In the second week of July 2026 the Potomac River reached 96 °F, the warmest reading anyone has on record. Menhaden, striped bass and brook trout washed up dead, along with 126 horseshoe crabs, and scientists named heat, drought and pollution together. But heat does not kill a fish by making it uncomfortable. It kills by changing what the water around the fish can carry.

A polar molecule, and what follows from it

Water's two O–H bonds are covalent bonds, and the oxygen atom pulls the shared electrons much harder than the hydrogens do. That uneven sharing leaves the oxygen end slightly negative and the hydrogen ends slightly positive. Water is a polar molecule, and its polarity governs everything that happens to oxygen in the Bay.

Polarity lets water molecules grab each other by hydrogen bonds, which is cohesion — the same attraction that gives the Bay's surface its surface tension and, out in the marsh grass at the edge, combines with adhesion to pull water up a stem by capillary action. Polarity also makes water an excellent solvent for salts and a poor one for oxygen gas, which is nonpolar. Even at its best the Bay carries only single-digit milligrams of oxygen per liter.

Fresh on top, salt below

An estuary is river water meeting ocean water. Fresh water is less dense than salt water, and warm water is less dense than cold. Through the summer both differences point the same way, so a warm, fresher layer rides on a cold, saltier one and the two stop trading places. Wind ruffles the top without turning the Bay over.

Oxygen only enters at the surface, from the air and from algae in the sunlit layer, so the bottom has no supply of its own. What it has instead is demand: bacteria down there decompose dead algae, and decomposition consumes oxygen. Once the layers separate, the bottom is spending what it cannot replace.

Solubility makes the arithmetic worse. Table 1 shows that cold fresh water at 5 °C holds 12.77 mg of oxygen per liter, while 30 °C sea water holds 5.90. Monitors call water hypoxic below 2 mg/L and anoxic below 0.2. At station CB4.2C in the middle of the Bay, bottom oxygen across the 42-year record averages 2.38 mg/L in May and 0.26 mg/L in July.

2026, a mild year

River flow from January through April ran 32 percent below average and carried 39 percent less nitrogen into the Bay, leaving less fertilizer for the algae whose remains feed the bacteria. Virginia and Maryland scientists forecast a dead zone 31 percent below the long-term average, among the smallest tenth since 1985, and the measurements agreed: early June hypoxia came in at 0.076 cubic miles against an average of 0.85. The layers still separated. Warm water still held less oxygen. What changed was how much there was to decompose.

In autumn the surface loses heat faster than the depths, becomes denser than the layer beneath it, and sinks. The whole Bay turns over, usually overnight, and the bottom breathes again until spring.