pH, Dissolved Oxygen, and What Kills a River

In August 1969, the Cuyahoga River in Cleveland, Ohio caught fire. A river burned. It wasn't magic — oil and industrial waste floating on the surface ignited from a passing train spark. But the deeper story is that the water underneath was already dead: almost no dissolved oxygen, choking pH swings, and no fish for miles. When scientists say a river is 'healthy' or 'sick,' they don't guess. They measure specific numbers — chemical parameters — that tell them exactly what the water can and cannot support. A trout needs different water than a catfish. A number below one threshold means life; below another, it means suffocation.

By the end of this lesson you will be able to: - Define pH and read a value on the 0-14 scale, explaining what acidic and basic mean for aquatic life. - Explain dissolved oxygen (DO), why it matters, and how temperature changes how much oxygen water can hold. - Describe how nutrient pollution triggers eutrophication and oxygen crashes. - Interpret a set of real water-quality readings and decide whether a stream is healthy.

pH measures how acidic or basic water is on a scale from 0 to 14. A pH of 7 is neutral (pure water). Below 7 is acidic; above 7 is basic (alkaline). The scale is logarithmic, so pH 5 is ten times more acidic than pH 6, and one hundred times more acidic than pH 7. Most freshwater fish thrive between pH 6.5 and 8.5. When acid rain (often around pH 4.2 in polluted regions) drains into a lake, it can push the water below pH 5. At that point trout eggs fail to hatch and aluminum leaches out of soil into the water, damaging fish gills. Entire lakes in the Adirondacks went silent this way in the 1970s and 80s before emission laws helped them recover.

Fish don't breathe the oxygen atoms locked inside H2O. They breathe O2 gas dissolved in the water, pulled across their gills. Dissolved oxygen is measured in milligrams per liter (mg/L). Healthy cold streams carry about 8-10 mg/L. Below 4 mg/L, sensitive species like trout begin to suffocate; below 2 mg/L the water is called hypoxic, and most fish either flee or die. A key twist: warm water holds less oxygen. Water at 5 degrees C can hold roughly 12.8 mg/L of oxygen, but at 25 degrees C it holds only about 8.2 mg/L. This is why a heat wave, or warm water discharged from a power plant, can quietly asphyxiate a river.

When fertilizer runoff or sewage adds extra nitrogen and phosphorus to water, algae explode in number — an algal bloom. It looks like green paint on the surface. The problem comes later: the algae die, and bacteria decomposing them consume enormous amounts of dissolved oxygen. This chain — nutrients, bloom, die-off, decomposition, oxygen crash — is called eutrophication. The Gulf of Mexico grows a 'dead zone' each summer, fed by nutrients from the Mississippi River. In 2017 it reached about 8,776 square miles, roughly the size of New Jersey, with too little oxygen to support most marine life.