River '99

March seems too early to be thinking about floods, especially when the weather turned cold and dropped a few snow flurries on us today. Not like the three feet of snow that Rochester, NY, got in the last week, but just enough to color the lawns out away from the trees. And enough to keep the gates locked at the Simplot Sports Complex, with signs saying No Play or Practice until further notice.

I didn't complain that I couldn't go out and run around with wet feet and wet hair in almost 40 F weather, for the first soccer game of the "spring" season. Next week will be soon enough.

But hey, crossing over the Fairview bridge the other day, we noticed that the Boise river was high and mighty, covering the gravel bars and the ankles of lots of trees and shrubs. In March?!

Last May was wet, this winter we've got mountains loaded with snow. Maybe, just maybe, the river masters will risk annoying irrigators this year, and keep as much space in the reservoirs as possible to prepare for the snowmelt. That is, maybe they'll err on the side of flood control for a change.

But a couple other things seem certain: the local media will report in astonishment on the flooding as if it were a force of nature, rather than an output of engineering. And as soon as the water recedes, developers will be developing just as close to the flotsam as they can get.

On tonight's 10:00 news, there was a nice video bite of some newly scraped land out in Eagle (that's just downriver from what the Boise River watermeisters care about) that was a little closer than planned to the river, now that it had been turned up. Ah, did we say that was out of the flood zone? Hmm, maybe you'd better wait a while before going ahead there.

Not like I'm laughing at misfortune or anything, but it cracked me up. It really does take the water crossing a property line around here to get anyone's attention. "Ah, we think you might be in the flood zone, now." But it'll probably dry out by July or August, and I'm sure there'll be plenty of willing buyers by then. Maybe some tarps and a few discarded concrete blocks could be fashioned into a little construction levee, so that we don't fall too far behind schedule...

Looks like a countdown to June to me, and a lot of folks who aren't going to be wanting to see that first heat wave.

I wrote up occasional reports on the river's doings from June to December last year, which, like most, was relatively uneventful, but with moments of nervous uncertainty.

Further reading (and pictures and charts!)

The Water Supply outlook, as of February 1, 1999, shows the Weiser, Payette and Boise River basins above average, but not all that much above average. The range is amazing to consider - everywhere from 2 or 3" as of February, to 10 times that much. Average is 20, we're about 24. In the early and late going, the "maximum" (observed?) is about twice the average. In the peak - April - it's about 160%, eyeballing their nice graph.

The USGS has Real time gaging station reports, with graphs going back a few days. On the 10th, you could see how they dialed the river up, midday on the 4th, 5th and 6th of March, from 5000 to 5500, then 6000, then 6300 cfs, and from 9.4 feet to 10.1 feet "above datum." This link is for the station at the Glenwood bridge, downriver from most of Boise, but still above many new subdivisions, and of course, Eagle.

http://idaho.usgs.gov/swdata/current/boise.htm has an index of gaging sites, and more general information.

This USGS site currently defines the flood threshold at 7000 cfs, with the March 10 flow at 6300 cfs. Given the development and vegetation growth along the channel, 7000 may be optimistic in general, and its certain to be optimistic in particulars. Controlled flow leads to vegetation growth, and vegetation growth reduces the carrying capacity of the river; less flow is required to cause flooding.

Restudy of the Boise River Flood Plain in Ada County, with the objective to "update the floodplain and floodway boundaries" for 26 miles of river from Barber Dam to Star, along with 6.5 miles of channel around the south side of Eagle Island. Good map of the study area.

The last "flood insurance study" (FIS) was conducted in 1978! The US Army Corps of Engineer defined flood stages in 1978 as 10-year flood, 7,200 cfs; 50-year flood, 11,000 cfs; 100-year flood, 16,600 cfs; and 500-year flood, 34,800 cfs. If you're still with me, I assume this is more than dry (ouch!) statistics for you. This 50-year flood would be an incredible event for the city. Such a 100-year flood would likely take out more than one bridge. 30,000 cfs happened in recorded history, but not since the big dams were put in. Knowing the river as I do (that is, as a close acquaintence of only 16 years), it seems incomprehensible to me.

They're in process of completing the work, this fiscal year. Their photo album shows the happy project workers gathering the data.

The Bureau of Reclamation offers a "teacup diagram," showing current storage reservoirs' status, with areas used to show their capacity, and current level. Streamflows are tabulated on the graph by gaging stations. (BIGI is the Glenwood bridge.) Varying the displayed width of the streams to show the flow would be pretty cool, but maybe tough to program. The wide blue line cutting across the lower left is the Snake River.

The Sacramento Bee's report, "Superfloods threaten West," quotes Susan Stacy, former city planner and author of the definitive history of the Boise River in this century, "When the River Rises":

"The National Flood Insurance Program actually invites development into the floodplain," she said. "It facilitates it. In Boise, we didn't have development in the floodplain until we had the federal flood insurance program."

The political counterpart to the mechanical cycle of flood control, vegetation growth, reduced capacity is insurance, development, disaster.


Tom von Alten      tva_∂t_fortboise_⋅_org

Monday, 09-May-2005 11:57:24 MDT