Flood — Harrison, Iowa

2011-08-01 to 2011-08-31 · near River Sioux, Harrison, Iowa

Event narrative

Flooding along the Missouri River gradually worsened over the area during June and July and continued into August as record releases from Gavins Point Dam brought widespread flooding along the River. The river near Blair climbed to a little under 33 feet by the end of June, flood stage is 26.5 feet, before falling slightly during July and a little more in early to mid August. However, by late August decreased releases from Gavins Point Dam had allowed the river near Blair to fall to around 28 feet. County roads and highways, some homes and farmland and parts of Interstate 29 were flooded along with several parks and recreation areas near the river. Sand bag operations were used near Missouri Valley, Modale and Mondamin where at least 100 to 200 voluntary evacuations of residences occurred. The flooding forced the post office in Modale to move operations to a nearby town. Large TrapBags placed along Highway 30 west of Missouri Valley to stop flood waters from backing up from a downstream levee breach would trap water from any heavy rain event causing flooding and its temporary closure, including a severe storm/heavy rain event on Aug. 18th. Although river levels started dropping by the end of August, minor flooding persisted into September. The Iowa Farm Bureau Federation estimated that the flood event caused total crop and economic losses of $36.7 million in Harrison county.

Wider weather episode

A record rain event in May in eastern Montana combined with high water from storms in April and May, plus snow melt from a much above normal snow pack, all contributed to bring record high water to the Missouri River chain of reservoirs by late Spring. Then residual snow melt and additional rains produced record 13.8 million and 10.0 million acre feet of runoff above Sioux City in June and July respectively. This helped contribute and sustain record releases from the Missouri River Reservoirs from mid June into early August. Releases from Gavins Point Dam, which is the last in the chain, reached around 160,000 cfs by the middle of June and remained that high into early August before dropping to around 90,000 cfs by the end of the month which help alleviate some of the flooding. The flooding continued into September but moderated greatly. By the end of September the Omaha district of the U.S. Army Corps of Engineers estimated it spent $56 million directly and indirectly in battling the river throughout its share of the basin. In western Iowa 46 miles of Interstate 29 were closed due to flooding and around 250,000 acres of farmland were inundated. The Iowa Farm Bureau Federation estimated that the flood event caused total crop and economic losses of $207 million in southwest Iowa.

View location on OpenStreetMap → (41.8287, -96.1276)

Source: NOAA Storm Events Database, event_id 322994. Narrative written by the NWS forecast office that issued the report.