|OMG, a gigantic river! The Umpqua River below Roseburg, Oregon|
noted in yesterday's post, the river reached flood stage last night, but just barely, rising a foot above the minimum considered to be a flood. But the amount of water! At the peak, the discharge at Elkton a few miles downstream was 131,000 cubic feet per second!
Here's why this is so ridiculous to me. In 1997 the Tuolumne River, which flows near my home, experienced its greatest flood in recorded history, with a peak flow of about 70,000 cubic feet per second. It could be such a rarity at that level that another such flood has but a one in three hundred or four hundred chance of happening in any given year. In other words, quite probably not in my lifetime or yours. And that extraordinary flood was barely more than half what I saw today on our journey down the Umpqua River to the coast.
I just don't often see rivers this big, or filled to this level. So my two hour drive turned into a four hours as we stopped to take pictures and stare at the flooding river.
A flood hydrograph, like the one above, is the tool that hydrologists use to analyze river flow and to predict possible flooding. It takes some time for rainwater and snowmelt to get into channels and flow downstream. By comparing the pattern, timing, and amount of precipitation in a given storm compared to storms of the past century (most U.S. rivers have been monitored for that long), they can predict hours in advance how high the floodwaters will reach, and for how long. For major rivers with huge drainage basins (like the Mississippi River), the warnings can be days ahead of the flood.
The Umpqua River flows into a large estuary at Reedsport, Oregon, and just upstream is a Roosevelt Elk viewing area. I would never have noticed such a thing on any other day, but the highway was actually a flood protection levee that prevented the river from flooding the meadows where the elk hang out. Driving along, I could see the Umpqua River just a foot or two below the level of the highway on the right, and the elk refuge on the left, about six or seven feet below river level! There was certainly some ponding as can be seen in the picture above, but if the highway weren't there, the entire scene above would be under at least four feet of water.
So this was a minor flood in the big scheme of things, barely rating mention in the local media. If you wonder what would really get their attention, how about the greatest flood ever recorded on the Umpqua River? It began on this very day fifty years ago, and the amount of water reached at least 260,000 cubic feet per second, about twice the flow of today's event. It's hard to even imagine.
So...your desert-living geoblogger got to get all fascinated about a river today. More rain is expected this week. If anything interesting happens, I'll let you know, and all my Pacific Northwest readers can quietly chuckle at my amateurish enthusiasm for BIG rivers.