Before Your Visit
What is a flood?
Too much water in too little time forces the water into places that are normally dry. Floods are the most deadly of all natural disasters, including tornadoes and hurricanes.
- What has to be in place for a flood to happen (the most important ingredient of a flood)? [Water! A lot of it in a shorter period of time than normal.]
- What can cause a lot of water in one place at one time? [Rain, rapid snow melt, ice jams, etc.]
- What can make flooding worse? [Lack of warning, no plan for evacuation, no emergency services, too much run-off because of erosion, development]
- Floods cause millions of dollars of damage every year in Pennsylvania. How do floods do their damage to people and to towns? [drowning, hit by debris, infected by polluted water; rushing water washes things away, debris crashes into buildings, things get wet that shouldn't get wet, polluted water contaminates furniture, clothing, food, etc.]
- What can make floods less damaging? [build houses and businesses away from the rivers, dredge and widen rivers, build flood control dams and levees, plantings to control run off and erosion, warning systems and evacuation plans]
Pennsylvania has always been especially vulnerable to floods
Ask students: Did you know that Pennsylvania is more prone to floods than any other state? Why do you suppose that is?
Then lead a discussion to elicit some of the reasons. The following are reasons found on various Pennsylvania state sites, the Army Corps of Engineers, and the National Weather Service.
Why Pennsylvania is more prone to floods than other states:
- Our hilly terrain creates thousands of creeks and rivers in every valley.
- Because of all the hills, heavy downpours drain quickly into the streams and rivers before they have a chance to sink into the soil.
- Our soils contain a lot of clay, which absorbs water slowly, making more run-off.
- When clouds coming from the west hit the mountains, the lift they get "squeezes" rain out so that the ridges -- including the Laurel Ridge -- receive more rain and snow than towns farther west receive.
To make matters worse, Pennsylvania suffers more damage when floods do happen:
- Steep hills and easy river transportation makes our flat floodplains the best place to locate towns and factories, which means they are right in the way of floods (that's why they call them floodplains!)
- Do you remember a flood ever occurring in your community? What kind of damage occurs in a flood?
Floods do damage by:
- Producing moving water with tremendous power
- People who have not experienced a flood often don't realize the dangers of moving water.
- The power of the moving water is magnified by the debris that the water carries: trees, vehicles, boulders, buildings, etc.
- Fast-moving water can sweep up everything in its path, throwing it against everything downstream and leaving terrible destruction behind.
- Leaving behind filthy dirt and debris
- The water itself can ruin things, like books, furniture, photographs, and electronic equipment that shouldn't get wet.
- The debris in floodwaters also contain chemicals and disease germs. This means that floods usually pollute drinking water supplies.
- The mud and debris floods leave behind not only cost a lot to clean up, but they are also a health hazard.
Man-made ingredients that can make floods worse:
- Obstructions in the rivers (bridges, natural obstructions like sand, silt, rocks, fallen trees);
- Changes in the shoreline;
- Changes in how fast the water runs off the land by building, mining, lumbering, etc.
Things that change the way water runs off the land:
- Buildings, roads, and development that cut away parts of hills and cover the soil with pavement. Water runs off quickly without being absorbed;
- Taking away vegetation, which absorbs tremendous amounts of water, keeps soil in place, and reduces erosion;
- Conduits that carry water into pipes below roads, etc., can get clogged with debris; without natural banks for extra water to escape, it becomes a dam instead of a conduit; water backs up and causes flooding behind it;
- Draining wetlands leaves fewer places for water to lay while it slowly soaks into the ground.
Overview of the Johnstown Flood of 1889
The worst flood in United States history happened when all the ingredients of floods combined in one big recipe for disaster.
As you read the story of the Johnstown Flood of 1889, make a note of every flood ingredient mentioned.
After reading the background article, discuss:
- Why were the waters already high (higher than anyone had ever seen before) in Johnstown? [very heavy rains]
- What was making the flooding even worse (even before the dam broke)? [clear-cut lumbering, river channel narrowing]
- What was the original purpose of the lake created by the South Fork Dam? [store extra water to release into the Pennsylvania Mainline Canal in Johnstown when the water got low (usually summer)]
Hints from a Modern Topo Map and Aerial Photo
Study the modern U.S. Geological Survey topographical map and aerial photography of the Conemaugh Valley (available in several formats -- see materials list above for links) and find the path of the flood:
- Locate South Fork Creek on the map. Mark which way the water is flowing. How can you tell?
- Find the location of the dam (hint: look for the Johnstown Flood National Memorial). Draw a rough outline of where the lake would have been, using the topographical lines as a guide.
- What path would the water from the lake naturally take when the dam broke?
- Follow the South Fork down to where it empties into the Little Conemaugh River.
- What kind of terrain does the water go through from the South Fork Dam to Johnstown? Where is the river channel widest? Where is it narrowest? What would happen to the water when it reached narrow places? What would happen if a bridge were over the river?
- Estimate the elevation of the South Fork Dam and of Johnstown. Which is highest in elevation? How much higher is South Fork Dam than Johnstown? What happens to water when it drops in elevation over a short distance?
- Find the towns along the rivers. Which towns look safest from floods? Which look to be in most danger? Why?
1889 Maps and Timeline
Maps from an 1889 Atlas and news article can give us more hints about the towns along the flood's path and obstacles it met along the way. Using the Caldwell Atlas maps and the flood timeline:
- Trace the route of the flood from the South Fork Dam to Johnstown.
- Using the timeline, mark on the map the time the flood reached the towns along the way.
- Try to estimate the elevation of each town using the cross section diagram on this map of the floodpath drawn in 1889. Mark the elevation of each town next to the time the flood arrived. On average, how many feet of elevation did the flood drop every minute?
- Using the Scale of Miles on the map, estimate the distance the flood had to travel from South Fork Dam to Johnstown. Estimate its speed in miles per hour. On average, how many feet of elevation did the flood drop every mile?
- What clues do these maps, the aerial photograph, and the timeline give about the ingredients of all of Johnstown's floods, not just the flood of 1889?
You will search for a lot more flood ingredients tomorrow at the Johnstown Flood Museum.
Side Trip: Pennsylvania Mainline Canal and Portage Railroad
The South Fork Dam was originally part of one of the great engineering feats of the early 1800s -- the building of a canal over the mountains of Pennsylvania. The water in its reservoir was pumped to Johnstown when the canal basin there went too low in the summer months. To learn more, visit these sites: