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Chapter 1: Terrain, Geography And Flood Risk

The Delaware River Basin and its Flood Potential


The risk of a major flood – an event where rivers and streams rise well beyond their banks – is an ever-present reality along most waterways.

To understand the inherent potential for floods in any given watershed, we need to look at the physiographic features of the watershed. This includes climatology, which looks at the patterns of rainfall, snowmelt, and evapotranspiration.

It also includes the hydrology of the watershed, which looks at the effects of its shape, terrain, soils, and vegetation on streamflow. In addition, we need to consider the influence of human activity in the watershed.

This includes impervious surfaces and development, as well as flood control structures and flood mitigation. The Delaware River marks the boundary of Pennsylvania, New York and New Jersey, and provides drinking water to more than 15 million people. The Delaware is the largest undammed river east of the Mississippi, though it does have tributaries that are dammed.

As we look at the risk of floods in the Delaware watershed, here are some primary questions to consider:

  • Does the watershed lie in proximity to large supplies of moist air and can wind and storm patterns interact with this moist air to produce very heavy rainfalls or snowfalls?
  • Are there any topographical triggers (mountains or regional plateaus) that can significantly increase the intensity of rainfall or snowfall in the watershed?
  • Are there steep valleys and thin soils that can move water downhill quickly?
  • Is there an historical record of large, damaging floods in the region?

When we look at the Delaware watershed, we see all these conditions:

First, this watershed is relatively close to the Atlantic Ocean. Strong low-level easterlies ahead of storms frequently brings in moisture from the Gulf Stream located off the mid-Atlantic coast. In addition, strong low-pressure centers and slow-moving frontal systems can bring in streams of very moist air from the Gulf of Mexico. These situations can produce heavy rainfall or snowfall. We also know from the historical record that this area lies in the path of tropical storms and hurricanes, and some of the biggest floods have come from these types of storms or their remnants.

Second, the topography of the upper watershed is dominated by the high terrain of the Catskills to the north and the Poconos to the west. These are the first major plateaus encountered by moist air that flows from the south or east. Even moist air flowing from the northwest is forced to rise somewhat as it flows over these highlands. Thinking of a layer of moist air as a sponge, it gets squeezed a little harder as it moves over the higher terrain and this makes it rain or snow harder there.

Third elements we look for are steep valleys and streams that can quickly channel the runoff from heavy rainfall into powerful currents. The map on this page clearly shows the upper Delaware watershed is dominated by steep hilly terrain that funnels into narrow valleys, which tend to shed water quickly. These conditions make this area especially prone to severe floods. On top of this, the region has generally thin permeable soil layers that can’t absorb a lot of water. The upper watershed is heavily forested, which helps slow down and absorb runoff during the growing season. Especially in the middle and lower portions of the watershed, development and land use patterns due to increasing population and suburban sprawl have increased the amount of impervious surfaces.

Unless good stormwater management techniques are utilized, this can cause an increase in the speed and amount of runoff to the river, and contribute to local flash flooding and river flooding.

Lastly, this area has a long history of devastating floods. We will be noting several of these events in this series.

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This website was prepared by NNF under the award number NA09NWS4670005 from NOAA, U.S. Dept. of Commerce. The statements, findings, conditions and recommendations are those of the authors and do not necessarily reflect the view of NOAA or the U.S. Dept. of Commerce.