Cyanobacteria in District of Columbia.

The sensor resolves at 300 m, and a lake needs several clear water pixels away from its own shoreline before anything can be read from it — roughly 900 m across. About 2,300 U.S. lakes across 46 states clear that bar, and no lake in District of Columbia does. Rivers, ponds and most reservoirs are absent from this map everywhere, and their absence says nothing about their condition.

A lake this satellite cannot see can still bloom, and blooms in small waters are common. District of Columbia's own cyanobacteria programme monitors the water; this map does not cover it.

This map is a pre-screening indicator that says where to look. It is not a health assessment and it is not a substitute for your state’s cyanobacteria advisory — your state programme is the authority on whether a lake is safe to swim in, drink from, or fish.

A satellite reads only the surface — the top of the water column. Wind mixes a bloom down through the lake, so a bloom that is genuinely present can be missed entirely, and this map can read clear on a day a lake is not. That is the failure direction that matters: an absent reading is never evidence of clear water.

The index measures a pigment common to cyanobacteria. It cannot distinguish a toxic bloom from a harmless one, and it says nothing whatever about microcystin or any other toxin. Only sampling the water can answer that.

Readings below roughly 10,000 estimated cells/mL are beneath the sensor’s detection limit. That is “we could not see anything”, not “there is nothing there” — those lakes are not verified clear.

Cell densities are converted from a pigment index, not counted. The published conversion carries roughly 29% mean absolute percentage error, so treat every figure here as an estimate with real spread around it rather than a measurement.

Cyanobacteria are bacteria, not algae, despite “blue-green algae” being the common name. The distinction matters because the toxins that make a bloom dangerous are bacterial, and treatments and advisories written for algae do not necessarily apply.

The underlying product is published by NASA as preliminary and for evaluation purposes only. It has not been through the validation an operational product would carry.

The sensor resolves at 300 m, and a lake needs a few clear water pixels away from its own shoreline to be read at all — roughly 900 m across. Smaller lakes and most rivers are absent from this map entirely, which says nothing about their condition.

This map has not been validated against state advisory records. Published agreement figures for satellite cyanobacteria screening come from the research literature and describe other lakes in other seasons — not this map’s performance, which nobody has measured yet.

District of Columbia publishes its own cyanobacteria guidance and advisories. Check it before using any water — advisories are issued from samples taken in the water, which is something no satellite can do.

DC DOEE — Algae Blooms in District Waters

NASA and EPA’s satellites read a pigment index over roughly 2,300 U.S. lakes large enough to resolve at 300 m. A pre-screening indicator, not a health assessment, and never a substitute for your state’s advisory.

Open the cyanobacteria bloom map →

Watershed Explorer

Click any point in the country and read what six federal datasets say about it — flow, drought, fire, flood forecast, satellite cyanobacteria screening and Clean Water Act impairment — each one labelled with what it is actually a fact about.

Wildfire & Smoke Map — District of Columbia

Official NIFC fire perimeters and incident points under NOAA’s HRRR-Smoke near-surface forecast, filterable by size, state, and containment.

Open the national wildfire and smoke map →

Streamflow Map — District of Columbia

Every reporting USGS stream gauge in the country, coloured by how today’s flow ranks against that gauge’s own record for this calendar day.

Open the national streamflow map →

U.S. Drought Map — District of Columbia

The U.S. Drought Monitor — the country’s official weekly drought assessment — drawn across five severity categories from D0 to D4.

Open the national U.S. drought map →

Flood Outlook Map — District of Columbia

NOAA’s river forecast centres call the next few days at every forecast gauge in the country, on the National Weather Service’s own flood categories — action, minor, moderate and major.

Open the national flood outlook map →

Impaired Waters Map — District of Columbia

Every subwatershed in the country against the states’ own Clean Water Act assessments — what is impaired, what is still on the 303(d) list, what is causing it, and how much of the country nobody has assessed at all.

Open the national impaired waters map →

Critical Habitat & Protected Waters Map — District of Columbia

Every river and stream the federal government has designated as ESA critical habitat, the listed species each unit is designated for, and who manages the protected land around it.

Open the national critical habitat map →