From the Streets of Washington blog. Full article with many illustrations is at: http://www.streetsofwashington.com/2020/11/the-great-flood-of-1889-in-dc.html
A massive storm system lumbered up the eastern United States from the Gulf of Mexico at the end of May 1889, dumping extraordinary amounts of rain everywhere it went. As it hammered the mountainous region of western Pennsylvania, the sheer quantity of water overwhelmed a poorly maintained dam, causing it to fail and send millions of gallons of water roaring down a narrow valley to destroy Johnstown, Pennsylvania. Over 2,000 lives were lost. While Washington fared far better than Johnstown, the flood of the Potomac river was still the worst the nation's capital had ever seen. A total of 4.4 inches of rain fell, and at the peak of the flood parts of Pennsylvania Avenue were covered with from 1 to 4 feet of water.
The disaster at Johnstown happened on May 31, the second of two days of drenching rain. As usually happens, riverine floods in Washington come about a day after heavy rains, because it takes time for the runoff to fill the Potomac and arrive downriver at Washington. In this case, the floodwaters came on June 1 and June 2, when the Potomac crested at a full 12.5 feet above flood stage.