T he James River will be a popular place to cool down during our hot summer weather, but the water will be warming up, too.
The temperature of the river doesn’t rise and fall as quickly as the air temperature does, but they do change in tandem.
Over the past week, the James River has been in a relatively mild 70- to 75-degree range. It’s likely to warm up once we get several days of hot weather next week.
The nearest real-time temperature observation comes from the U.S. Geological Survey station in Cartersville, about 45 miles upstream from downtown Richmond.
“The water temperature measured at Cartersville is typically representative of the temperature you would expect in the James through Richmond,” according to John Jastram, a local USGS hydrologist.
The USGS periodically takes samples of the water at the Boulevard Bridge to see how it compares with the Cartersville measurements. The temperatures are a close match despite the distance.
During the course of a year, the James River cycles between an average peak water temperature of 91 degrees in the summer and a minimum of 33 degrees in the winter, but it zigzags along with the weather patterns.
“You see pretty rapid responses, but the water is buffered somewhat,” Jastram said.
During a typical day, the water temperature usually fluctuates by just a few degrees.
On a summer afternoon, the water is usually somewhat cooler than the air. The opposite is often true at night.
Sometimes the difference between air and water temperature is negligible. When the weather suddenly turns hot, the water can be 10 or 15 degrees cooler than the afternoon high.
Since the USGS started measuring the river temperature at Cartersville in 2003, the warmest reading was 97 degrees on July 22, 2011. That day, Richmond’s official high temperature was 102 degrees.
During intense cold spells, like the one in Feb. 2015, the surface of the river can freeze.
The warmth of the water affects the behavior of the fish that live in the river. The spring warmup dictates the timing of the shad run. Warm summer temperatures mean that there’s less dissolved oxygen in the water, which can make fish less active.
In 2014, Jastram and another researcher found evidence that the rivers and creeks in the Chesapeake Bay watershed have warmed up over the past several decades. As the climate continues to warm, that trend would alter the balance of nutrients and pollutants in the bay and shift the range of aquatic species.
If you do plan to go for a swim soon, remember that life jackets are required when the level is above 5 feet at the Richmond-Westham gauge. According to the National Weather Service forecast, that reading will hover near the 5-foot level through Saturday, but may drop slightly by Sunday.