Maps and Data: A Reader's Guide to Public Environmental Tools
Some of the best environmental information in the country is free and a few clicks away. Stream gauges report around the clock, satellites scan for wildfires several times a day, and soil scientists have mapped most of the country down to the scale of a farm field. The catch is that many of these tools feel built for specialists. Here are eight we use all the time, each with a question it answers and a tip for reading it.
Each tool is built and run by the agency named; Hollow & Headwater is an independent publication with no connection to any of them. We explain how to read the maps, while the agencies own the data and its official interpretation. If a page has moved, start from the agency's main site. In a warning or an emergency, follow official alerts and local authorities.
The tools at a glance
| Tool | Run by | Good for |
|---|---|---|
| USGS Water Data | U.S. Geological Survey | Current and past streamflow, water levels, water temperature |
| National Water Prediction Service | NOAA / National Weather Service | River forecasts and flood stages |
| weather.gov | National Weather Service | Forecasts, radar, watches and warnings |
| AirNow | U.S. EPA and partner agencies | Air Quality Index, forecasts, smoke |
| How's My Waterway | U.S. EPA | Reported condition of streams and lakes |
| Wetlands Mapper | U.S. Fish and Wildlife Service | Where wetlands are, and what kind |
| Web Soil Survey | USDA Natural Resources Conservation Service | Soil types, drainage, suitability for uses |
| FIRMS | NASA | Satellite detections of active fires |
| The National Map | U.S. Geological Survey | Topo maps, elevation, streams, imagery |
A little map literacy first
Layers. Most of these tools are web GIS maps that stack layers: points (a gauge), lines (a river), polygons (a wetland or soil unit) and rasters, which are grids of values such as elevation or imagery. Layers often come from different sources and years. Toggle them one at a time, and look for an "about" or metadata link saying where each came from.
Scale. At 1:24,000, one inch on the map equals 24,000 inches, or 2,000 feet, on the ground. That's called large scale because features look large; 1:250,000 is small scale, more country in less detail. Zooming in on a web map makes lines bigger, not more accurate. A wetland boundary traced from county-wide aerial photos won't line up with your fence.
Legends and units. Read the legend before the map. Colors are choices: one tool's red means flooding, another's means unhealthy air. Streamflow comes in cubic feet per second, river level in feet, fine particles in micrograms per cubic meter, and mixing them up is easy.
GPS and dates. A phone's GPS is often good to a few meters under open sky but wanders under thick canopy, in deep valleys and beside cliffs; the circle around your dot is an estimate. Older paper maps may use an older datum, so copied coordinates can be noticeably off. And check the date on everything.
USGS Water Data
What it shows. Readings from the U.S. Geological Survey's national network of stream gauges: gage height (the water level at the gauge, in feet), discharge (cubic feet per second) and, at many sites, water temperature, specific conductance, dissolved oxygen or turbidity. Groundwater wells and long historical records are there too.
A question it answers. "Is the creek I want to paddle Saturday running high, low or about normal?" Compare the current flow with what that gauge usually carries at this time of year.
Tip. Gage height is not depth; each gauge's zero point is set for convenience. Paddlers learn the numbers for their own rivers, as our guide to reading a stream gauge explains. Recent readings are provisional and can be revised.
NOAA and National Weather Service: river forecasts and radar
What they show. The National Water Prediction Service maps river forecast points. Click one for a hydrograph: observed levels, forecast levels, and colored bands for action stage and minor, moderate and major flooding at that spot. The National Weather Service site carries local forecasts, radar, and watches and warnings.
A question they answer. "It's been pouring upstream of our campsite. Will the river come up overnight?" The hydrograph shows the expected crest and timing; radar shows whether more rain is coming.
Tip. River forecasts depend partly on expected rainfall, so they shift as storms develop. A watch means conditions favor a hazard; a warning means it's happening or imminent. On uncertain days, read your local office's forecast discussion, where forecasters explain their reasoning.
AirNow
What it shows. The Air Quality Index from monitors run by state, local and tribal agencies, mainly for ozone and fine particles, plus forecasts. Its fire and smoke map adds smoke plumes, fires and low-cost sensor readings.
A question it answers. "Smoke is in the forecast. Is this afternoon all right for soccer practice?"
Tip. The AQI has six color-coded categories: Good (0–50, green), Moderate (51–100, yellow), Unhealthy for Sensitive Groups (101–150, orange), Unhealthy (151–200, red), Very Unhealthy (201–300, purple) and Hazardous (301 and up, maroon). The headline number reflects whichever pollutant is worst. Sensors fill gaps between monitors but are less certain. More in our piece on the AQI on smoky days.
How's My Waterway
What it shows. This EPA tool gathers what states, territories and tribes report about their waters under the Clean Water Act. Enter an address or a waterway to see nearby streams and lakes by reported status (good, impaired, condition unknown) and the problems identified, such as bacteria, nutrients or mercury in fish.
A question it answers. "Is the lake down our road listed as impaired, and for what?"
Tip. "Condition unknown" means nobody has assessed that water, not that it's clean. Assessments run in cycles and can rest on data several years old, so a listing reflects the state of knowledge more than today's water. For beach or fish advisories, check the current notice where you live.
National Wetlands Inventory mapper
What it shows. The U.S. Fish and Wildlife Service's Wetlands Mapper shows wetlands and deepwater habitats mapped mostly from aerial imagery. Each polygon has a code: PFO1A, for instance, is a freshwater forested wetland of broad-leaved deciduous trees that floods only temporarily.
A question it answers. "Where are the marshes and wet meadows in this valley worth checking for spring frogs?"
Tip. It's a scientific inventory, not a legal boundary, and it misses small wetlands, forested wetlands hidden under canopy, and places that have changed since the imagery was taken. Questions about land use or construction belong with the responsible public authorities, not this map or us.
Web Soil Survey
What it shows. Draw an area of interest and the USDA Natural Resources Conservation Service's survey shows the soils mapped there: texture, drainage class, depth to bedrock and to a seasonal water table, plus ratings for uses from gardens and ponds to septic fields.
A question it answers. "Why does the low end of my yard stay soggy until June, and would a rain garden work there?"
Tip. A map unit is named for its main soils but nearly always holds patches of others too small to draw. Use it for the general setting, then dig a hole. Gray, mottled subsoil usually means the ground stays saturated part of the year.
NASA FIRMS
What it shows. NASA's Fire Information for Resource Management System maps heat detections from the MODIS and VIIRS satellite instruments, usually within a few hours of an overpass. Each mark is a spot much hotter than its surroundings.
A question it answers. "There's a smoke column on the horizon. Where is it coming from?"
Tip. A detection is a hot pixel, not a fire outline, and the dot sits at the pixel's center, so the fire may be a few hundred meters away. Clouds and smoke hide fires, satellites pass only a few times a day, and gas flares can show up too. It is not an evacuation tool; follow local officials for that.
The National Map
What it shows. The USGS base map of the country: elevation (much of it now from lidar), streams, boundaries, roads, place names, land cover and imagery, viewable online or downloadable, including free US Topo maps as PDFs that work offline.
A question it answers. "How steep is the climb from the creek to that ridge, and where does the creek start?" Trace the blue line uphill to its headwaters.
Tip. Turn on a lidar hillshade where available. Stripped of trees, the ground reveals old logging roads, stone walls, mine benches and sinkholes you'd walk right past. Check the contour interval first: tight lines at 40-foot intervals mean a far steeper slope than the same spacing at 10 feet.
Putting them together
The tools work best in combination. For a fall paddle, take the put-in and take-out from The National Map, current flow from USGS Water Data, then the river forecast, the weekend weather and, if there's smoke around, AirNow. Ten minutes at the kitchen table can save a wasted drive. For what you might see once you're out there, our field almanac has the month's highlights.
Tools and longer guides
- Latitude/longitude ↔ UTM converterConvert GPS positions for maps and field notes, right in the browser.
- Air quality at home: particles, ozone and cleaner wood smokeA plain guide to household air quality: fine particles, ozone, wood smoke and cleaner burning, how monitors feed the AQI, and basics of healthy indoor air.
- Kick-net stream monitoring: technique, bug groups and simple scoresHow volunteer kick-net sampling works: setting the net in a riffle, sorting the tray, sensitive and tolerant bug groups, simple scores, pebble counts, safety.
- Erosion and sediment control on building sites: what good looks likeHow silt fence, check dams, sediment basins, seeding and stone construction entrances keep mud out of streams, and how to spot controls that are failing.
- What an impaired stream means: 303(d) lists and TMDLs, explainedWhat it means when a stream is listed as impaired: designated uses, 303(d) lists, TMDL pollution budgets, common causes, and how to look up your own creek.
- How a watershed assessment works, from creek chemistry to land useA plain guide to watershed assessments: chemistry, habitat scores, stream bugs and land use, how to read the findings, and how towns turn them into plans.
- Inside an environmental lab: why water test results can be trustedHow environmental labs keep results honest: blanks, duplicates, spikes, detection limits, holding times, chain of custody and how to read a well water report.
- Shale gas and water: wells, wastewater and what is still debatedA balanced look at shale gas and water: how fracturing uses water, what flowback holds, wastewater options, risks to streams and wells, and baseline testing.
- Illegal dumping and roadside trash: why it happens and what helpsWhy illegal dumping happens, how it harms streams and wildlife, what communities do to prevent it, and how to run a safe cleanup without touching hazards.