U.S. streamflow map: every gauge, against its own normal.

Daily mean discharge

Each dot is a U.S. Geological Survey stream gauge reporting discharge — the volume of water moving past it, in cubic feet per second. The figure shown is the daily mean, the statistic USGS publishes once a day's record closes, and the same statistic the historical comparison is computed from. Comparing a daily mean against a daily baseline is what makes the two numbers on this map directly comparable.

Percentile of the historical record

A raw discharge figure means nothing without context: 200 cubic feet per second is a drought on one river and a flood on another. So every gauge is ranked against its own record for this calendar day — all the August 13ths USGS has ever measured there, sorted, and today's reading placed among them. A gauge in the 5th percentile is running lower than 95% of the days on record for this date.

The class boundaries are USGS's own — the 10th, 25th, 75th, and 90th percentiles, the same cuts the agency's streamflow maps have used for two decades. Inventing our own thresholds would mean publishing a national water signal that disagrees with the agency's, so only the colours here are ours.

The five classes

The scale diverges from the middle: normal conditions are drawn small and dim, and both directions brighten as they get further from normal. Roughly two in five gauges sit in the normal band on any given day, so dimming them is what lets the dry and wet anomalies read at a national glance.

• Much below normalBelow the 10th percentile
• Below normal10th to 25th percentile
• Normal25th to 75th percentile
• Above normal75th to 90th percentile
• Much above normalAbove the 90th percentile
• No baselineToo short a record to rank

Why some gauges are hollow rings

A percentile is only as good as the record behind it. A gauge installed three years ago can technically be ranked — against three numbers — and calling the result "much below normal" would dress up noise as a drought signal. Any gauge with fewer than ten years of record for this date is drawn as an empty ring instead: present on the map, visibly not classified.

Folding those into the normal band would have been easier and would have looked cleaner. It would also have meant the map quietly asserting that conditions are fine at hundreds of sites where nobody actually knows.

Updated daily, and dated on the page

This map is rebuilt once a day rather than queried from your browser on every visit. That is a deliberate choice: assembling the national picture takes roughly sixty-five requests against a public federal API that charges nothing and rate-limits nobody, and pushing that cost onto USGS once per visitor would be an abuse of a service everyone depends on. Harvesting once and serving the result from our own CDN means USGS sees a single pass a day no matter how many people open the page.

Little is lost by it. The map compares a daily mean against a daily percentile, so the classification genuinely only moves once a day. The date the data was gathered is stamped on the map itself — and for the live hydrograph, provisional-data caveats, and sub-daily readings at any single gauge, the detail panel links straight through to that gauge's page at USGS.

Whether a river is runnable

Paddle trails and water trails live and die on flow. Below a reach's minimum, boats drag and portages multiply; above its safe maximum, the same reach becomes a hazard for the recreational paddlers it was designed for. Programs that manage public access are making a go/no-go call against gauge readings constantly, and usually by hand.

What fish and restoration work can survive

Sustained low flow concentrates fish into shrinking pools, raises water temperature, and drops dissolved oxygen — the combination behind most warm-season fish kills. It also strands newly planted riparian work and shifts the timing of spawning runs. High-flow extremes scour redds and move sediment through reaches that restoration crews have just stabilised.

Where a watershed is trending

A single gauge below its 10th percentile is a fact. Twenty adjacent gauges below their 10th percentiles for three weeks is a watershed heading somewhere, and it is visible on a national map well before it shows up in a drought declaration.

The same feed, wired into the daily work

Driftwise watches these same USGS gauges as one of more than a dozen federal inputs on every monitored river reach, alongside NOAA flow and flood forecasts, EPA water-quality data, wildfire proximity, and air-quality readings. A reach drifting toward its low-flow tail crosses an edge, and that routes recommended actions to the crews responsible for it — which is the difference between a map you check and a system that tells you when to act.

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

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

U.S. Drought Map

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

Flood Outlook Map

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.

Impaired Waters Map

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.

Cyanobacteria Bloom Map

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.

Critical Habitat & Protected Waters Map

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.