A fertilizer spill in Iowa this month wiped out much of the aquatic life across a 60-mile stretch of rivers in two states, officials said, leaving an estimated 789,000 fish dead in one of the region’s most ecologically devastating chemical spills in recent years.

A Missouri official who surveyed the damage said that the banks of the Nishnabotna River had been lined with fish carcasses, and that dead fish were visible through the water.

“I refer to this one as ‘the big one,’” said the official, Matt Combes, an ecological health unit science supervisor for the Missouri Department of Conservation. He added: “Calling something a near-total fish kill for 60 miles of a river is astounding and disheartening.”

While fish kills on that scale are unusual, smaller kills are common. Comparing the scope of fish kills across different states is difficult because of limited data and tracking, experts said.

The latest die-off started, Iowa officials said, when a valve was left open over a weekend on a storage tank at NEW Cooperative, an agricultural business in Red Oak, in southwestern Iowa. The Iowa Department of Natural Resources, which learned of the spill on March 11, said this week that 265,000 gallons of liquid nitrogen fertilizer spilled into a drainage ditch and into the East Nishnabotna River, which flows into the Nishnabotna River and then the Missouri River.

Iowa officials estimated that more than 749,000 fish died in that state. Most of them were small species, such as minnows and shiners, but thousands of larger fish, including catfish and carp, also perished. Mr. Combes, the Missouri official, estimated that around 40,000 fish died in his state. He said he saw large catfish dead, as well as shovelnose sturgeon.