An early morning rower glides through the glare of the rising sun on the Potomac River on Saturday. (J. David Ake/AP)

The 12-day heat wave that just ended in Washington turned the Potomac River to veritable bath water.

On the river at Little Falls, Sunday’s 93.7 degree reading 4.1 feet from the shoreline is the highest on record in that location by a half degree and almost 10 degrees hotter than average (84.2 degrees). That temperature tops the highest mark of 93.2 degrees set in 2011 and 2012, two of Washington’s hottest summers on record.


The top four years for water temperatures near the middle of the Potomac River at Little Falls. (USGS)

While the nearly 94-degree water temperature measurement is eye-opening, the period of record for these measurements is rather short, dating to 2008. That said, it coincides with five of Washington’s six hottest years on record dating back to 1871.

The Potomac River temperatures observed at other locations were also much higher than normal, but some were not quite as hot as measured in 2011.

During this year’s heat wave, water temperatures in the Chesapeake Bay also surged to abnormally high levels, rising well into the 80s to around 90.

Such high temperatures can be detrimental to water quality and aquatic life. “Warm water holds less dissolved oxygen than cool water, and may not contain enough dissolved oxygen for the survival of different species of aquatic life,” writes the U.S. Geological Survey. “Some compounds are also more toxic to aquatic life at higher temperatures.”

The Environmental Protection Agency notes that increases in water temperatures, expected because of climate change, can also result in increased pathogens, nutrients, invasive species and harmful algal blooms.

As if the air wasn’t hot enough, the steamy water pushed humidity levels through the roof along adjacent shores. The heat index, a measure of how hot it feels factoring in the humidity, topped out as high as 115 to 120 degrees near the water’s edge.

Relentless heat over the past four weeks pushed water temperature this high, culminating in the hottest weather of the year this past weekend.


Maximum temperatures during the recent heat wave. (Ian Livingston/The Washington Post)

Washington hit 99 degrees, while many locations in the region reached at least 100. A small list of the local century club inductees includes Sterling, Oxon Hill and the National Arboretum. Downtown Baltimore also reached 101 degrees, as did The Washington Post’s weather station (on the roof of 1301 K St.). Even areas out toward the mountains were scorching; Winchester surged to at least 100.

From June 25 to July 22, 23 days hit at least 90 degrees. Only 1977 saw more days (24) that hot in that time frame.

For some additional perspective, 23 days at or above 90 degrees in four weeks is more than the most on record for June (18 days in 2010) and in the neighborhood of the July record (25 days in 2011).

The 12 days in a row of 90-degree (or hotter) weather to conclude this stretch, including five days at 95 or higher, was lengthy but three days more than average longest such streak each summer. Longer streaks occurred as recently as 2016 (13 days), and 1980 and 1988 strung together a record 21 straight days of 90-degree weather.


90-degree days in Washington this year. (Ian Livingston/The Washington Post)

Washington has now reached at least 90 on 31 days this year, almost a month ahead of normal. The 31 days ranks as the sixth most year-to-date. Just five more 90-degree days are needed for Washington to match its 1981 to 2010 annual average of 36.

While the next several days should remain below 90 degrees, the odds of such hot weather returning increases this weekend into early next week.