The sun poked out at times Wednesday, signalling that the worst of the flooding is over.
“We do have additional rainfall in the forecast over the next few days, but it is not going to be anything like we’ve seen so far. The heaviest rain is over at this point, and from here we look for rivers to be receding over the next few days,” said Jeremiah Pyle, a meteorologist for the Weather Service in Portland.
Although some comparisons were being made to the historic February 1996 flood, river levels, rainfall and the extent of flooding this week “paled in comparison” to the damage and significance of the storm nearly two decades ago, Pyle said.
That storm was so severe in part because the warm, rain-laden storm system fell on and melted an ample snow pack, making runoff rage into rivers. The equivalent of about 40 inches of rain ran off the June Lake area near Mount St. Helens in that earlier event, for example.
Comparisons aside, the storms that just passed made for a significant weather event. Longview had nearly 5 inches of rain over the past three days, and Castle Rock had more than 6 inches. Rainfall in the south Cascade Range, which drains into the Lewis, Cowlitz and Kalama rivers, was staggering. Sheep Canyon on the west slope of Mount St. Helens had nearly 13 inches.
Swelled by the runoff, the Cowlitz River crested slightly above 52 feet at Castle Rock Wednesday morning, nearly matching the peak level recorded when mudflows from Mount St. Helens sluiced through the river on May 18, 1980. However, it fell more than 3 feet below the record crest set during the 1996 floods. Hydrologists are tentatively calling this week’s storm a 15-year flood on the Cowlitz, one with a 15 percent of happening annually.
The river rose faster and higher than hydrologists had expected, even though Tacoma City Light had reduced flows out of Mossyrock and Mayfield dams upstream a day earlier. Tacoma maintains flood storage space behind Mossyrock Dam, the state’s highest. Nearly the full amount of flood storage space — an amount set by the utility’s federal license — was still available Wednesday despite the heavy rains of the past half week.
There would be one other lingering down side to the storm: Scientists who monitor the Cowlitz noticed a large amount of volcanic sediment washing out of the Toutle and into the Cowlitz, said Diana Fredlund, a spokeswoman for the U.S. Army Corps of Engineers in Portland. Hydrologists will have to assess if it significant amount settle in the Cowlitz and diminished the water-carrying capacity of the Cowlitz.