What is the principal cause of flooding along Northwest rivers?
Many factors affect flooding; however, the principal cause of flooding in Western Washington is intense rainfall associated with storm systems originating over the Pacific ocean. Typically from October through March, low pressure systems form in the Aleutian area of the Pacific and draw warm moisture laden air from the tropical areas into the Pacific Northwest. This warm moist air has come to be known as the "Pineapple Express" as its origin can often be traced to the vicinity of the Hawaiian Islands. Heavy rain occurs when the warm moist air is cooled and lifted as it passes over the Olympic and Cascade mountains.

What are some other factors which affect flooding?
The list is long. There are meteorological variables such snowpack, temperature, precipitation distribution and the intensity and orientation of the jet stream. It includes many natural factors such as soil permeability, initial soil moisture content, ground slope, ground cover and basin size and shape. The activities of people such as paving, clearcutting, road building, farming, stream channelization, and levee and dam building all have effects.

Does rain melting the snowpack have a major effect?
Rain is the major source of runoff in virtually all floods west of the Cascade mountains. Snowmelt is a very significant factor in most of the larger floods. However, it is warm temperatures and very high freezing levels (8000-12,000 feet) during the "Pineapple Express" events that melt most of the snow. Contrary to popular belief, the energy content (heat) in rain is not sufficient to melt large amounts of snow during a flood event.

What effect does development and clear cutting have on flooding?
In areas that are clear cut or covered by impervious surfaces the volume of runoff tends to be larger and the peak of the runoff is higher and occurs more quickly than would be the case in a totally natural basin. The effect is proportionally greater in small to medium size flood events. In severe flood events such as occurred in November 1995 and February 1996 rainfall is so intense that all portions of the basin tend to contribute runoff regardless of the level of development, so the percentage of increase in runoff attributable to development is reduced.

Were the floods of November 1995 and February 1996 made dramatically worse by clear cutting and development?
It is extremely difficult to quantify the increase in flooding attributable to clear cutting and development. Most of the severe flooding in Western Washington occurs near the mouth of rivers having drainage areas of hundreds of square miles. Most of the runoff is the result of intense precipitation in the mountainous areas that are essentially undeveloped. Clear cutting is a factor and clear cutting increased significantly in the 1970s and '80s. The effect of clear cutting on runoff from a clear cut area diminishes over time as the ground cover and forest is reestablished. Also, as was stated earlier, proportionally the effect of clear cutting is greatest in small to medium size flood events. The preponderance of evidence is that the increase in flooding in most basins last year attributable to development and clear cutting was small.

Is the frequency and magnitude of flooding in Western Washington increasing?
Certainly the flooding last year was unusually severe. In some basins, for example the Green and Puyallup, two floods occurred each of which exceeded all previous floods in the basins. This is an extremely unusual occurrence. However, in those same basins no significant floods occurred in the years of 1991 through March 1995. Some statistical evidence indicates that the magnitude of floods is increasing. However, because of the natural variability of rainfall and runoff, it is very difficult to prove conclusively that an increase has occurred.

What was the effect of the flood control dams on the floods last year?
Flood control dams prevented catastrophic flooding in the Puyallup, Green and Skagit basins. Damages downstream of the Corps of Engineers Mud Mountain dam on the White and Puyallup rivers and Howard Hanson dam on the Green river were very minor. Damages prevented on the White/Puyallup are estimated at $146 million and $268 million on the Green. The combination of Seattle City Light's Ross dam and Puget Sound Power's Upper Baker dams prevented catastrophic levee overtopping in the Skagit basin and damages prevented by the dams exceeded $100 million.

What is the effect of levees on flooding?
Levees can be very effective at containing a river and preventing flooding in low lying areas which would otherwise be flooded. However, they can aggravate flooding at downstream locations by increasing the rate at which the flood wave passes downstream and reducing the natural reservoir effect that occurs when a river overflows its banks and routes some of its flow to overbank storage areas.

Is dredging of river channels an effective way to diminish flood damage?
In most cases dredging a river channel is not a practical solution. In most cases it must be done regularly over relatively long stretches of a river to significantly reduce the flood level in a river. Most rivers in Western Washington move large volumes of sand, gravel and other sediments. It is quite possible for a river to refill most or all of a dredged channel during the next significant flood. The cost of periodic dredging and disposal of dredged material often make dredging prohibitively expensive. The economics of dredging are much improved if a buyer, such as a concrete producer, can be found for the dredged material.