Using USGS River Data
Anglers, paddlers, and other river users now have access to more real-time information than ever before. The United States Geological Survey (USGS) has provided river discharge and gauge height data for many years from a gauge on the Rappahannock River in Fredericksburg, VA just below Motts Run (maybe you’ve seen the parking spot reserved for USGS staff right at Banks Ford as you walked in to climb Rapp Rocks or drove to Motts Run Landing). In September 2016 they started providing information from additional sensors that will be useful for individuals planning trips on or around the river. The new data includes temperature, specific conductance, dissolved oxygen, pH, and turbidity.
To view data for the past week, check the boxes corresponding to the measurements you want. You can see past data by changing the date range. Be sure to check out other gauges across the country or just upstream in the Rappahannock River watershed.
Let’s take a look at each of these parameters, what they can tell us about the river, and what they mean for our adventures. Note that these graphs show data from September 23, 2016 to September 30, 2016. The first few days show normal, late summer conditions with low, warm water. Between September 27 and 29, nearly 4.5” of rain fell, changing all of the parameters drastically.
Water Temperature is shown in both degrees Celsius and Fahrenheit. The water temperature generally follows the seasonal changes we experience in air temperature, but water doesn’t change temperature as fast as air, so we don’t see drastic daily swings. By knowing the water temperature, anglers will be able to predict fish activity levels and paddlers can bring the proper gear to stay safe and comfortable on the river.
Because water changes temperature slower than air, knowing the water temperature can be important in the spring and fall as air temperature is changing rapidly. We’d all see people lining up to fish on the first warm day of the year, but if the water temperature hasn’t risen, the fish may still act like it’s winter. The water temperature and fish activity are closely related because fish are cold blooded. During the winter and early spring, low temperatures lead to lethargic fish that don’t need to move or eat much. As the temperature (and day length) increase during the spring, fish become more active and prepare to reproduce by spawning. During the summer, high water temperatures can stress fish when combined with low dissolved oxygen. As the water cools in the fall, fish begin to feed aggressively to store enough food for the long winter ahead leading to great fishing opportunities.
Discharge is the total volume of water flowing past the gauge measured in cubic feet per second (CFS). This unit is the standard for measuring and comparing the flow of rivers at different times or places. Basically when it rains, CFS increase, when it’s been dry, CFS decrease. I’d use it to compare to other streams (the Rappahannock River at Fredericksburg has average 1,680 CFS over the past 100 years. Without power diversion, 212,000 CFS flow over Niagara Falls) or if you are used to CFS units, but gauge height may be more intuitive if you’re just getting started using USGS data.
Gauge Height is the depth of the water above the gauge sensor and is measured in feet. On the graph you can see it rise very quickly once the rain begins to fall. These data can be misleading depending on where the gauge is located. If the gauge is in a deep pool, there may normally be a lot of water above it and a high gauge height. If it is in a shallow area, it may normally read a low number. Another consideration is how the width of the river changes. The Fredericksburg USGS gauge is at a relatively wide part of the river. If the river rises 6” there, it may be a much larger rise where the river narrows to run through the rapids just above Route 1. For this reason, gauge height is not comparable between sites, but is only useful to compare between different times at the same site.
American Whitewater considers 2.45’ to be the minimum for paddling the Mott’s Run to Route 1 trip, and anything from 3.5’ to 6.5’ to be barely runnable and should only be attempted by experienced paddlers with proper safety equipment. For anglers, wading at 3’ can be dangerous in some places and you shouldn’t be wading over 3.5’. If in doubt, stay out and be sure to wear you PFD. If the water is much over 3.5’, the turbidity is probably too high to successfully fish anyway.
Note that USGS gauge heights are different from Randy Carter minimum flows often painted on bridge pilings at canoe launches or take-outs.
Specific conductance is a measure of the purity of water. Measured in microseimens, it shows the amount of ions in the water. In the graph you can see it dropping as more rainwater enters the river. These data are less useful for anglers and paddlers, but drastic changes could be indicative of water contaminants.
Dissolved Oxygen (DO) is a measure of the amount of O2 that is in the water, measured in milligrams per liter (mg/l). Dissolved Oxygen is critical for organisms that use gills to respire and levels below 4 mg/l are dangerous. DO levels can drop in hot water because warmer liquids can’t hold as much dissolved gas (think about what happens when you open a warm carbonated drink compared to a cold one) and organisms are more active. If DO levels drop to these levels, consider giving the fish a break or looking for areas with rapids and waterfalls, the turbulence and splashing can increase the DO levels. These levels often drop to dangerous levels in Chesapeake Bay, creating dead zones.
The graph shows a daily increase in the afternoons. This is because of the oxygen given off by aquatic plants as they photosynthesize. As the storm passes, muddy water keeps plants from photosynthesizing as effectively and the extra splashing of flood waters keeps DO in the water.
pH is a measure of the amount of acidity in the water. Lower numbers are more acidic, seven is neutral, and higher numbers are basic. The daily pattern shown in the first few days on the graph is caused by aquatic plant photosynthesis raising the pH during the day.
This is another gauge that is less useful for paddlers and anglers but just like specific conductance, drastic changes can mean a problem for the river. Virginia hasn’t seen as many problems with acid rain as states to the north, but it can cause some issues in headwater streams that flow into the Rappahannock River.
Turbidity is measured in formalin nepholometric units (FNUs) and is basically a measure of the cloudiness of the water. In the Rappahannock River, most of our turbidity comes from erosion of exposed soils on land. When it rains, those sediments get washed into the river, increasing turbidity. Excess sediment is one of the biggest forms of in the Rappahannock River and Chesapeake Bay (along with nutrients) and can cause a slew of environmental problems by blocking sunlight from aquatic plants, abrading fish gills, smothering fish eggs and aquatic invertebrates, and accumulating as mud in slower moving parts of the river.
I’m very excited about this gauge! While fish can be found in high water, muddy water makes it hard to get fish to bite a fly. Fishable water clarity is affected by more than just turbidity, but it can be a good start. For snorkeling in the river, I’d hope to see turbidity around 2 FNU or less (and have a bright overhead sun). I would target water with turbidity less than 25 FNU for flyfishing. Water over 2000 FNU is completely opaque and no light can pass through. You can see in the graph that our clear summer water was less than 5 FNU but the storm water raised it to over 50 FNU.
Below each graph is an option to subscribe to WaterAlert. This allows you to set a certain level for any of the parameters and receive an email or text. This can be useful in many ways. Paddlers may wish to get a warning when the gauge height reaches 4’ and backside is running. Anglers can set a temperature alert over the winter for 50 degrees Farenheit and the shad show up. Let me know what else you set alerts for.
The addition of new data from the USGS gauging station on the Rappahannock River upstream of Fredericksburg, VA will be great for river enthusiasts and adventurers, allowing advanced knowledge of river conditions to plan for a safer and more enjoyable experience on the Rappahannock River.