By Jeff Zogg, National Weather Service
Quad Cities, Iowa / Illinois

The spring of 2008 brought widespread flooding to much of our region. In fact, this flood event was our region’s worst flood event since the spring of 1997. Although the flood of 2001 brought higher crests at the Mississippi River, that event was mainly confined to the Mississippi River itself. This spring, however, the Mississippi River, as well as many of its tributaries, saw major to near-record flooding. In addition, ice jams led to record flooding on the Rock River in Illinois early in the season.
What caused the flooding?
The combination of heavy rainfall and snowmelt produced the flooding. Snow pack across our region was much above normal. Some places reported record snow depths. In addition, much of our region saw heavy rainfall in April. Since our area rivers typically see their snow melt-related crests in April as well, the heavy rainfall combined with snowmelt to produce widespread flooding.

Many locations in eastern Iowa into southwestern Wisconsin saw April rainfall totals of six or more inches, which was 150 to 400 percent of normal.
How severe was the flooding?
As stated earlier, this flood event was the worst such event in our region since 1997. While many rivers experienced flooding, the Mississippi River, as well as tributaries in east central Iowa, were hit the hardest. The table shows the preliminary crests for selected locations on our area streams.
It is interesting to put into perspective the amount of water involved during the height of the flooding. In the Quad Cities, when the Mississippi River crested at 19.2 feet, the volume of water passing by was 236,000 cubic feet per second. At that rate the Mississippi River would be able to fill two and two-thirds Olympic-size swimming pools every second, or over 160 Olympic-size swimming pools every minute. It is also important to note that some locations saw prolonged flooding. One extreme example was the Rock River near Joslin, Ill. The Rock River was above flood stage there from March 3 through May 16, or for 74 consecutive days.
How well did the spring flood outlooks predict the flooding?
This spring we issued three spring flood outlooks. We issued them on Feb 22, March 6 and March 28. The March 28 outlook called for a 50 to 60 percent chance of the Mississippi River reaching flood stage upstream of the Quad Cities, and a 70 to 80 percent chance downstream. The outlook called for varying chances of the tributaries reaching flood stage. In addition, our outlooks called for a higher than average risk of flooding.
Upon considering the fact that the entire Mississippi River as well as many of its tributaries not only reach flood stage but also experience major to near-record flooding, one may conclude that our last outlook under-forecast the rivers. That is, the outlooks assume near normal temperatures and precipitation during the outlook period. Both temperature and precipitation are important in the spring because 1) temperature affects the rate and amount of snow melt; and 2) precipitation can have a significant impact on the severity of flooding. If temperatures or precipitation are not near normal, then the actual risk of flooding may be different than what our outlook indicates.
In the case of this spring, temperatures were nearly ideal and minimized the impact of snowmelt on our rivers in spite of the record snow pack that existed. Precipitation, on the other hand, was much above normal. In fact, some locations reported record rainfall amounts. It was the heavy rainfall that led to the severe flooding. If precipitation had been near normal, then most locations would have experienced minor flooding at worst. Due to the heavy rainfall, however, many locations experienced major flooding.
Another way of looking at this situation is that the snowmelt alone plus normal rainfall would have led most area rivers, including the Mississippi, to crest near flood stage. The differences in river levels between flood stage and the observed crests were due mainly to the heavy rainfall.