Methodology & data sources

Every number on this site comes from a government agency. We collect it on a schedule that respects each agency's limits, store it, and compare today's value with the same date in past years.

How we compare with normal

"Normal for the date" is the middle 50% (25th to 75th percentile) of past values for that day of the year. Lakes use our stored daily history (at least 5 years). River gauges use the U.S. Geological Survey's own day-of-year statistics from the full approved record.

Full pool and datums

Lake elevations can be measured against different vertical datums (for example NGVD29 and NAVD88, which differ by up to several feet). We only compare a level with a full-pool value when both come from the same datum.

What we never do

We never estimate missing values, and we never issue flood warnings. Flood status is always NOAA's, with a link to the official page.

Data sources

USGS Water Data OGC API

Licence: US public domain (USGS). Updates: TBC.

USACE CWMS Data API

Reservoir data: U.S. Army Corps of Engineers (CWMS).

Licence: US public domain (USACE). Updates: see source.

USACE Access2Water (Internet of Water hub, usace-edr)

Reservoir data: U.S. Army Corps of Engineers via Internet of Water

Licence: TBC. Updates: see source.

Bureau of Reclamation RISE

Reservoir data: U.S. Bureau of Reclamation (RISE, data.usbr.gov).

Licence: US Government Works (public domain, 17 U.S.C. 105). Confirmed in writing by the RISE Team, 2026-10-20: reservoir/lake/river monitoring data falls under US Government Works.. Updates: daily, 1-3 day lag; values flagged provisional.

NOAA National Water Prediction Service

River forecasts and flood categories: NOAA National Weather Service

Licence: US public domain (NOAA). Updates: see source.

NWS api.weather.gov

Weather: NOAA National Weather Service

Licence: US public domain (NOAA). Updates: see source.

Recreation.gov RIDB (Recreation Information Database)

Campground data source: ridb.recreation.gov

Licence: Free reuse, no permission needed (recreation.gov 'Use our data'); data.gov lists it as Creative Commons Attribution. Use means accepting the RIDB API Access Agreement (no illegal/misleading use, no use that seeks to capitalize on a natural disaster).. Updates: bulk export rebuilt daily; we refresh weekly (campground lists change slowly).

USACE National Inventory of Dams (NID)

Dam facts: U.S. Army Corps of Engineers, National Inventory of Dams

Licence: US Government data, no licence restrictions stated (public). Fields NID marks private (hazardId, eapMapSourceDate) are never stored; we also never store or publish hazard potential, condition assessment, inspection, EAP or risk text.. Updates: continuous (per-dam dataUpdated); we refresh monthly; full records only re-fetched when dataUpdated changes.

GBIF occurrence API (fish occurrence records)

Species records: Derived dataset GBIF.org (21 October 2026) Filtered export of GBIF occurrence data https://doi.org/10.15468/dd.5jr6vx (CC0 / CC BY 4.0 records from the contributing datasets)

Licence: Per record. We keep only CC0 1.0 and CC BY 4.0 records (never CC BY-NC). GBIF Data User Agreement: cite GBIF.org with a DOI and credit the publishers.. Updates: continuous (new records daily); refresh species lists monthly.

Minnesota DNR LakeFinder, stocking and ice-out

Lake surveys, stocking and ice-out: Minnesota Department of Natural Resources

Licence: . Updates: see source.

OpenStreetMap slipways (Geofabrik extracts)

Licence: ODbL 1.0. Updates: see source.

Bureau of Meteorology Water Data Online (KiWIS web service)

Water data: Bureau of Meteorology Water Data Online, from data supplied by [owner]. CC BY 3.0 AU.

Licence: CC BY 3.0 AU (page states: unless otherwise noted, all material is licensed under CC BY Australia). Updates: see source.

WaterNSW API portal (Water Data product)

Licence: WaterNSW copyright notice (waternsw.com.au/copyright): may copy, distribute, display and freely deal with the information for any purpose, on condition the copyright notice is included. Permission needed only to charge for access, put a document in advertising or a product for sale, or modify a document.. Updates: see source.

Utah Division of Wildlife Resources fish stocking

Fish stocking data: Utah Division of Wildlife Resources (https://dwrapps.utah.gov/fishstocking/Fish)

Licence: Written permission from Utah DWR (Faith Jolley, Public Information Officer), 2026-10-20. Updates: updated as fish are stocked (several times a week in season).

New York State DEC fish stocking lists (data.ny.gov)

Fish stocking data: New York State Department of Environmental Conservation, via data.ny.gov

Licence: data.ny.gov Terms of Use (open data, reuse allowed with attribution to the State agency). Updates: actual list once a year, about mid-May, for the PREVIOUS year; spring trout list once per season.

California CDFW Fish Planting Schedule + BIOS ds778 stocked waters

Fish planting data: California Department of Fish and Wildlife (Fish Planting Schedule; BIOS ds778, CC BY 4.0)

Licence: BIOS ds778 layer: CC BY 4.0. Fish Planting Schedule page: CDFW Conditions of Use (public information, reuse with credit). Updates: updated in real time by hatchery staff; the page keeps a rolling ~12 months, so we must collect at least weekly to build history.

OpenStreetMap slipways (leisure=slipway, seamark slipways, service=slipway) via the Overpass API

Licence: Open Database License (ODbL) 1.0. Updates: OSM is edited continuously; refresh ramps monthly (python3 -m collectors.ramps fetch-osm ALL).

Minnesota DNR ice-out records (State Climatology Office / LakeFinder)

Ice-out dates: Minnesota Department of Natural Resources, State Climatology Office (dnr.state.mn.us)

Licence: No open licence stated on the DNR site or service (MN DNR website terms only).. Updates: spring: daily as volunteers and DNR staff report ice-out; history is static.

Minnesota DNR LakeFinder (lake list and fish surveys)

Fish survey data: Minnesota Department of Natural Resources, LakeFinder (dnr.state.mn.us/lakefind)

Licence: No open licence stated (MN DNR website terms only).. Updates: surveys added after each field season (summer/autumn); refresh monthly.

USGS National Hydrography Dataset (NHD high resolution), waterbody layer

Lake outlines: U.S. Geological Survey, National Hydrography Dataset

Licence: US public domain (USGS). Updates: static (NHD is no longer actively updated; 3DHP replaces it over time).

ECCC GeoMet OGC API: hydrometric real-time (Water Survey of Canada)

Contains information licenced under the Data Server End-use Licence of Environment and Climate Change Canada.

Licence: ECCC Data Server End-use Licence v2.1 (https://eccc-msc.github.io/open-data/licence/readme_en/). Worldwide, royalty-free, commercial use allowed. No ECCC logos or crests, no implied endorsement, no warranty. Raw data may not be resold as-is (Water Office disclaimer); value-added use is allowed.. Updates: most stations every 5 min (some 15/60 min), posted with a median 1 h lag; the API keeps only the last 30 days. Collect hourly..

HYDAT, National Water Data Archive (Water Survey of Canada)

Contains information licensed under the Open Government Licence – Canada. Source: Water Survey of Canada, HYDAT National Water Data Archive.

Licence: Open Government Licence – Canada (https://open.canada.ca/en/open-government-licence-canada). Updates: officially quarterly, in practice irregular (current: 2026-07-17); check monthly. Approved data end in 2025 (some stations into early 2026), so most stations have no public data from about Jan to Sep 2026: our own real-time archive fills it..