Give a fly angler a cloudy day with a slight chill in the air and at least once in the day they’ll think, “Perfect day for fishing.” Sunny, bluebird days are great for baseball games, yard work, or a backyard barbeque with friends. Rain and snow provide the necessary hydrology to recharge our groundwater and feed our streams, however intense or prolonged rainfall or snowmelt can overflow stream banks, fill floodplains and switch water clarity from off color to a turbid mess in a matter of minutes.   

Muddy Water from a Tributary Enters a Main Creek

Water from precipitation impacts the ground and either soaks in or runs off the surface downhill. The steeper and more impervious a surface, the faster the water flows into low lying streams, rivers, and other water bodies. The United States Geological Survey (USGS) maintains over 11,000 gages nationally to help monitor stream flow and keep our communities safe from floods. The website displays data from each of these gages in real time. 

The website provides site information, mapping, and hydrologic data for water depth, groundwater, discharge, precipitation, and water quality parameters. The data is arranged by state and watershed with an interactive map, allowing the user to zoom into watersheds and find specific gages. The legend is color coded to indicate relative streamflow as compared to the statistical analysis of historic records, with red indicating flood levels. 

USGS Stream Flow Data for Maryland

Generally, I will check stream gage levels before I go fishing, especially if there have been recent storms, to educate myself on the flow levels at any stream I want to fish. I will navigate the Maryland or Pennsylvania pages and find the specific gage I want to investigate for stream flows. The specific gage webpage provides an interactive graph of stream depth, stream discharge, temperature, or other water quality parameters per time, and the user can change time duration from seven days to 30 days to a year. The site provides tables of statistical frequency of different flows (above or below average for this time of year or per year), and a map of nearby sampling stations. 

If you are inquisitive like me, you may be asking yourself, how is discharge calculated? At each gage location, a pressure transducer is placed on the bottom of the stream bed, typically encased in an open-ended pipe to protect it from debris and keep it anchored to the bottom. The transducers are powered by a battery or solar array and can measure depth, temperature and turbidity in 5- or 15-minute intervals. Water depth is translated to discharge using the equation Discharge = Velocity X Area. At different times throughout the year, USGS Engineers and Scientists will measure velocity across the cross-section of the channel with uniform distances between measurements. Water depth is measured at each velocity measurement location (using a doppler or rotating cup velocity meter). The area of water at each sampling location is multiplied by the measured velocity and these values are summed to provide the total discharge of the channel. A relationship between depth and discharge is created, known as a rating curve, so for each depth of flow, discharge can be calculated. These values must be verified frequently and can be changed if the cross-sectional shape of the channel changes due to erosion or deposition.   

Measured streamflow is important for Departments of Public Works (DPW), The Federal Emergency Management Agency (FEMA), and State and Local Emergency Managers. They can set depths or stages of concern for residents living near streams and plan road closures, detours, or temporary evacuations if needed. The flood stages are often referred to as Action, Bankfull, Flood, Moderate, and Major. 

As a fly angler, stream gages are also very useful tools. They allow us to know what discharges or stages created muddy conditions and/or unsafe wading conditions. Stream reports from local fly shops will often reference discharges from USGS and they correlate the values with safety and turbidity levels. Creating a mental or written reference on what the fishing conditions are like at specific discharges can help you adjust your fishing at different flows in the future. For instance, I understand that with discharges over 300 cfs at the Gunpowder River, wading starts to get treacherous and streamer fishing will produce the best results. At discharge over 400 cfs, the Gunpowder River is not safe for me to wade. In addition, following a storm event, you can look at gages across the region you are near to see what stream flows are rescinding and rising, allowing you to find streams that are fishable faster, without driving across a state. 

USGS Stream Gages are critical tools for Emergency Management and are very helpful for anglers. I recommend anglers build habits of checking gages before they head out to fish to help you prepare for your time on the water and to reduce the chance of wasting time by driving to a blown-out river. There are several apps for smartphones where you can list your favorite gages and have the data quickly summarized on your phone. 

Keep Mending…

One Reply to “Helpful Tips for Understanding and Reading USGS Gage Data”

  1. This is something I never knew!
    I can see how it would be helpful in deciding when and where it was safe to fish!
    Great tool!

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