Flooding in the Susquehanna River Basin

The Flood-Prone Watershed
The Susquehanna basin is one of the nation's most flood-prone areas. Additionally, the main stem Susquehanna River is more prone to ice jams and subsequent flooding than any other river east of the Rocky Mountains.

The basin's topography and geology and nearly 30,000 miles of streams are some of the contributing factors. The following are two distinct ways that the basin's topography and geology can cause flooding.

The first situation occurs when a section of river is very wide, but then is suddenly squeezed into a steep, narrow gorge. During heavy rainfall events or when the winter ice begins to break up, the increased flow of water or ice backs up in the narrow gorge, causing the river to overflow its banks. Also, when the ice jam breaks, a sudden surge of water can cause downstream flooding.

The second situation occurs when a river flows through an area with very little slope, and shallow banks. In this topography, which is fairly common in the basin, the river levels out and flows slowly. During heavy rainfall events, the river quickly swells and overflows its banks. When winter ice breaks up, the slow-moving flow causes the ice to jam easily, creating obstacles and backing up the water.

Flood Loss Reduction
Of the 1,400 communities in the Susquehanna basin, about 1,160 have residents located in flood-prone areas. Roughly 30 percent of the basin's population lives along major rivers.

While no one can prevent floods, the resulting damages can be reduced through: (1) proper planning to avoid building in flood-prone areas; and (2) flood management and protection.

Due to the diverse conditions and flood-prone nature of the Susquehanna basin, flood management programs work best when structural and nonstructural measures are combined.

Structural Flood Control
Structural flood control devices include:

- dams and reservoirs
- floodwalls and levees
- channel excavation and modification

Flood control dams and reservoirs store significant amounts of floodwater to reduce or prevent downstream flooding. Floodwalls and levees prevent floodwaters from inundating designated areas. These structural devices substantially reduce the basin's average annual flood damages.

Large-Scale & Flash Floods
The people of the Susquehanna River Basin have long experienced large-scale floods of devastating proportions. The American Indians first told of serious floods occurring about every 14 years along the Susquehanna River. Since the early 1800s, the main stem Susquehanna River has flooded on average once every 20 years.

The Susquehanna basin also is vulnerable to frequent, localized flash floods every year. These flash floods, usually affecting smaller tributaries, can occur with little advance warning.

Large-scale and localized flash floods both cause significant property damages and often result in the loss of lives. In June 1972, Hurricane Agnes caused the worst recorded flood in the Susquehanna basin - 72 people were killed throughout the basin and damages were estimated at $2.8 billion.

At the time, the Agnes flood was the nation's most destructive and costly natural disaster. Other years when large-scale, devastating floods hit the basin include 1865, 1936, 1955, 1975, and 1996.

Nonstructural Programs
Nonstructural flood protection programs include:
- flood forecast and warning systems
- flood insurance
- relocation
- flood education and training
- floodproofing
- flood plain management

Nonstructural flood measures can be extremely cost-effective. Among the most cost-effective is the Susquehanna Flood Forecast and Warning System. The flood damages the system prevents annually have averaged 12.5 times the cost of operating it. As shown below, radar and a network of rain and stream gages provide the data that are used to forecast river levels and issue more accurate early flood warnings. Early warnings give people and businesses time to secure their property and get themselves out of harm's way.