Egeria densa Planch.

Common Name: Brazilian waterweed

Synonyms and Other Names:

Anacharis densa (Planch.) Victorin, Elodea densa (Planch.) Caspary, Philotria densa (Planch.) Small & St. John, leafy elodea, dense waterweed, Brazilian elodea

Kristian Peters ( Info

Identification: Egeria densa is a dioecious, submersed perennial found in lentic and lotic freshwater systems. Only male plants are found in the U.S. and in its native range male plants outnumber female plants by as much as 6:1 (Cook and Urmi-König 1984). Leaves and stems are generally bright green and the short internodes give it a very leafy appearance. Leaves which are are minutely serrated (needing magnification) and linear, are 1-3 cm long, up to 5mm broad, and found in whorls of four to eight. The lowest leaves may be opposite or in whorls of 3; middle and upper leaves are in whorls of 4 to 8. Stems are erect, cylindrical, simple or branched, and grow until they reach the surface of the water where they form dense mats. Flowers have three petals which are white (18-25 mm) and float on or rise just above the water's surface on a slender peduncle. Slender roots are unbranched and typically a white to pale color. Adventitious roots are freely produced from double nodes on the stem (Washington State Department of Ecology 2003). It can be distinguished from related species by the absence of turions (shoots from underground stems) and tubers, and by the presence of showy white flowers that float on the water (Hoshovsky and Anderson 2001). It is usually rooted in the bottom mud, but can be found as a free-floating mat or as fragments with stems near the surface of the water.

Size: 3-5 m long, 2-2.5 mm diameter.

Native Range: South America (central Minas Gerais region of Brazil, coast of Argentina, and coast of Uruguay)

Nonindigenous Occurrences: Egeria densa is declared a weed in Argentina (Cabrera Walsh et al. 2013) and in Tasmania, Australia (Parsons and Cuthbertson 2001). Egeria densa has been reported in Bogakain Lake, Bangladesh in 2010 (Alumujaddade Alfasane et al. 2010). This species was historically introduced outside its native range through the aquarium trade and is considered naturalized in parts of Central America, Chile, England, Mexico, New Zealand, Spain, United States, and the West Indies (Cabrera Walsh et al. 2013, Curt et al. 2010, Flora of North America Editorial Committee 2000).

Egeria densa was first reported outside of its native range in 1893 in Long Island, New York (Yarrow et al. 2009). 

It occurs in Alabama (Flora of North America Editorial Committee 2000), Arkansas, in Siera Nevada, Central Valley, central coast of San Francisco Bay, and Jan Jacinto Mountains of California (Hoshovsky and Anderson 2001), California (California State Parks 2014), Colorado (Flora of North America Editorial Committee 2000), Delaware, Florida, and Georgia. Egeria densa occurs private ponds in Lake County, Illinois (Illinois Database of Aquatic Non-native Species, Lake County Health Department and Community Health Center 2009, New Invaders Watch Program 2014), which is near Lake Michigan; however, dispersal from these ponds is limited because it does not occur in waters connected to the Great Lakes. It was first reported in Griffy Lake near Bloomington, Indiana in 2004, but was controlled successfully by herbicide (Jones 2006). This species is found in eastern Massachusetts (Magee and Ahles 2007). Egeria densa has been reported in Powderhorn Lake, Minnesota (City of Minneapolis 2013). It is found in New Jersey (Flora of North America Editorial Committee 2000), New Mexico, Long Island, New York (Magee and Ahles 2007), North Carolina (Flora of North America Editorial Committee 2000), Oklahoma, Oregon, Pennsylvania, South Carolina, Tennessee, Texas, southern Vermont (Magee and Ahles 2007), Virginia (Flora of North America Editorial Committee 2000), Washington (Washington State Department of Ecology 2013), and West Virginia (Flora of North America Editorial Committee 2000).

This species is not currently in the Great Lakes region but may be elsewhere in the US. See the point map for details.

Table 1. States/provinces with nonindigenous occurrences, the earliest and latest observations in each state/province, and the tally and names of HUCs with observations†. Names and dates are hyperlinked to their relevant specimen records. The list of references for all nonindigenous occurrences of Egeria densa are found here.

State/ProvinceYear of earliest observationYear of last observationTotal HUCs with observations†HUCs with observations†
Alabama196620179Apalachicola; Coosa-Tallapoosa; Guntersville Lake; Lower Chattahoochee; Mobile-Tensaw; Pickwick Lake; Upper Alabama; Upper Black Warrior; Upper Choctawhatchee
Arizona194319844Lower Colorado Region; Rillito; Upper San Pedro; Upper Santa Cruz
Arkansas197420157Beaver Reservoir; Lake Conway-Point Remove; Lower Arkansas-Maumelle; Lower Sulpher; Ouachita Headwaters; Upper Ouachita; Upper Saline
California1935201633Antelope-Fremont Valleys; Big Chico Creek-Sacramento River; Central California Coastal; Clear Creek-Sacramento River; Crowley Lake; Honcut Headwaters-Lower Feather; Lower American; Lower Pit; Lower Sacramento; Mad-Redwood; Middle Kern-Upper Tehachapi-Grapevine; Middle San Joaquin-Lower Chowchilla; Monterey Bay; North Fork American; Owens Lake; Russian; Sacramento-Stone Corral; Salton Sea; San Diego; San Francisco Bay; San Francisco Bay; San Jacinto; San Joaquin; San Joaquin Delta; San Pablo Bay; Suisun Bay; Tomales-Drake Bays; Upper Coon-Upper Auburn; Upper Kern; Upper Klamath; Upper Merced; Upper Mokelumne; Upper Tuolumne
Colorado198419841Big Sandy
Connecticut199220093Lower Connecticut; Quinnipiac; Saugatuck
Delaware194119943Brandywine-Christina; Broadkill-Smyrna; Nanticoke
Florida1937201725Apalachee Bay-St. Marks; Apalachicola; Aucilla; Blackwater; Caloosahatchee; Chipola; Crystal-Pithlachascotee; Florida Southeast Coast; Hillsborough; Kissimmee; Lake Okeechobee; Lower Ochlockonee; Lower St. Johns; Lower Suwannee; Nassau; Oklawaha; Peace; Santa Fe; South Atlantic-Gulf Region; Southern Florida; St. Andrew-St. Joseph Bays; St. Marys; Tampa Bay; Upper St. Johns; Withlacoochee
Georgia197920078Lower Chattahoochee; Lower Flint; Middle Chattahoochee-Lake Harding; Middle Chattahoochee-Walter F; Middle Savannah; Middle Tennessee-Chickamauga; Spring; Upper Ocmulgee
Idaho200720082Lower Boise; Palouse
Illinois1978201612Big Muddy; Cache; Des Plaines; Lower Ohio; Lower Ohio-Bay; Lower Wabash; Saline; Skillet; Sugar; Upper Fox; Upper Mississippi-Cape Girardeau; Wabash
Indiana200320042Blue-Sinking; Lower White
Iowa201720171South Skunk
Kansas197319841Lower Kansas
Kentucky198620124Lower Levisa; Lower Ohio-Salt; North Fork Kentucky; Upper Cumberland
Louisiana1960199316Atchafalaya - Vermilion; Bayou Teche; Black Lake Bayou; Castor; Central Louisiana Coastal; East Central Louisiana Coastal; Eastern Louisiana Coastal; Lake Maurepas; Louisiana Coastal; Lower Mississippi Region; Lower Ouachita; Lower Ouachita; Lower Red-Ouachita; Red-Sulphur; Vermilion; West Central Louisiana Coastal
Maryland193820166Chester-Sassafras; Choptank; Middle Potomac-Anacostia-Occoquan; Middle Potomac-Catoctin; Patuxent; Tangier
Massachusetts193920012Charles; Narragansett
Minnesota200720071Twin Cities
Mississippi196720103Lower Pearl; Mississippi Coastal; Upper Leaf
Missouri197020085Cahokia-Joachim; Eleven Point; Little Chariton; Sac; Upper Mississippi-Cape Girardeau
Nebraska197719841Missouri Region
New Hampshire200120161Merrimack River
New Jersey199020135Hackensack-Passaic; Lower Hudson; Mullica-Toms; Raritan; Sandy Hook-Staten Island
New Mexico196119961Upper Gila
New York189320158Hackensack-Passaic; Long Island; Lower Hudson; Middle Delaware-Mongaup-Brodhead; Middle Hudson; Northern Long Island; Rondout; Southern Long Island
North Carolina1968199713Albemarle; Albemarle-Chowan; Cape Fear; Middle Roanoke; Neuse; New River; Northeast Cape Fear; Pamlico; Roanoke Rapids; South Atlantic-Gulf Region; Upper Broad; Upper Catawba; Upper Neuse
Ohio199020172Black-Rocky; Little Miami
Oklahoma196919855Arkansas-White-Red Region; Cache; Red-Little; Red-Washita; West Cache
Oregon1934200819Alsea; Clackamas; Coos; Lower Columbia; Lower Columbia; Lower Columbia-Clatskanie; Lower Willamette; Middle Columbia-Hood; Middle Willamette; Necanicum; Oregon-Washington Coastal; Pacific Northwest; Pacific Northwest Region; Siletz-Yaquina; Siltcoos; Siuslaw; Sixes; Upper Willamette; Willamette
Pennsylvania191720044Crosswicks-Neshaminy; Lower Delaware; Raystown; Schuylkill
Puerto Rico198219833Cibuco-Guajataca; Eastern Puerto Rico; Puerto Rico
Rhode Island200920092Pawcatuck-Wood; Quinebaug
South Carolina1936201612Cooper; Enoree; Lake Marion; Middle Savannah; Saluda; Santee; Santee; Tyger; Upper Broad; Upper Savannah; Waccamaw; Wateree
Tennessee194619977Buffalo; Caney; Holston; Lower Cumberland-Sycamore; Lower French Broad; Middle Tennessee-Chickamauga; South Fork Forked Deer
Texas197019997Caddo Lake; Denton; Lake O'the Pines; Lower Angelina; Lower Neches; San Marcos; Toledo Bend Reservoir
Virginia1946201613Albemarle-Chowan; Appomattox; French Broad-Holston; James; Lower James; Lower Potomac; Lower Rappahannock; Middle New; Middle Potomac-Anacostia-Occoquan; Nottoway; Roanoke; Roanoke Rapids; Upper Roanoke
Washington1977201518Duwamish; Grays Harbor; Hood Canal; Lake Washington; Lower Chehalis; Lower Columbia; Lower Columbia-Clatskanie; Lower Columbia-Sandy; Lower Cowlitz; Lower Skagit; Nisqually; Puget Sound; Puget Sound; San Juan Islands; Snohomish; Strait of Georgia; Upper Chehalis; Willapa Bay
West Virginia201520151Tygart Valley

Table last updated 6/9/2018

† Populations may not be currently present.

Ecology: Egeria densa is an aquatic plant in the waterweed family that inhabits mild to warm freshwaters, such as slow flowing streams of warm, temperate, and tropical regions (Parsons and Cuthbertson 2001). It occurs at depths as deep as 7 m. It grows in thick mats of intertwining stems (Parsons and Cuthbertson 2001), which alter the light and nutrients available to the biota where it occurs (Yarrow et al. 2009), acting as an ecosystem engineer (Jones et al. 1994).

Egeria densa can inhabit waters with a wide range of temperatures, low CO2 levels, and low light levels. This species can survive in waters with temperatures of 3-35°C (Yarrow et al. 2009). The plant can overwinter as seeds, dormant shoots, or semi-dormant shoots until temperatures rise above 15°C (Parsons and Cuthbertson 2001). Egeria densa exhibits the C4 pathway and utilizes HCO3-; thus it is able to photosynthesize in waters with low CO2 levels (Casati et al. 2000). Egeria densa can tolerate high phosphorous levels, but is susceptible to iron deficiency (Parsons and Cuthbertson 2001). This species has a low light requirement and can thrive in turbid environments (Parsons and Cuthbertson 2001). Optimal light intensity is about 100 lux. Egeria densa cannot tolerate high light intensities or high levels of ultra-violet and blue light, as it experiences chlorophyll damage to light levels of 1250 lux. Egeria densa cannot tolerate high UV-B radiation, as it can damage the enzymes involved in photosynthesis and can reduce photosynthetic capacity (Casati et al. 2002).

Flowering occurs in November-February in Australia, which float above the water surface and are pollinated by insects (Parsons and Cuthbertson 2001). It reproduces asexually in Australia (Parsons and Cuthbertson 2001) and in California (Hoshovsky and Anderson 2001), where only the male plant has established. Egeria densa is capable of vegetative fragmentation; stems of at least two nodes can break off from the parent colony and disperse by stream flow (Parsons and Cuthberson 2001). Stem fragments that break off can take root in bottom mud or grow as free-floating mats (Hoshovsky and Anderson 2001). Fragmentation can occur as a result of the mechanical shearing of water flows, wave action, waterfowl activity, and boating. Seeds of E. densa have not been documented in California.

Means of Introduction: Egeria densa has a high probability of introduction to the Great Lakes (Confidence level: Low).

Potential pathway(s) of introduction: Hitchhiking/fouling, unintentional release

Introduced world-wide through the aquarium trade - sold widely as good "oxygenator" plant and dispersed secondarily by boat trailers and vegetative dispersal downstream. As a popular ornamental plant, E. densa is planted in water gardens (Indiana Department of Natural Resources 2013) and can be purchased (e.g. in Ohio; however, there is not enough information available to determine the frequency of E. densa plantings.  Egeria densa is cultured in Florida for the ornamental aquatic plant industry (Fenner); however, there is no indication that E. densa is commercially cultured in the Great Lakes region.

Egeria densa is one of the most common species sold in the Montreal aquarium trade (Cohen et al. 2007). The sale and transport of E. densa is prohibited in Illinois, Indiana, Michigan, Minnesota, and Wisconsin (Great Lakes Panel of Aquatic Nuisance Species 2012); but there are no regulations on the sale or transport of E. densa in New York, Ohio, Ontario, Pennsylvania, or Quebec. A survey performed from 2002 to 2003 on aquarium and pet stores near Lakes Erie and Ontario found that 35% of stores surveyed sold E. densa (Rixon et al. 2005). Due to the availability of E. densa in stores near the Great Lakes, and the ability of the species to overwinter, Rixon et al. (2005) predicted that it has the potential to be introduced to the Great Lakes. Based on the number of aquarium stores in Montreal, Quebec that sold E. densa, the number of E. densa sold by each store, and the disposal pathways for aquatic plants, Cohen et al. (2007) estimates that 188 E. densa individuals are released into the St. Lawrence Seaway each year.

This species may be transported by hitchhiking on recreational gear; E. densa grows in thick mats that can become entangled on boat propellers and trailer wheels, or can be captured in bilge water (Washington State Department of Ecology 2013). Attached fragments can be transported between water bodies. Egeria densa is not known to be taken up in ballast water.

Nonindigenous E. densa populations in Río Cruces, Chile have similar genotypes as populations in Western Oregon, suggesting that the two populations experienced similar bottlenecking events at introduction, or there is a lack of genetic diversity in the native population (Carter and Sytsema 2001).


Status: Established in North America, but not including the Great Lakes.

Egeria densa has a moderate probability of establishment if introduced to the Great Lakes (Confidence level: High).

Egeria densa has broad physiological tolerances. It can tolerate low light conditions (Lara et al. 2002). Although a tropical plant, it is able to adapt to seasonal changes and overwinter (Indiana Department of Natural Resources 2013, Parsons and Cuthbertson 2001, Rixon et al. 2005, Yarrow et al. 2009). It can tolerate low levels of CO2 (Casati et al. 2000), nitrogen, phosphorus, and nutrients (Yarrow et al. 2009). This species can survive in waters with salinities up to 8 ppt (Hauenstein and Ramirez 1986).This species is known to have a relatively fast growth rate (Yarrow et al. 2009).

Moreover, the introduced ranges of Egeria densa have similar climate and abiotic conditions as the Great Lakes. Egeria densa has established near the Great Lakes previously (e.g. Griffy Lake, Indiana), in areas that have similar water temperature and dissolved oxygen levels (Aquatic Control 2008, NOAA CoastWatch 2014). This species can survive in freshwater habitats of varying temperatures, light levels, and CO2 levels; thus suitable habitats are readily available in the Great Lakes region. Turbidity and nutrient levels of the Great Lakes is likely suitable for E. densa to obtain sufficient light, phosphorus, and nitrogen. It is likely that Egeria densa will benefit from the effects of climate change, including warmer temperatures and shorter duration of ice cover. Increased salinization may negatively impact this species’ establishment if salinities exceed 8 ppt (Hauenstein and Ramirez 1986). Grass carp preys on E. densa and occurs in the Great Lakes region (Anderson et al. 2008, Hoshovsky and Anderson 2001, Parsons and Cuthbertson 2001), but it is not likely that it will prevent the establishment of E. densa in the Great Lakes. Each year, it is estimated that 188 E. densa individuals enter the St. Lawrence Seaway through disposal of aquarium plants (Cohen et al. 2007).

Egeria densa reproduces asexually via vegetative fragmentation (Hoshovsky and Anderson 2001), which may aid its establishment in the Great Lakes region. This species spreads rapidly by vegetative fragmentation and recreational activities, resulting in dense mats (Indiana Department of Natural Resources 2013).

E. densa has established extensively in 27 countries beyond its native range (Curt et al. 2010). In Australia, E. densa spread quickly; over a period of two years, it doubled its biomass and doubled the area it occupied in the Hawkesbury-Nepean River (Roberts et al. 1999). It is difficult to control once established (Yarrow et al. 2009) and it is recommended to correctly identify the plant due to its similarities with native plants (Indiana Department of Natural Resources 2013). Grass carp is used as a biological control in California (Hoshovsky and Anderson 2001). In Griffy Lake, Indiana, fluridone successfully removed E. densa, but also caused the mortality of all aquatic plants (Jones 2006). Physical removal is not recommended as it can result in vegetative fragmentation and encourages dispersal of E. densa (Parsons and Cuthbertson 2001).

Impact of Introduction:  



Great Lakes Impacts: Egeria densa has the potential for high environmental impact if introduced to the Great Lakes.

The model developed by Rixon et al. (2005) predicts that E. densa poses a threat to the Great Lakes. Egeria densa acts as an ecosystem engineer by preventing the resuspension of sediments and controlling light and nutrient availability (Yarrow et al. 2009). The dense growth of E. densa can retard water flow and reduce turbidity (Parsons and Cuthbertson 2001). This species can reduce the abundance and diversity of native plant seeds in lake bottoms due to increased sediment accumulation under its weed beds (Hoshovsky and Anderson 2001). Egeria densa removes nutrients from the water column, thereby decreasing the standing stock of phytoplankton (Yarrow et al. 2009). Furthermore, Egeria densa forms mats that can shade out phytoplankton. 

Egeria densa can outcompete native species. In Duck Lake, Washington, E. densa displaced native stonewart, elodea, and pondweed in a period of 3 years (Washington State Department of Ecology 2013). In Hawkesbury-Negean River, Australia, evidence suggests that E. densa outcompeted native vallisneria (Vallisneria americana) for light (Roberts et al. 1999) and that floods were responsible for the rapid spread of E. densa.

Egeria densa is not known to pose a threat to the health of native species. This species is not known to alter predator-prey relationships or genetically affect native populations.

Egeria densa has the potential for high socio-economic impact if introduced to the Great Lakes.

Dense stands of E. densa may restrict water movement, trap sediment, and cause fluctuations in water quality. The dense growth of E. densa can interfere with irrigation projects, hydroelectric dams, and urban water supply (Hoshovsky and Anderson 2001, Parsons and Cuthbertson 2001). In New Zealand, there was an infestation of E. densa in the Wikato River that clogged the water intake pipes resulting in the shut-down of an electrical plant (Washington State Department of Ecology 2013). In Brazil , E. densa (as well as E. najas, Ceratophyllum demersum, and Eichhornia crassipes) have severely infested hydropower reservoirs. It was estimated that 48,000 cubic meters of aquatic weeds were removed from water intake structures in Jupia Reservoir (Marcondes et al. 2000).

Egeria densa can inhibit recreational activities as a nuisance for navigation, fishing, swimming, and water skiing (Washington State Department of Ecology 2013). The removal of E. densa is costly; Washington local and state governments spend thousands of dollars each year to control the species. Egeria densa may pose a risk to human safety. In 2006, police reports indicate that E. densa may have contributed to the death of a physician in San Joaquin County, who drowned after becoming entangled in the “tentacle-like Delta weeds trap” in attempts to save his nephew (Breitler 2006, Victorian DPI 2013).

Egeria densa has gained widespread recognition by parks departments and local and state governments as a nuisance species (e.g. Great Lakes Panel on Aquatic Nuisance Species 2012, Lake County Health Department and Community Health Center 2009, Mcglynn 2013). Resources have been devoted in various cities and states to remove infestations of E. densa due to its costly impacts on water supply, infrastructure, and recreation (e.g. California State Parks 2014, Indiana Department of Natural Resources 2013).

Egeria densa has the potential for moderate beneficial impact if introduced to the Great Lakes.

It is an ornamental plant for aquariums and small ponds. Egeria densa has been recommended as a submerged oxygenator plant for water gardens (Creative Homeowner 2010). Egeria densa is also utilized in plant biology classes for students to study photosynthesis (Berkely 2014).

Management: Regulations (pertaining to the Great Lakes region)

The sale and transport of E. densa is prohibited in Illinois, Indiana, Michigan, Minnesota, and Wisconsin (Great Lakes Panel of Aquatic Nuisance Species 2012); but there are no regulations on the sale or transport of E. densa in New York, Ohio, Ontario, Pennsylvania, or Quebec.

Note: Check federal, state/provincial, and local regulations for the most up-to-date information.


White amur or Chinese grass carp, and Congo tilapia (Tilapia meanopleura) can be biological control agents for Egeria densa (Hoshovsky and Anderson 2001, Parsons and Cuthbertson 2001). Sterile triplorid grass carp usage to control E. densa is permitted with California Department of Fish and Game authorization in several counties in California (Hoshovsky and Anderson 2001). In Argentina, the leafminer fly Hydrellia sp. is an herbivore that specializes on E. densa (Cabrera Walsh et al. 2013). Peking ducks (Anas platyrhynchos) were experimentally used to remove E. densa, but they were not effective (Curt et al. 2010).

Hand-pulling, cutting, and mechanical removal provide temporary control, but can encourage dispersal by vegetative fragmentation (Curt et al. 2010). Flowing water treatments are used in New South Wales and Victoria, Australia which can help control other aquatic nuisance plant species (Parsons and Cuthbertson 2001).

In California, herbicides are used with the consultation of a specialist (Hoshovsky and Anderson 2001). Contact-type Diquat, copper, acrolein, and fluridone are used in California (Hoshovsky and Anderson 2001). In Sacramento-San Joaquin Delta, fluridone treatment started in June 2014 and will continue through September 2014 (California State Parks 2014). In an attempt to control E. densa in Griffy Lake, Bloomington, Indiana, fluridone treatments started in April 2006 and continued until September of that year (Jones 2006). The dose needed to control E. densa was high enough to kill all aquatic plants in the lake. Post-treatment surveys found only fragments of native species.

Note: Check state/provincial and local regulations for the most up-to-date information regarding permits for control methods. Follow all label instructions.

Remarks: Egeria densa can be  detected using digital imagery (Mandvikar and Huan Liu 2004). It is often sold in the name “Anachris” and is advertised to aquarium customers as oxygenators.


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Washington State Department of Ecology. 2013. Non-Native, Invasive, Freshwater Plants – Egeria densa. Available Accessed 2 May 2013.

Victorian Department of Industries. 2013. Impact Assessment - Dense waterweed (Egeria densa) in Victoria. Available Accessed 13 August 2014.

Yarrow, M., V.H. Marín, M. Finlayson, A. Tironi, L.E. Delgado, and F. Fischer. 2009. The ecology of Egeria densa Planchon (Liliopsida: Alismatales): a wetland ecosystem engineer? Revista Chilena de Historia Natural 82: 299-313.

Other Resources:
Common waterweed (Center for Aquatic and Invasive Plants, University of Florida)
Global Invasive Species Database
Technical Information about Egeria densa (Brazilian elodea) (Washington Department of Ecology)

Author: Morgan, V.H., E. Baker, C. Stottlemyer, and J. Li

Contributing Agencies:

Revision Date: 1/28/2015

Citation for this information:
Morgan, V.H., E. Baker, C. Stottlemyer, and J. Li, 2018, Egeria densa Planch.: U.S. Geological Survey, Nonindigenous Aquatic Species Database, Gainesville, FL, and NOAA Great Lakes Aquatic Nonindigenous Species Information System, Ann Arbor, MI,, Revision Date: 1/28/2015, Access Date: 7/18/2018

This information is preliminary or provisional and is subject to revision. It is being provided to meet the need for timely best science. The information has not received final approval by the U.S. Geological Survey (USGS) and is provided on the condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from the authorized or unauthorized use of the information.