OHD - Dis_Svy: Red River Flood 97 Event Overview

EVENT OVERVIEW

Floods on the Red River of the North occurred in the context of unusual conditions that led to serious flooding throughout much of the upper Midwest. Beginning in the fall months in 1996, much of the region received substantial precipitation in late October and November. For many areas, this amounted to 4 or more inches above normal. Much of the precipitation occurred as rainfall during October and early November. The soil profile was generally wet to 5 feet even though it was a dry summer; the rains that occurred during November were very efficient in increasing the soil moisture since most, if not all, the rain that fell was absorbed by the soil. Following the wet fall, the north-central U.S. experienced horrific conditions over the winter of 1996-97. Blizzard after blizzard during the second half of November through January built up an enormous snowpack; many areas had more than 100 inches of snowfall. These amounts were as much as 2-3 times the normal annual snowfall. Although February and March were quite dry, frigid conditions throughout much of the winter ensured that as much as 10 inches of snow water equivalent remained on the ground at the start of the spring melt period.

Early March brought below normal temperatures, delaying the onset of snowmelt. By mid-March, the snow line had moved north to the vicinity of the northern Iowa state line. Mid-March saw significant rises and ice jams on the Redwood and Cottonwood Rivers in Minnesota. Additionally, the Blue Earth and Le Seur Rivers began rising, resulting in flood stage being reached on the Minnesota River at Mankato. As this flow from the tributaries of the Minnesota River continued, the Minnesota River rose above flood stage at all gaging locations.

Significant melt of the deep snow cover started with particularly warm conditions at the end of March and into early April. At this time, many rivers in South Dakota, southern Minnesota, and southern North Dakota were rising, in some cases well above flood stage. Conditions changed dramatically over the weekend of April 5-6, with heavy rain in the areas already experiencing melt, followed by more blizzard conditions that brought a foot or more of snow to the northern portions of the Red River of the North Basin in the U.S. This event increased the amount of water destined to flow into rivers by 2-3 inches and all but cut off melt due to temperatures dropping well below freezing; the second week in April saw temperatures that averaged as much as 20 degrees below normal. The cold temperatures effectively halted snowmelt runoff, with shallow and/or slowly moving water freezing rapidly--in some cases, seizing anything that remained, including cars and some livestock. However, the cold did not halt flood flows already moving through rivers in southern Minnesota, eastern South Dakota, and southeastern North Dakota. In addition to the havoc wrought by the snow and cold on transportation and, in some cases, on power distribution, sandbagging became extremely difficult. The reduced runoff and freezing as well as the added snow water equivalent significantly complicated the forecast problem. While temperatures moderated in mid-April, they continued below normal until April 17 when they rose rapidly to above normal with no overnight low temperatures below freezing. Within a week after the early April blizzard, melt resumed, with additional rises on most of the area's rivers and streams. Some of the most serious flooding in the U.S. occurred in the Red River of the North Basin within North Dakota and Minnesota--the area of coverage of this report. (Serious flooding also occurred in nearby areas of southern Minnesota, particularly on the Minnesota River, on the James River in eastern South Dakota, and on portions of the Mississippi River.)

Flooding on the Red River of the North in 1997 established (twentieth century) records at most locations and was particularly devastating in the towns of Grand Forks, North Dakota, and East Grand Forks, Minnesota. The summary below provides a comparison of differences in the new flood records with previous floods of record (from the twentieth century). With the exception of Grand Forks, which exceeded the previous (1979) record by over 5 feet, observed crests at forecast locations on the Red River of the North were within approximately 2 feet of the previous records. Estimated damages for the complete event, including all United States portions of the Red River of the North, totaled approximately $4 billion, of which $3.6 billion occurred in the immediate vicinity of Grand Forks / East Grand Forks. No deaths directly attributable to the flooding resulted from this event.

Table 1. Red River of the North Flood of 1997 Summary.

* Wahpeton, North Dakota, at the southern end of the Red River of the North, established a new record on April 6, then another crest at or above this on April 15; the high water mark from these two crests is 19.44 feet.