Clear Lake

Clear Lake is the largest, natural freshwater lake in California and perhaps the oldest lake in North America. It has a surface area of 43,790 acres (~68 square miles) and contains 1,115,000 acre-feet of water when full. Clear Lake stretches diagonally across the landscape from northwest to southeast and is composed of three "arms" (Upper, Lower, and Oaks) joined by a narrows.

Rumsey Gauge

The lake level is measured in reference to the Rumsey Gauge which was established by Captain Rumsey at Lakeport in 1873. Zero Rumsey is equal to 1318.256 feet above mean sea level and is considered to be the natural low water level of Clear Lake. By definition, a "full lake" is one that measures 7.56 feet (1,325.816 feet above mean sea level) on the Rumsey Gauge.

Prior to the building of the Cache Creek dam in 1914, outflow from Clear Lake into Cache Creek was controlled by a rock ledge called the Grigsby Riffle. The riffle is a rock located at the confluence of Cache and Seigler Creeks, about three miles from Clear Lake. Before the dam was built, water would normally cease to flow over the riffle during the summer months.

Captain Rumsey decided to register the lake level. He decided that when water ceased to flow over the riffle it would be called zero Rumsey. When water was above the riffle it would be called plus Rumsey. Below the riffle, the lake level would be measured as minus Rumsey. He also installed an actual gauge in Lakeport and it's used as a reference to the actual depth at the riffle, not the depth at Lakeport. The actual location of the Rumsey Gauge is on our pier, here at the Lake County Vector Control District (USGS 11450000). This is the only gauge on the entire Lake. The gauge was read and recorded each day by county personnel, but nowadays the U.S. Geological Survey Agency (USGS) reads the gauge by remote telemetry.

Since 1914, the Cache Creek Dam has regulated the level of Clear Lake. The Dam is owned and operated by Yolo County Flood Control and Water Conservation District. Two court decrees, Gopcevic Decree (1920) and Solano Decree (1978, revised 1995), outline the operation of Clear Lake. The Gopcevic Decree regulates winter water levels by setting a lake level below which water may not be released and above which water must be released to reduce flooding (0 - 7.56 feet Rumsey, with exceptions). The Solano Decree regulates summer water levels by establishing the allowable releases based on the spring water level. If the lake level equals or exceeds 7.56 feet Rumsey on May 1, Yolo County may withdraw 150,000 acre-feet. If the lake level is below 3.22 feet Rumsey on May 1, then no water may be released by Yolo County. Additionally, Yolo County can not take its full allotment at once and no water can be taken after Oct 31.


"History of Clear Lake." County of Lake, California. 27 May 2009. Web. 9 Dec 2013.

Knight, Terry. "It's called the Rumsey Gauge for a reason." Record-Bee. 2 Feb 2010. Record-Bee Outdoors Web. 9 Dec 2013.

A Midge or Mosquito?


Midges, aka "gnats" or "rice flies", are common names that generally define any number of small, non-biting flies with two wings. Chironomid midges (non-biting flies in the family Chironomidae) are the most common midges encountered in Lake County. These pestiferous, but harmless insects are often confused with mosquitoes. This is because midges look similar to mosquitoes and their immature stages share many of the same water sources. Like mosquitoes, midges can survive in polluted, stagnant water. Here in Lake County our primary source for midges is Clear Lake. In fact, the Clear Lake Gnat (Chaoborus astictopus) or Phantom Midge, is native to Clear Lake and was a major pest concern due to its sheer abundance in the 1940s-1970s.

Commonly Encountered Midges

  • Clear Lake Gnats (Chaoborus astictopus): Clear Lake gnat adults have reduced mouthparts and are not known to bite. They are much smaller than mosquitoes and other midges of Clear Lake and can pass through large window screens. These gnats are also called "Phantom Midges", because the larvae are transparent making them very difficult to see. There are 4 larval stages (instars) and 4th instar larvae migrate up and down in the water column. During the day they stay in the sediment and at night they move to the surface. Historically, they emerged in large numbers off of Clear Lake, creating a nuisance for local residents.
  • Chironomids (various species, most common is Chironomus plumosus): This is the largest midge in North America and is

common around Clear Lake in the spring and summer. Larvae, commonly called "Bloodworms", are usually reddish in color as a result of hemoglobin in the lymph system. Larvae live in the mud, so they filter feed on organic debris on the lake bottom. They also build "horseshoe" shaped tubes around their bodies and circulate water through these tubes. Adults emerge in large plumes and can form mating swarms in the evening that can be readily seen by residents living near the lake.

Another mosquito look-alike

Crane Flies (aka "mosquito hawks" or "mosquito eaters"): These delicate flies with stilt-like legs are commonly seen in and around homes. They vary in size, but can be as large as 1.5 inches in length. They are often confused with mosquitoes or as "mosquito eaters", however they do not bite people and do not eat mosquitoes. In fact, many adults have short life spans and do not eat at all. Crane fly larvae are commonly found in loose soil or organic matter, although some species can emerge from aquatic sources.

Is it a midge or a mosquito?

Midges, like the one pictured, raise their forelegs at rest, while mosquito adults do not. Chironomid

The wings of midges are shorter than their body, while mosquito wings are slightly longer than their body.

Midges have nonfunctional (reduced) mouth parts, while mosquitoes have a long proboscis (needle-like projection).

Midges form large mating swarms in the evening, which may occur over serveral days. While male mosquitoes may swarm when mating, they are typcially in a defined location and difficult to see.

Midges only live long enough to mate and lay their eggs, while certain species of mosquitoes can live for months at a time.


Midges are highly attracted to light, so minimizing the use of outside lights will help to reduce their presence near your home. Should you choose to use a "Bug Zapper", keep it as far away from your residence as possible to maximize its effectiveness. Keep doors and windows closed or tightly screened to prevent midges from entering your home. Using insecticides to control midges is NOT recommended, as more gnats will quickly re-enter the area that was treated. Remember, these midges are harmless and have very short life spans so any nuisance they pose will be temporary and intermittent. Midges are important to the food chain as they provide food for fish and other aquatic animals.

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Clear Lake Gnat Monitoring

As part of the Lake County Vector Control District's ongoing surveillance of the Clear Lake Gnat (Chaoborus astictopus) population, we have been monitoring various nuisance pests (midges), associated organisms (plankton), and inshore fish in Clear Lake since 1954. We sample the lake each month for benthic (lake bottom) organisms and plankton. In additon to these "lake checks" we also evaluate water quality parameters: temperature, pH, total hardness, water transparency, and specific conductance. The District has also continued to conduct beach seines (inshore fish sampling) on Clear Lake.

Ekman Dredge

Monitoring is conducted on all three arms of the lake (Upper, Lower, and Oaks). Presently, we sample 14 stations in the Upper arm, 6 stations in the Oaks arm, and 9 stations in the Lower arm. Two 6-in cubed Ekman dredges are used to sample the lake sievingbottom at each of these specified stations. Samples are collected by lowering each Ekman dredge to the lake bottom. Dredges are spring loaded with a "messenger" that travels down the rope and triggers the jaws of the dredge to close. The dredges are hauled up and the "mud" samples from each dredge are sieved to remove the sediment, leaving the benthic organisms behind. The organisms collected at each station are identified and counted at our laboratory. We also record top and bottom water temperature and water depth at each station. Additional water quality parameters are only taken once in each arm of the lake.

What's identified in the benthic samples?

Chaoboridae: Clear Lake Gnat (Chaoborus astictopus) immatures; larvae are transparent and nearly invisible in the water; larvae feed on plankton and other small aquatic animals; adults are non-biting.

Chironomidae: Known as chironomids or non-biting midge immatures; these midges are ubiquitous in freshwater worldwide and are often bright red because of hemoglobin content; larvae live in the muddy bottom of Clear Lake (over 50 per square meter) and eat decaying matter; adults are non-biting.

Two subfamilies are counted within this group:

Hirudinea: Leeches; these are small (1-3 cm) leeches with 2 suckers on each end on the bottom of the body; some feed on the blood of fish or turtles, but most are free-living predators; free-living leeches avoid light and generally hide in the detritus at the lake bottom; they feed mostly on insects, molluscs and oligochaetes, or are scavengers, feeding on dead animal matter.

Plankton Tows

Clear Lake plankton (phytoplankton and zooplankton) are collected via the Vertical Tow method.  One sample per arm, per month is typically collected throughout the year. Vertical Tow samples are collected by lowering a conical vertical tow netplankton net to the bottom of the water column and hauling it though a vertical path back to the surface.  The net must be hauled slowly to minimize the possibility of positive water pressure developing at the mouth of the net.  Positive water pressure will push water out of the path of the net and prevent capture of some of the sample. Because of the potential for this method to decrease plankton capture, vertical tow samples are best regarded as qualitative rather then quantitative.  A maximum of 36 plankton samples are collected and analyzed annually through the District's monitoring of Clear Lake. The collected plankton are a minute representative sample of the total population.  The absence of a species from these data should not be considered conclusive evidence of its absence from Clear Lake.

For more information on Blue-Green Algae (i.e. Lyngbya) in Clear Lake: Click HERE


Beach Seining

The District's inshore fish beach seine haul sampling is routinely done twice a summer.  We use a 30 x 6 ft seine net to sample eleven beaches and launch ramps throughout Clear Lake.  Three seine hauls are taken at each sampling station and the fish collected are measured (standard length of the first 25 of each species), counted, and returned to the lake.

Swimmer's Itch

Swimmer's itch (sometimes called schistosome dermatitis or cercarial dermatitis or sedge pool itch) is a skin irritation caused by immatures (cercariae) of certain species of nonhuman blood flukes (parasitic flatworms of the Phylum Platyhelminthes, Class Trematoda) belonging to the famly Schistosomatidae. The cercariae can penetrate the outer layer of skin of people who are wading, swimming, or floating on inner tubes in infested waters. During skin penetration, an itching or stinging sensation may develop and persist for up to one hour. A skin eruption may occur at the site of penetration and a more diffuse reddish rash may occur during this phase. After penetration, the cercariae die, causing an allergic reaction characterized by intense itching and the formation of papule irritations or small blisters in sensitized individuals. The condition usually subsides after several days to one week. However, problems can persist for more than a week if secondary infection occurs.

The small (less than 0.04 inches in length) cercariae which penetrate human skin die because people are not suitable hosts. However, various birds and small mammals are suitable hosts for these flukes. If cercariae successfully penetrate a suitable host (i.e. duck), they enter the circulatory system, where they grow and become adult schistosomes. These adults are wormlike and usually less than 0.5 inches in length. The adult female may mate and produce eggs which are passed with the duck's feces. If the droppings fall into the water, then the egg hatches into a larval form called a miracidium. The free-swimming miracidium can only survive if it penetrates an aquatic snail (the intermediate host). Miracidia give rise to sporocysts (in the snail tissue) which can produce cercariae. The cercariae burrow out of the snail and swim away. If the cercariae contact a warm-blooded animal, they penetrate the skin. If the animal is a suitable host, the life cycle is continued.

If medical personnel have not identified cercariae in the skin of individuals who have developed swimmer's itch symptoms after being in lake or pond water, other possible causes of skin irritation might be considered. Some species of algae reportedly can irritate the skin but this has not been identified as a problem in Lake County. In other areas of the United States (i.e. Louisiana) roundworms (nematodes) in the water can penetrate human skin and cause severe inflammation and itching (sometimes called "nutria itch"). These roundworms probably do not occur in Clear Lake. Backswimmers (Notonectidae), toe-biters (Belostomatidae) and some other aquatic insects which can inflict painful bites on swimmers occur in some ponds and lakes in Lake County. These insect bites can cause acute temporary pain, but swimmer's itch cercariae can cause more prolonged allergic reactions.


  • Do not feed ducks or geese near swimming areas. Increased numbers of waterfowl in an area can result in increased swimmer's itch problems.
  • Do not swim in areas frequented by large numbers of ducks or geese.
  • Avoid swimming in areas where recent swimmer's itch problems have been reported.
  • Swim in relatively deep water. The cercariae are usually found in shallow waters and most easily attach to inactive bodies. Babies sitting along the shoreline are especially vulnerable to swimmer's itch.
  • Swim in open water areas and do not approach areas infested with aquatic weeds and snails.
  • Upon exiting the water, rubbing the skin with a rough dry towel may dislodge cercariae.
  • Swim for short periods of time (i.e. 10 minutes or less). If the skin becomes soft due to excessive time in the water, it may be easier for cercariae to penetrate.
  • If skin develops symptoms of swimmer's itch, antiseptic anesthetic lotions or ointments may provide some relief.
  • Keep affected skin clean and do not scrach to avoid the possibility of secondary infection.
  • If swimmer's itch symptoms persist, consult your family physician.
  • Be aware that schistosome dermatitis cannot be transferred to other parts of the body or to other people by skin contact.