Every public sensor on this platform whose owner ticked “List on the open map” on their Views page, from every group, on one map. Data comes from the open data API; a sensor that goes private leaves the map. No GIS overlays here — those live on each group's own maps.

The USGS gauges layer is every active USGS stream gauge, drawn at once from a catalogue of published USGS coordinates this platform refreshes daily. Each gauge is coloured by how its latest flow compares with the flow recorded on the same day of the year over its approved record — the classes USGS WaterWatch used. The latest value is fetched from USGS by this platform's server once an hour and held one per gauge, never as a history; the ranking is computed here from USGS day-of-year statistics, and a gauge needs at least ten years of record to be ranked. The two-week charts a click opens are fetched by your own browser straight from the USGS Water Services Instantaneous Values API and do not pass through this platform. USGS instantaneous values are provisional and subject to revision. Source: U.S. Geological Survey, waterdata.usgs.gov. A gauge's number is its USGS site number, which is not a sensor on this platform.

The NWS river forecasts layer is every gauge and forecast point of NOAA's National Weather Service National Water Prediction Service, drawn at once from a catalogue this platform refreshes hourly: the observed stage and flow, the flood category the observation falls in against the stages the local forecast office maintains, and the official forecast where the NWS issues one. The two-week series and forecast a click opens are fetched by your own browser straight from the NWPS API and do not pass through this platform. The same NWS flood categories are what colour a USGS gauge “above flood stage”. Source: NOAA / National Weather Service, water.noaa.gov. NWS data is in the public domain; forecasts are the NWS's, not this platform's.

The CDEC layer is every active station of the California Department of Water Resources' Data Exchange Center — about 3,100 reservoirs, river gauges, rain gauges, snow courses and weather stations across the state — from a catalogue this platform refreshes daily. CDEC's servers refuse connections from this platform's hosts, so both the catalogue and the readings a click opens travel through a relay the operator runs; a reading is fetched by this server only for the station you open, passed straight back, and never stored. Source: California Department of Water Resources, cdec.water.ca.gov. Data is provided as published by DWR and the agencies that operate each station.

The Monitor My Watershed layer is community and research monitoring — EnviroDIY loggers and volunteer sites run by 133 separate groups, on the portal operated by Stroud Water Research Center and LimnoTech. Where the stations are, and whether each was reporting, is a catalogue this platform refreshes once a day. Readings are fetched only for the station you open: unlike the USGS charts they are relayed by this server rather than by your browser, because Monitor My Watershed sends no cross-origin headers — nothing is stored, cached or written down on the way through. These are not regulatory-grade measurements and nobody here quality-assures them. Data by Monitor My Watershed contributors, CC BY-SA 4.0, via monitormywatershed.org.

The Rain & snow (radar) layer is the National Weather Service's NEXRAD base-reflectivity mosaic — every WSR-88D radar in the country composited onto one grid and refreshed every five minutes — published as map tiles by the Iowa Environmental Mesonet of Iowa State University. The tiles are fetched by your own browser straight from that publisher and do not pass through this platform, which stores none of them. It is off until you switch it on. Reflectivity is what the radar beam sees rather than what reaches the ground: snow, hail, virga and a beam overshooting a distant storm all read differently from the rain gauge underneath, and the colour says how hard it is falling, not whether it is rain or snow. Coverage is the United States radar network, and blank is not the same as dry. Source: NOAA / National Weather Service, via the Iowa Environmental Mesonet. IEM's materials are in the public domain and NWS data is too; attribution to the Iowa Environmental Mesonet of Iowa State University is given here at their request.

The weather on a station's card — the sky, the temperature and the rain that has fallen in the last hour, day and three days — comes from Open-Meteo, one request made by your own browser for the one station you opened. It is modelled, not measured: Open-Meteo answers from a weather model's grid cell of roughly 1–25 km rather than from a gauge at the site, and the card prints how far away the cell that answered actually is. No model is named in the request, so Open-Meteo chooses per point; inside the United States that is NOAA HRRR at 3 km blending out to GFS. Weather data by Open-Meteo, CC BY 4.0, from the national weather services' own models. Nothing about your reading is stored: this platform is not in the request at all.

The Watershed answer control is the only thing on this map that composes several sources into one answer, and the care it takes about that is the point of it. Click anywhere and this platform resolves the point to a river reach at the USGS Network Linked Data Index, draws the land draining to that reach, and then asks every source it holds about that basin at once — the Corps' national dam inventory, EPA's Clean Water Act assessments, this platform's own nine station catalogues, USGS mineral records and state geologic mapping, the USFWS and NOAA Fisheries endangered-species screens, EPA's permitted dischargers, and California's stream-condition scores. The polygon is the land above the OUTLET of the reach you snapped to, not above the exact point you clicked; on a long reach those differ, and the panel says so above everything counted inside it. Each section says which question it actually answered — counted inside the basin outline, found on the reaches upstream, answered for the outlet alone, or answered for a circle of a stated size around it — because almost none of these sources can answer "for this watershed" and presenting them as though they could would put a number next to a claim it does not support. Each also carries what kind of true it is: a designation with legal force, a measurement, a model, or a published compilation. A section that is still loading says so, a section whose source failed says so, and a section this deployment has not loaded the data for says that — none of them ever quietly disappears, because a missing “Dams” heading would read as “there are no dams here”. Every section shows how old its own answer is, since they are cached separately and for different lengths of time.

The sea state on a coastal station's card — wave height and period, sea surface temperature, and the speed and set of the surface current — comes from Open-Meteo's marine API, fetched by this server for the one station you opened and held briefly. It is offered only where the nearest tidal NOAA CO-OPS station is within 25 km, which is how this platform decides a station is on tidal water; everywhere else it is not shown at all. It is modelled on a grid of about 8 km, so the card prints how far the sea cell that answered is from the station, and Open-Meteo's own caveat travels with it: “Accuracy at coastal areas is limited. This is not suitable for coastal navigation and does not replace your nautical almanac.” Wave direction is where the waves are coming FROM and current direction is where the water is going TOWARDS — the card says which, on each. Sea state by Open-Meteo, CC BY 4.0; no model is named in the request, so Open-Meteo choose per point from the wave and ocean models they republish — Météo-France MFWAM, ECMWF WAM and NCEP GFS-Wave for waves, and the Copernicus Marine Service SMOC analysis for sea surface temperature and surface currents.

The Ocean waves layer is the same marine model, drawn as a field: significant wave height and mean wave direction for a grid of points, fetched by this server, cached for three hours and shared by every reader, and animated in your browser as particles that travel the way the waves are going. The particles' speed stands for wave height and not for anything moving at that speed. Wave direction is where the waves are coming FROM, so a westerly swell runs east across the picture. The model answers only over water, on a grid of about 8 km: land is blank, and so is the last cell along many coasts, inside bays and up estuaries — blank means no data, never a flat sea. The grid is sampled no finer than 0.25° however far you zoom, because a finer sample would spend the shared Open-Meteo allowance to draw interpolation. It is a model, not a buoy; the NDBC layer is the measured sea state. Waves by Open-Meteo, CC BY 4.0, from the Météo-France MFWAM, ECMWF WAM and NCEP GFS-Wave models.

The Active flood warnings layer is the only thing on this map that is neither a measurement nor a model: it is what the National Weather Service has actually issued, right now, drawn as the area it was issued over. Flood warnings, flash flood warnings, flood statements and flood watches, from api.weather.gov, refreshed by this server about once a minute and served from one cached answer, so a thousand readers are still one request to the NWS. A watch is not a warning. A warning means flooding is happening or imminent; a watch means it is possible. Watches are drawn with a dashed outline and a much fainter fill, and there is a second difference worth knowing: the NWS issues a warning over a polygon a forecaster drew, but issues a watch over whole forecast zones with no polygon at all, so a watch's shape here is the zone boundary this platform fetched separately. Outlines are simplified to about 550 m for drawing; whether a particular gauge falls inside a warning is answered on the server against the full-precision area, not against what you see. Flood advisories — the nuisance-flooding tier — are not shown, because they carry a generic product code this layer's filter cannot select. If the status line under the map says the National Weather Service could not be reached, an empty map means nothing is known, not that nothing is active; the line always says how old the answer is. Source: NOAA / National Weather Service. NWS data is in the public domain; the warnings are the NWS's, not this platform's.

The Active flood warnings layer is the only thing on this map that is neither a measurement nor a model: it is what the National Weather Service has actually issued, right now, drawn as the area it was issued over. Flood and flash flood warnings, watches, statements and advisories, from api.weather.gov, refreshed by this server about once a minute and served from one cached answer, so a thousand readers are still one request to the NWS. A watch is not a warning. A warning means flooding is happening or imminent; a watch means it is possible; an advisory is nuisance flooding, an inconvenience rather than a threat to life. The tier is drawn on two channels rather than one, so it reads without having to tell the colours apart: warnings and statements have a solid outline, watches a dashed one and a fainter fill, advisories a dotted one and the faintest fill of all. There is a second difference worth knowing: the NWS issues a warning over a polygon a forecaster drew, but issues a watch over whole forecast zones with no polygon at all, so a watch's shape here is the zone boundary this platform fetched separately. Outlines are simplified to about 550 m for drawing; whether a particular gauge falls inside a warning is answered on the server against the full-precision area, not against what you see. Advisories are fetched by product name rather than by product code, because an advisory carries the same generic code as an air quality alert — so if that half of the answer fails, or the NWS renames the product, the status line says which half is missing instead of quietly drawing the other one. If the status line says the National Weather Service could not be reached, an empty map means nothing is known, not that nothing is active; the line always says how old the answer is. Source: NOAA / National Weather Service. NWS data is in the public domain; the warnings are the NWS's, not this platform's.