![[Visit Client Website]](/img/gsa/banner.jpg)
Paper No. 1
Presentation Time: 8:05 AM
RESPONSE OF THE NORTH FORK TOUTLE RIVER TO NOVEMBER 2006 RAINFALL AND FLOODING
Heavy rainfall over a 3-day period from November 5-7, 2006, caused major flooding along many rivers in the Pacific Northwest, including the N. Fork Toutle River near Mount St. Helens, WA. Weather stations in the vicinity of Mount St. Helens reported 24-hr precipitation totals of more than 25 cm, and 3-day totals of more than 50 cm. The streamflow response to precipitation and runoff was highly variable in this region; for most rivers with more than 10 years of gage record, the 2006 peak discharge was less than 2 times the mean annual flood, and the return period was less than 20 yr. Locally, however, the response to flooding was spectacular, particularly in channels draining the northwest side of Mount St. Helens. Measurements taken before and after the 2006 flood at 15 different localities in the headwaters of the N. Fork Toutle River document continuous incision along the upper 15 km of the channel, followed by continuous aggradation for at least another 10 km downstream. Field evidence suggests that headwater incision rapidly followed at least one, if not two, debris flow(s). Measurements of the channel indicate that there were no major changes in reach-average slope at individual localities, thus incision was not accompanied by major changes in grade. However, the channel formed by the 2006 flood is, on average, 3 times the width and 2 times the depth of the pre-existing channel, and the bed material is now somewhat finer than before. Preliminary estimates of flood discharges were made for each locality by coupling equations for continuity and flow resistance, with a criterion for bank erosion. These estimates were then compared with values of the mean annual flood predicted from a regional hydrologic relation. The results suggest that the magnitude of the 2006 flood decreases systematically downstream in relation to the mean annual flood, which is consistent with the observed pattern of erosion and deposition along the N. Fork Toutle River.