DISTRIBUTION OF MAMMUTHUS AND ERRATIC FINDS RELATIVE TO THE SIZE OF ICE AGE FLOODS IN SOUTHEASTERN WASHINGTON STATE

The number, frequency, and relative size of Ice Age floods in southeastern Washington State have been the subject of much recent debate. Two independent studies―one on the distribution of ice-rafted debris and another on the distribution of mammoth (Mammuthus) finds―provide supporting evidence that most of the so-called Missoula floods were substantially smaller than the largest of these floods.

Repeated cataclysmic floods that crossed southeastern Washington State encountered a hydraulic constriction at Wallula Gap, causing the floodwaters to backup and form temporary Lake Lewis. Erosional trim lines and spillover channels indicate the largest of these temporary lakes reached an elevation of about 1250 ft in the Pasco Basin. These floodwaters carried with them a tremendous amount of debris including icebergs containing exotic rocks (erratics), soil and vegetation ripped up by the floods, and animal carcasses.

Detailed mapping of over 2000 ice-rafted erratics and bergmounds on a portion of Rattlesnake Mountain, found erratics at a maximum elevation of 1,189 ft, while the predominance of ice-rafted debris rested between an elevation of 600 and 1,000 ft. The mean elevation for bergmounds was found to be about 800 ft, while the elevation of erratics was bimodal with a major peak at an elevation of about 700 ft and a lesser peak at an elevation of about 1,000 ft.

Recent efforts to catalog the distribution of mammoth finds in southeastern Washington State have identified 24 locations in the Lake Lewis area where mammoth skeletal elements have been found in Ice Age flood deposits. Of these, the maximum elevation of these finds was approximately 1190 ft (just outside the Pasco Basin), while almost all other sites are at elevations below 1000 ft. The median elevation of these sites is 607 ft.

Benito and O’Conner (2003) calculated the discharges of 25 floods downstream of Wallula Gap and found that most of these floods were relatively small, with discharges between 1 x 106 m3/s and 6 x 106 m3/s, well below the maximum flood discharge of 10 x 106 m3/s through Wallula Gap. The distribution of ice-rafted debris and mammoth skeletal remains at elevations well below the maximum Lake Lewis levels supports the contention that most floods were considerably smaller than the largest floods.