The snowmelt flooding of two weeks ago was particularly damaging along those rivers and streams flowing into the Ohio River and the Chesapeake Bay. On the Potomac River in Washington D.C., the water level was the fifth highest in the last one hundred years, and this was perhaps the greatest wintertime flood along the Potomac.

One of the remarkable things about this flood was how fast the river waters rose from relatively low levels to near-record levels. Most of the flooding occurred over a two day period. For such rapid flooding to occur, a number of conditions had to come together at about the same time. First, much of the northeast received upwards of 20 in.(50 cm) of snow from Jan. 6-12, and in higher elevations snow totals of over 30 in. (75 cm) were not uncommon. On the 18th and 19th of Jan., daytime temperatures were in the 50s and 60s F(10-15 C), and even nighttime temperatures were above freezing on the 18th. The warm temperatures were accompanied by strong winds and heavy rains and in some cases thunderstorms. From 1-2 in.(40-80 mm) of rain fell in many areas of the Ohio Valley and Mid Atlantic states. This rain fell onto a snowpack that was in the process of melting (there was probably an equivalent of about 2 in.(80 mm) of water already stored in the snow). In addition, fog was prevalent over much of the area as a result of the combination of warm air and cold but melting snow. When fog (water vapor) condenses on the snow, heat is released (the latent heat of vaporization) which accelerates snowmelt. This "hidden" heat cannot be felt, but is released when water changes from one state to another, such as from a liquid to a solid (freezing) or from a vapor to a liquid. Furthermore, strong winds from the south transported warmer air (advection) across the entire northeastern U.S., so that snow was melting simultaneously over very large areas.

The primary reason why melt water reached streams so quickly was because the ground was frozen. The frozen surface enabled the meltwater to run off immediately. Very little water penetrated into the ground. On some rivers the flooding was made worse by the presence of ice jams (on the Susquehanna River, for example). Water moving downstream was blocked by ice that had not yet melted. When the ice jam broke, the surging water rushed downstream. At Port Deposit on the lower Susquehanna River in Maryland, flooding occurred because the Conowingo Dam was forced to release water quickly from the resevoir behind the dam, after an ice jam gave way.

Last week the northeastern U.S. experienced near-record flooding. Contributing to the floods were heavy rains which fell onto deep snowpacks, and temperatures that were in the 50s and 60s (F). What other factors were responsible for the flooding?

2/2/96