to alter lake ecosystems, impact ecosystem food webs, and increase potential for algae blooms. The quagga mussel has already impacted water infrastructure and facilities within the lakes, and necessitated active management programs to prevent spread to additional lakes. The recently discovered non-native gizzard shad may impact lake fisheries. There are additional non-native species of concern on management watch lists for the lakes.

  • How is climate change predicted to affect the lakes?

    How is climate change predicted to affect the lakes?

    Studies predict that warmer temperatures along with changing rain and snow patterns will affect the amounts of water available to the Colorado River. Warmer springs, with more rain instead of snow, grow more vegetation that can that utilizes water along rivers. These warmer temperatures also require more water in order to grow crops meaning that larger amounts of water will evaporate from Lake Mead and other large reservoirs. Studies indicate that based upon climate change, the amount of water available in the Colorado River will be reduced anywhere between 5% and 20% by the year 2050.

    The most noticeable climate change impact on lakes Mead and Mohave is the greater likelihood that low water levels will persist. Lower water levels combined with increasing air temperatures will create greater risks of harmful algae growth as well as other disease- or illness-causing organisms. Fish and wildlife will need to adapt to changing habitat conditions with the loss of some habitats such as site-specific fish spawning beds, to the creation of new habitats such as newly exposed mudflats for shorebirds.

    Intake towers

    Lower water levels can affect not only water availability but also the functioning of dams and pumps.

  • What is being done to protect the water quality in lakes Mead and Mohave?

    What is being done to protect the water quality in lakes Mead and Mohave?

    Making sure our water is safe for people and supports a healthy environment is a high priority. Lakes Mead and Mohave benefit from an active team of scientists, managers and agencies making sure water quality is protected for people, fish and wildlife. The Nevada Department of Environmental Protection has the authority to regulate the treated wastewaters and surface waters of the Las Vegas Wash, and they set water quality standards to protect the Las Vegas Wash, Boulder Basin (where the Wash enters Lake Mead), and all of lakes Mead and Mohave.

    The wastewater treatment departments of Clark County, the City of Las Vegas, the City of North Las Vegas and the City of Henderson work to ensure that the treated wastewater meets strict environmental and water quality standards. The Bureau of Reclamation, the Southern Nevada Water Authority and the National Park Service partner in order to provide monthly water monitoring at 22 locations around Lake Mead as well as 5 locations around Lake Mohave. Furthermore, water quality in the Las Vegas Wash is monitored in real-time using advanced monitoring technology. The US Geological Survey, US Fish & Wildlife Service, Nevada Department of Wildlife, Arizona Game & Fish Department, University of Nevada – Las Vegas and Reno, and Desert Research Institute are all involved in various research and assessment of water quality and ecosystem health.

    All of these agencies meet frequently to discuss findings, emerging issues and suggested actions to protect water quality. The National Park Service, in its role of providing for recreation across the two lakes, incorporates information from this extensive interagency team into management guidelines and public information to protect park visitors. This coordinated effort ensures our waters are constantly being checked in order to provide the highest (and safest) water quality.

    Patrol boats

    The National Park Service routinely samples and surveys the lakes.


Last updated: April 5, 2017

Contact the Park

Mailing Address:

601 Nevada Way
Boulder City, NV 89005


(702) 293-8990

Contact Us