Forks residents to purchase flood insurance, but only a thousand of the 52,000 residents did so. Later, researchers at the University of North Dakota determined that although more than 90% of residents knew about the option of purchasing flood insurance, only a small percentage purchased it since they believed the National Weather Service had projected that the river would not crest high enough to warrant a flood. On March 31, the North Dakota National Guard used helicopters to dump sand on ice covering the Red River in an attempt to use the sun's thermal energy to thaw the ice and prevent ice jams. The "dusting" was the most extensive attempt in North Dakota's history. Starting on April 3 and continuing every day up to the flood, sandbagging and dike-building began. Residents, local university students, and Grand Forks Air Force base personnel placed about 3.5 million sandbags at various points around the river. On April 16, at a public meeting, Grand Forks mayor Pat Owens recommended that residents voluntarily evacuate the city. The next day, about 500 airmen and women from Grand Forks Air Force Base were assigned to assisting with monitoring the dikes around the river. The flood hits The flood in Fargo The Red River crested in Fargo at on April 17, but dike-building efforts were able to prevent the water from flooding into a majority of the city. An emergency dike was built and volunteers had to add sandbags to increase the height of a dike that had settled. The mayor of Fargo at the time, Bruce Furness, later stated: "We came very, very close to losing parts of Fargo...it took a lot of effort by a lot of people—a great many of them high school and college students—to save the city. I was very proud of them. I think it was one of Fargo's finest moments." The flood in Greater Grand Forks There was some sense of imminent threat in Grand Forks, but the cities could not prepare for such an enormous flood. The National Weather Service (NWS) had a long-standing forecast for the river to crest at 49 feet (14.9 m), which was the river's highest level during the 1979 flood. The cities had been able to get their dikes to this level, but the river continued to rise past it, to the astonishment of the NWS (which did not upgrade its forecast until April 16,