Iowa's flood risk about normal, despite persistent drought
Weather Service: Below normal soil moisture likely to be 'neutralized' by frozen ground
The anticipated drought dividend of greatly reduced flood risk this spring has not fully materialized.
Despite thirsty soil and what until recently had been low stream flows and subnormal snowpack, flood risk in the weeks ahead will be normal to only slightly below normal, the National Weather Service said Thursday in its first spring snowmelt flood outlook of the season.
Winter precipitation has been above normal in Eastern Iowa, returning most streams to normal winter flows, and “below normal soil moisture will likely be neutralized by frozen ground,” said Maren Stoflet, a hydrologist with the National Weather Service in the Quad Cities.
Exceptionally deep frost, in the 15- to 30-inch range, will likely repel most snowmelt and spring rain, leading to increased runoff into creeks and rivers, “even more so than in a normal spring,” said the National Weather Service in La Crosse, Wis., which monitors streams in northeast Iowa.
Soil has been made even less permeable, the Weather Service said, by a condition called “concrete frost,” which occurs when the top several inches of soil soak up moisture from rainfall and snowmelt and freeze solid before the arrival of appreciable snow cover.
Precipitation falling after the soil thaws – typically in mid-March in Eastern Iowa – would be more likely to soak in rather than run off, State Climatologist Harry Hillaker said.
Although much of western Iowa remains extremely dry, the 10-county east-central Iowa region, which includes Linn and Johnson counties, has had a wetter than normal winter, according to Hillaker.
From Dec. 1 through Feb. 25, the region has recorded an average of 5.07 inches of precipitation – almost an inch more than the region's 4.24-inch average for the period, Hillaker said.
Among Eastern Iowa locales, some of the season’s lowest flood risks are on the Cedar River at Cedar Rapids and on the Iowa River at Iowa City.
At Cedar Rapids there is just a 6 percent chance the Cedar will reach even the 12-foot minor flood stage, while the likelihood of the Iowa reaching its 22-foot minor flood stage in Iowa City is less than 5 percent.
Iowa River flood chances are much higher upstream at Marengo (a 49 percent chance of reaching minor flood stage) and downstream at Lone Tree (a 26 percent chance).
Iowa City’s comparatively secure status is due to the water storage capacity of Coralville Lake, according to Jim Stiman, chief of the Army Corps of Engineers’ Water Control Section in Rock Island, Ill.
Coralville Lake is at normal winter pool, 683 feet above sea level, and will be drawn down another 4 feet before March 20, Stiman said.
The Corps had been considering skipping the spring drawdown until winter precipitation somewhat alleviated drought concerns, he said.
Stiman said flood risk on the Mississippi River along Iowa’s eastern border is “a little below normal right now.”
At McGregor, for example, there is a 29 percent chance the Mississippi will reach minor flood stage during the snowmelt period, while the likelihood is 21 percent at both Guttenberg and Dubuque. In a normal year, there would be about a 50 percent chance of reaching minor flood state in each of those three river towns.The Eastern Iowa rivers with the highest risk of reaching minor flood stage are the Turkey at Elkader (52 percent), the North Skunk at Sigourney (50 percent), the Iowa at Marengo (49 percent) and the English at Kalona (39 percent).