Flooding occurs in both natural and developed watersheds. When the rate of rainfall or snowmelt exceeds the rate of infiltration to the ground, the excess water, called runoff, moves across the ground surface toward the lowest section of the watershed. As the surface runoff enters stream channels, stream levels increase. If the rate of runoff is high enough, water in the stream overflows the banks and flooding occurs. This area of over-bank flow is called the flood plain. All natural watersheds have flood plains. Structures located in these flood plains are subject to damage. In a natural watershed, flooding can be affected by ice jams, the accumulation of debris at channel constrictions, and even the dam-building activity of beavers.

Human activity has profound impacts on flooding. The two major activities which impact flooding are land use change and the building of flood control structures.

Land Use Change- Hundreds of years ago, the Delaware River Basin was covered by forests. This maximized the infiltration of rainfall and slowed the movement of runoff. As the land was cleared for agriculture, infiltration rates were reduced and runoff rates increased. The increase in runoff rates widened flood plains and stream channels in many of the basin's watersheds. With gradual urbanization and the increasing use of asphalt and concrete paving, in addition to densely spaced buildings, infiltration rates were further reduced with corresponding increases in runoff rates. Because of these land use changes, flood flow rates in many areas are much higher than they would naturally be for a given rain storm. Although some land that was formerly in agricultural use has been reforested, the runoff reduction benefits have been offset in many areas by continued urbanization.

The transportation network associated with land use change also affects flooding. In addition to the impacts of impervious paved surfaces, bridges and culverts usually constrict stream channels and flood plains. This aggravates upstream flooding, especially when the constrictions become clogged with ice or debris.

Flood Control Structures- The purpose of flood control structures is to physically constrain or to convey flood waters. Flood control structures include dams, levees, lined stream channels, and storm sewers. Dams and levees have been used for centuries to open flood plains to agriculture and settlement, and in the case of dams, to detain flood waters for gradual release or for use for water supply, recreation, and the generation of hydroelectric-power. Dams and levees are highly effective in flood loss reduction. For information about cumulative flood damages prevented by dams in the Delaware River Basin, visit the U.S. Army Corps of Engineers' Damages Prevented by Philadelphia District Reservoir Projects web page.

Though effective, one drawback to the use of dams and levees for flood loss reduction is that they are very expensive. Secondly, local cost sharing requirements and environmental issues have slowed construction of new facilities in recent years. Flood control dams and levees are not necessary where there is no flood plain development.

The Delaware River has flooded many times in the past. Just one of many examples is the Flood of August 1955 which produced record-high flows for the Delaware River at Trenton, New Jersey. For additional information on historic flooding in the Mid-Atlantic Region, visit the Middle Atlantic River Forecast Center's web site. The Delaware River will continue to periodically overflow its banks for as long as it flows through the basin. With this in mind, it is important that we understand why floods occur and the impacts of human activity so that we can be better prepared for initiating flood loss reduction actions and policy.

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