Floods cause bridge collapses in a few different ways. Severe floods can cause rivers and creeks to overflow, picking up debris like trees, cars and parts of houses. When the river passes under a bridge, the high water level smashes the debris into the bridge. If the impact doesn't destroy the bridge immediately, the weight of the piled up combined with the force of the flowing water pushing on it can bring the bridge down. This is what happened to the Conemaugh Viaduct in 1889, when the South Fork Dam in Pennsylvania collapsed, unleashing a massive torrent of water down the Little Conemaugh River [source: NPS].
Flooding can collapse bridges in a far more insidious way -- by gradually wearing away the earth around and underneath the bridge piers. This process is known to bridge engineers as scour, and occurs whenever bridge foundations are placed underwater. The natural flow of the water can produce scour over many years, but bridges are built to withstand that type of erosion. Engineering techniques such as laying riprap, or layers of heavy rocks, can prevent scour. However, floods dramatically increase the force and volume of water affecting the bridge, and the damage to sediments can cause a bridge to collapse immediately or even days or months later. A study by the American Society of Civil Engineers determined that 53 percent of all bridge collapses are caused by flood and scour [source: Wardhana].
The Schoharie Creek Bridge is an example of a collapse caused by flood and scour. The bridge carried the New York State Thruway over the creek. In 1987, spring flooding caused high water levels. This washed sediment out from under one of the bridge piers, causing it to fall into a hole nearly 10 feet (3 meters) deep. Ten people died in the resulting bridge collapse [source: Storey & Delatte].