"The Great Johnstwon Flood of 1889 "

Melissa Watson , Undergraduate Geographer/EIUWC Weather Analyst
February 10, 2012


The great Johnstown Pennsylvania flood of May 31 1889 is considered to be the worst flood tragedy in US history. This catastrophic flood had a 36-40 foot wall of water associated with it when it destroyed Johnstown.

In this report I will discuss how this flood happened through a meteorological view point, facts about this flood and what has come about because of this flood. War Department Weather maps will be used in this synoptic analysis.

31 May 1889, A Meteorological View Point

According to the war department weather map for 8 A.M (figure 1) a low pressure system is sitting on top of Lake Ontario. By 8 PM the low pressure has moved over to Lake Michigan and a new low is situated over the North Carolina, Virginia region (figure 2). As the low pressure system moved by, cooler air sank in bringing with it threatening weather and a shift from northwesterly to southeasterly winds. Through the course of the day, large amounts of precipitation fell in eastern portion of Pennsylvania. According to figure 3, Harrisburg Pennsylvania, which is located just southeast of Johnstown, received 4.34 inches of rain by 8 PM on May 31. Looking at the week ahead of the flood, Harrisburg had a total of 1.34 inches of precipitation before the flood event. That means that a total of 5.68 inches of rain fell in Harrisburg that week. With such a high amount of precipitation in such a short amount of time, this raised the risk for flooding. Not only because of the natural tendencies for streams and rivers to flood, but also because the pressure behind the dam was very large.

The Great Johnston Flood Facts

Even though Johnstown received a lot of rain, the cause for the catastrophic flood was actually due to an earthen dam breaking. The dam, formally known as the South Fork Dam, was located 14 miles up the Little Conemaugh River on the side of a mountain. The dam was there to hold a 3 mile long lake, with approximately 20 million tons of water in it, for the use and enjoyment of the South Fork Fishing and Hunting Club. This club had such prevalent members as Andrew Carnegie and Andrew Mellon, Pittsburg steel and coal industrialists. While the club did make minor repairs to the dam, they also made some modifications that eventually had devastating consequences. The members wanted to keep the more expensive fish from escaping their lake, so they decided to install fish screens across the spillway. This caused debris to pill up along with keeping the spillway from draining the lakes’ overflow. They also decided to lower the dam by a few feet in order to have two carriages pass each other. This act resulted in the dam now only being 4 feet higher than the spillway.

Having said that, once the dam broke the water rushed down stream, demolishing Johnstown, and flowing into the Stony Creek River where it eventually pilled debris as high as 40 feet on the Stony Creek Bridge. The water rushed towards Johnstown approximately at 40 miles per hour with the water being described as “The Great Wave” that was 35 to 40 feet high. The Johnstown flood killed approximately 2,200 people and 1,600 homes were destroyed. The highest flood line associated with this flood was at 89 feet above river level.

Affects of the 1889 Johnstown Flood

In July 1992 the National Oceanic and Atmospheric Administration National Weather Service came up with a preparedness guide on floods. Even though this was not a direct affect of the Johnstown flood, it did play a part in the construction of this guide. It is also featured in the historical data portion of the guide, to help shed light on what has happened and what could happen again.

This flood did result in the first expressions of fury towards the giant corporation powers that had formed in the post Civil War period. Eventually Johnstown made a Johnstown Flood Museum that has exhibits about the club and its members. At the Johnstown Flood National Memorial, the reservoir, dam and part of the clubhouse can still be seen.


On May 31, 1889 an earthen dam broke 14 miles north of Johnstown Pennsylvania. Approximately 20 million tons of water came rushing towards Johnstown, destroying the town. The week of May 31, Johnstown had received large amounts of precipitation, the largest amount occurring on the 31. This resulted in a lot of pressure behind the dam, and since the dam was not well maintained, the dam broke on the 31. The flood killed approximately 2,200 people and 1,600 homes were destroyed. After the flood, Johnstown made a Johnstown Flood Museum and a Johnstown Flood National Memorial in honor of the flood.

Figure 1. Surface Chart 8 AM May 31, 1889 Photo from NOAA.
Figure 1.Surface Chart 8 AM May 31, 1889. Photo from NOAA.
Figure 1. Meteorological Data for region May 31, 1889. Photo from NOAA.


References and Further Information

“A Roar Like Thunder,” www.johnstownpa.com/History/hist19.html
Johnstown Flood Museum, www.jaha.org/FloodMuseum/history.html
National Weather Service, www.weather.gov