LITTLE FALLS, MARYLAND — Water temperatures in the Potomac River north of Little Falls, Maryland reached record highs in early July, preceding a large fish kill that spanned approximately 14 miles. A biologist from the Maryland Department of the Environment recorded a water temperature of 94 degrees in the river on July 4.

USGS data indicated that water temperatures in the same area peaked at 98.4 degrees. This recorded temperature marks the warmest ever at this location since record-keeping began in 1988, according to the FOX Forecast Center. Officials stated that the fish kill extended between Whites’s Ferry and Violette’s Lock in Montgomery County, and most of the deceased fish were golden redhorse suckers.

Scientists determined that extreme water temperatures created stressful conditions for the fish, leading to the high number of deaths. Warmer water contains less dissolved oxygen, which reduces the amount of oxygen available for fish to survive. The Maryland Department of the Environment reported no evidence of a chemical spill or pollution in the area where the fish kill occurred.

Scientists believe the fish kill resulted from a 'summer turnover' event, during which warmer surface water and cooler lower water rapidly mix. Authorities noted that most of the dead fish washed ashore near Sycamore Island, located south of Glen Echo Park.

Washington, D.C., experienced three consecutive days of record-high temperatures during this period. On July 4, the city recorded a new Independence Day high temperature of 103 degrees. The fish kill took place upstream of a previous Potomac Interceptor failure, which released millions of gallons of raw sewage into the river.

Why It Matters

The record water temperatures and subsequent fish kill in the Potomac River highlight the ecological impacts of extreme heat events on aquatic environments. Such events can reduce dissolved oxygen levels, threatening fish populations and potentially affecting the broader ecosystem. The absence of chemical pollution points to temperature as a primary factor in this incident. Understanding these events is part of ongoing efforts to manage water quality and ecological health in the region.