St Marys River At Great Mills flow report
As of July 13, 2026, St Marys River At Great Mills is flowing at 66 cfs with a gage height of 2.21 ft, rising 448% over the past 24 hours. Source: USGS gauge #01661500, refreshed throughout the day.
Historical Data
St Marys River At Great Mills at a glance
How St Marys River At Great Mills is running right now, where it sits on the map, and the key gauge stats.
St Marys River At Great Mills is flowing at 66 cfs, with the water sitting 2.21 ft at the gage. Flow is up 448% since yesterday — a rising hydrograph.
This is USGS gauge #01661500 in Maryland. Over the past 10 days the average has been 28 cfs, peaking at 104 cfs.
Over the next 5 days, St Marys River At Great Mills is expected to recede from today's 12 cfs, toward roughly 8 cfs by 2026-07-17 (likely range 3-25 cfs) -- drier than normal for the date.
For real-time updates and historical context, see the realtime view or the historical comparison. Browse other gauges in the Maryland flow report.
Streamflow Forecast
Powered by PULSE — Snoflo’s Predictive Unified Learning & Simulation Engine, which learns from how this river has answered every past storm, snowmelt, and dry spell to forecast where it’s headed with a precision generic models can’t match.
How does this compare to past years?
Year-over-year overlay, annual peak discharge, the full distribution of daily flows on record, and the gauge's rating curve.
Weather Forecast
Next 5 days, hour by hour
Temperature line with weather symbols on top, snow + rain accumulation as columns, humidity as a dotted line.
5-day forecast table
Every 3 hours, broken out across temperature, snow, rain, humidity, and wind.
| Time | Condition | Temp (°F) | Snow (in) | Rain (in) | Humidity (%) | Wind (mps) | Wind dir |
|---|---|---|---|---|---|---|---|
| Loading detailed forecast… | |||||||
15-day forecast
Daily temperatures, snow, and rain projected over the next two weeks.
St Marys River At Great Mills
Marys River at Great Mills, MD streamgauge measures the water flow of the St. Marys River, which is primarily fed by groundwater sources and precipitation. The river runs through several tributaries, including the lower Patuxent River and the Potomac River. The flow of the river tends to be highest during the winter and spring months due to increased precipitation and snowmelt. The river is also influenced by the nearby Great Mills Dam, which was constructed in the early 1900s to provide flood control and power generation. Interestingly, the St. Marys River is known for its high quality water, which supports a variety of aquatic life, including blue crabs and striped bass.
Nearby streamflow levels
Cross-check St Marys River At Great Mills's discharge against nearby gauges to spot whether the change here is local or regional.
| Gauge | Streamflow |
|---|---|
| St Marys River At Great Mills | 66 cfs |
| St Clement Creek Near Clements | 38 cfs |
| Zekiah Swamp Run Near Newtown | 32 cfs |
| Piscataway Creek Near Tappahannock | 12 cfs |
| Chicamacomico River Near Salem | 4 cfs |
| Mattawoman Creek Near Pomonkey | 6 cfs |
Plan a trip
Reservoirs, boat launches, river runs, and fishing spots within driving distance of St Marys River At Great Mills.
Boat launches
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River levels & flood safety
- Read the level before you go
- A river that's runnable at one flow can be deadly at another. Check current discharge and gage height — like the values shown above — against the flood-stage thresholds, and remember levels can spike fast after rain or a dam release.
- Respect cold water
- Snowmelt rivers run cold even in summer. Sudden immersion triggers cold-water shock and saps strength within minutes. Wear a PFD, dress for the water temperature (not the air), and never wade or paddle alone.
- Watch for swiftwater hazards
- Strainers (downed trees), undercut rocks, and low-head dams are the deadliest features on moving water. High, fast, muddy water hides them. If in doubt, scout from shore and portage.
- Mind flash floods & releases
- Narrow canyons can flood from a storm miles upstream, and dam-controlled reaches can rise without warning. Know the forecast, the release schedule, and your exit before you launch.
Track St Marys River At Great Mills in the Snoflo app
Save this gauge as a favorite, set push alerts when streamflow crosses a threshold (e.g. "alert me when St Marys River At Great Mills crosses 5,000 cfs"), and Snoflo's iOS app will push the moment USGS reports the crossing.
About St Marys River At Great Mills
Where does the streamflow data for St Marys River At Great Mills come from?
Discharge, gage height, and water temperature come directly from the USGS streamflow gauge 01661500. Snoflo refreshes the time series throughout the day. Forecasts come from the NOAA / yr.no feed Snoflo's iOS app uses.
How often is the report updated?
USGS gauges report continuously (typically every 15 minutes). Snoflo pulls fresh values throughout the day — look for the "as of" timestamp on the streamflow hero card.
What's the difference between discharge and gage height?
Discharge (cubic feet per second, or cfs) is the volume of water flowing past the gauge each second. Gage height is how high the water sits at the gauge (feet). They're related by a rating curve specific to each gauge — higher water means more flow, but the exact ratio depends on channel shape.
How is "percent of median" calculated?
Today's discharge is compared to the historical median discharge on this calendar day across the gauge's full record. 100% = right on median; 200% = a very high year; 30% = a drought-level low.
What are flood stages, and is this river safe right now?
Flood stages are NWS-defined gage-height thresholds — Action, Minor, Moderate, Major — marking when nearby roads or floodplains start to be affected. "Safe" depends on your activity and skill: a level that's a fun paddle for an expert can be lethal for a wader. Always check the current level against the thresholds above and the safety links, and when in doubt, stay off the water.
Can I get alerts when St Marys River At Great Mills rises?
Yes — flow alerts are managed in the Snoflo iOS app. Favorite this gauge, set a streamflow threshold (e.g. "alert me when discharge crosses 5,000 cfs"), and you'll get a push the moment USGS reports the crossing.
Access the free St Marys River At Great Mills report
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