Blame it on the Ice Age?

Seasonal ice and flooding in the Pittsburgh Region is not just a present-day phenomenon. Floods have been plaguing the region for 20,000 years, since the last Ice Age. That was when the Kansan and Wisconsin glaciers invaded the district and advanced southward.

Prior to the glaciers, the pre-historic Monongahela and Ohio rivers flowed northward into Lake Erie. They were joined northward by three streams that over time merged to become the Allegheny River. After the glaciers advanced into the region, the southern shores of Lake Erie became icebound. The streams and rivers flowing into the lake were forced to find other outlets.

Gradually, huge pools formed at the stream mouths causing back flooding. In the vicinity of what is now known as Moundsville, West Virginia, the Ohio River began flowing towards the southwest. The Monongahela River had always flowed north, but because of the flooding in the Lake Erie area it began to drop its suspended silt into its lower reaches. When the ice cap began to melt and retreat northward, the Allegheny Basin filled with deposits of glacial outwash left by the ice sheet.

These glacial deposits contributed to the present problems of flooding. In some areas, these deposits are over 150 feet deep. The land reacted when the glacial pressure eased, elevating up to 350 feet in the unglaciated areas. This upheaval formed the divide just south of Lake Erie and ever since has caused the drainage of the basin to be southward towards Pittsburgh and the fork of the three rivers.

Flood Stage - 24 feet

The river level at the forks of the Ohio generally hovers slightly above the 15 to 16 foot mark. Flood level at the Point is set at 24 feet. It's normal for Pittsburgh to reach flood stage at least once a year, either as the result of heavy winter snowfall or a nasty summer low-pressure system that dumps an abnormal amout of rainfall along the river basins.

On November 21, 2003, the river crested at 25.86 feet. The result is pictured above. The Mon Wharf and the Allegheny Riverfront are submerged. The stretch of the Parkway East along the riverfront, heading towards the Fort Pitt Tunnels, is under water and the Point is flooded up to the top of the steps leading to the fountain. The outer ring of the fountain is totally submerged and obscured from view, with only the elevated spout above the water level. This is the result of a rather average yearly flood, with the water almost two feet above flood level.

Now try to imagine what the result would be if the river waters rose to the level of 30 feet or more. It has happened over 25 times since the inhabitants of the city began charting the river levels in the 1760s. Several of these floods have been absolutely devastating, with waters cresting above 40 feet twice, in 1763 and in 1936.

The 100-Year Flood and the 500-Year Flood

Flood experts generally describe floods statistically. For example, some flooding is expected to occur on an average of one and a half years. Large, damaging floods occur statistically once every 100 years, and truly devastating floods occur once every 500 years.

In reality, floods occur at very irregular intervals, and the severity of flooding varies greatly as well. In downtown Pittsburgh, where the normal pool elevation of the three rivers is 710 feet above sea level, a “100-year” flood would raise the water level 20 feet or more (35 feet). But downtown Pittsburgh has suffered from at least four “100-year” floods in the 20th century alone, and many others rose to within a foot or two of the classification. The disastrous St. Patrick’s Day Flood of 1936, which peaked at 30.2 feet above normal pool level (45 feet), was a “500-year” flood.

Over the course of Pittsburgh's history, the city has been the scene of 15 floods over the 35 foot level. That's fifteen "100-year" floods in 250 years. Luckily, due to the flood controls in place, the city has only suffered two "100-year" floods in the last 68 years.

Attempts at Flood Control

The development of flood controls on the rivers following the St. Patrick’s Day Flood in March, 1936 has helped alleviate the worst flooding problems along the rivers. For example, record flooding during Hurricane Agnes in 1972 occurred in the upper Allegheny River drainage basin in New York and northern Pennsylvania. However, the severity of flooding was greatly reduced in Western Pennsylvania by the Kinzua Dam in McKean County and the locks-and-dams system along the river. Along the Ohio River at Sewickley the peak flood stage was 10.3 feet lower than that of the 1936 flood. A lot of water went down the river, but not as much as would have if the controls were not in place. Flood stage was also lower during the flood of January, 1996.

Perhaps the most convincing example of the effectiveness of these flood control measures occured during the one-two punch of Hurricane Francis and Hurricane Ivan during the summer of 2004. Francis dumped a record rainfall amount of four inches, followed a few weeks later by Ivan, which surpassed that record by dropping a whopping six inches on the already saturated region. On September 19 the rivers crested at 31 feet. Although surrounding municipalities such as Carnegie suffered catastrophic consequences, the downtown area was spared, relatively speaking. Although the flooding was bad in the city proper, it could have been equal to or worse than the Great Flood in 1936.

As effective as the controls are, however, even they cannot completely alleviate flooding problems. Man-made flood control systems are designed primarily to maintain floodwaters within a narrower area than they would cover naturally. To compensate for that part of the floodplain protected by the flood control system, backup of the floodwaters will occur, thus increasing the elevation of floodwaters upstream.

Despite the dangers to upstream dwellers, the system works. After the flood of 1907, Pittsburgh petitioned the government to begin the system of flood control measures. The plans stalled, and it took the catastrophic St. Patrick's Day Flood 26 years later to spur the government to action. The Pittsburgh Chamber of Commerce estimated that the persistent flooding had caused between $150,000,000 and $200,000,000 in damages to the city.

Within weeks of the disastrous 1936 flood, which also totally obliterated the town of Johnstown, President Franklin Roosevelt signed into law a bill that would provide funds for the systems of dams, locks and reservoirs that provide a measure of safety to the city.

Record Floods Over the Past 250 Years

Other Disasters - Snowfalls, Tornados and Earthquakes

"Let It Snow, Let It Snow, Let It Snow"

It snows in Pittsburgh. That's another fact of life in the Northeast. Sometimes it snows quite a bit. A ten inch snowfall can temporarily paralyze the city, and make life difficult for those who have to clear the snow and return the roadways to normal. The Department of Public Works and PennDot have their work cut out for them, and when they don't respond quickly, residents start to complain.

Snowfalls exceeding one foot can really cause things to grind to a halt, and those approaching two feet can rival the disastrous consequences of the "100-year" flood. Pittsburgh has seen a couple 20+ inch snowfalls in the last century. The Blizzard of March 1993 dumped 24.6 inches of snow on the city, and on February 15, 1940, the city was buried under a 20 inch blanket of fluff.

But, as terrible as these snowfalls were, they could not compare with the grand-daddy of all arctic downfalls, the Thanksgiving Blizzard of 1950. The city was buried under 30.5 inches of snow, and for three days city workers and the National Guard attempted to clear the mess and keep order in the paralyzed streets.

Trolley service ground to a halt on November 25, 1950
as the city was buried by the Thanksgiving Blizzard

Automobile traffic on Brookline Boulevard - November 26, 1950

Twisters and Macro-Bursts

As the clock ticked towards the end of the 20th Century, a new atmospheric phenomenon burst onto the scene, causing further concern for weather weary Pittsburghers. Large and dangerous storms have begun to produce tornados and macro-bursts. Twisters are a fairly common occurance north of the city in places like Butler County, but a Golden Triangle Twister is a true rarity. Prior to 1998, the last recorded tornado in Pittsburgh occured near Lincoln Place back in 1944.

Pittsburgh dodged a bullet back on May 31, 1985, when a line of violent storms with multiple funnel clouds left 65 dead, destroyed 1,009 homes and caused an estimated $375 million in damage in Western Pennsylvania. The town of Wheatland, Pa was totally obliterated by the only F5 tornado ever recorded in the state of Pennsylvania. Another funnel cloud associated with that storm system did touch down in Southwestern, PA, but it hit just north of the city.

Pittsburghers developed a certain sense of protection from the hills surrounding the city. Those hills would act as a barrier and keep Mother Nature's fury away. It was a false sense of security. In the past decade, beginning in 1998, Pittsburgh has been the target of three twisters and a series of macro-bursts after a 54-year calm.

The first occured on June 2, 1998, sixteen people were injured by a twister that touched down west of the city, then proceeded down the path of the Parkway West towards Mount Washington. The tornado soon reached the city, and the hillside was blanketed in a large black cloud. Moments later, when the cloud dissipated, stunned residents emerged to find many homes damaged along Grandview Avenue and the surrounding streets. Roofs and walls were torn from several structures, trees were uprooted, and debris littered the area. Several of the large trees lining Herschel Field were just sheared away, broken in half like twigs. The tornado was the first known twister to breach the city limits, and it registered as an F1 on the Fujita Scale.

Governor Tom Ridge and Mayor Tom Murphy
inspect damage to a home on Mount Washington.

The following links to Post-Gazette articles describe the Mount Washington twister.

<Fronts created Tornado Alley> <> <Residents Weather Stormy Evening>
<Region Torn Asunder By Hard-Hitting Storms>

<><><><><><><><><> <><><><><><><><><> <><><><><><><><><>

On June 1, 2002, Kennywood Park in West Mifflin was the scene of a macro-burst that ripped the roof off of the pavilion covering The Whip, a ride in the Lost Kennywood section, hurling the structure onto a crowd of frightened onlookers. The powerful storm produced wind gusts registering over 80 mph. One person was killed and over 50 injured in the tragedy. Several trees in the historic Trolley Park were damaged or destroyed. The fierce storm also damaged buildings in the Lawrenceville, Homestead, East End and Bloomfield areas.

Firefighters inspect storm damage in Kennywood Park.

The following links to Post-Gazette articles describe the Kennywood macroburst.

<Kennywood Park Witnesses Describe Chaos at Whip Ride>
<Woman Dies in Kennywood Collapse as Fierce Storms Tear Through Region>

<><><><><><><><><> <><><><><><><><><> <><><><><><><><><>

A year later, on June 12, 2003, a thunderstorm over Pittsburgh produced funnel clouds. The vortex travelled from Greentree to Mount Washington, as seen from Flagstaff hill in Schenley Park. The following day the National Weather Service confirmed that the funnel cloud was indeed an F-0 tornado with 75 mph winds, the first tornado in Pittsburgh in five years. The rare twister is shown in the photos below.

The following links to Post-Gazette articles describe the June 2003 tornado:

<Tornado-like Winds Fell Trees Throughout Region>
<Storm Damage Wasn't Too Bad, Considering It Was A Tornado>

<><><><><><><><><> <><><><><><><><><> <><><><><><><><><>

Most recently, on August 9, 2007, a storm system blew through Pittsburgh and produced multiple funnel cloud sightings, accompanied by an 85mph microburst in the Uptown district. In Sheraden, near Chartiers Avenue, trees were uprooted and projectile damage was done by an F-0 tornado. The twister was confirmed the following day by meteorologists from the National Weather Service. Read more about the latest twister in the following Post-Gazette article:

<It's Official: Tornado Hit Sheraden>

"Did Somebody Say ... Earthquakes ???"

And last, but not least, who ever thought that the city of Pittsburgh was in an earthquake prone area? Well, it is not. But, on March 1, 1935 the city did experience a slight tremor. That's one for the record books.

The Calm After The Storm ... A Rainbow Over Pittsburgh

Not all inclement Pittsburgh weather is bad, and
sometimes a spring rain can have fascinating results.

* Compiled from multiple sources - December 2004 *
Last Updated: October 23, 2007