Portneuf River May 14 Update

 So whazzup with the so-called Portneuf River flooding this spring.  Well, in a word: Nothing.  We thought we were staging to see a classic flood in a small watershed's little river.  Nothing of that sort happened....at all.  Yes, the Portnuef technically rose into so-called moderate flood level.  However, as you can see from the Pocatello Greenbelt webcam below, that's not exactly what we normally consider a "flood" in the traditional sense of the word.  

The Portneuf came up to a peak level of 10.54 feet and has been flated pretty much ever since...going on maybe two weeks now.  Looking at the USGS webcam, we can see the river rise or fall a tenth of an inch now and then.  real scintillating stuff, eh?

We first became interested in the Portneuf along about the tip of the red arrow.  Since then we've realized there's absolutely no sense of urgency to show up at any particular time to see the Portneuf in "flood."  It's going to look just about the same every day.  And it's going to stay at this level for awhile.

You can see that the flow forecast shows the Portneuf rock solid at its current (sic) level./

This is Mount Putnam as seen from the south side of Idaho Falls about 35 miles away from out camera.
There is still a lot of snow left on Mount Putnam.  (The black line is an irrigation pivot pipe.)

The top of Mt. Putnam as seen in our photo is Point "X" on this map.  The Upper Portneuf Watershed is enclose in the approx. red boundary.  The Ghost town of Chesterfield is at point "I".

This map clip shows Putnam, Chesterfield and Lava Hot Springs (Point "N"). We're hoping to finally do a short Road Trip down thataway this week, maybe leaving Tuesday and returning Friday. Our main goal is to soak in Lava Hot Springs.  However, we will definitely take time to drive as far as we can into the Upper Portneuf and see what can be seen.  It's all a "learning experience," right? RIGHT!

Portneuf River May 4 Update

Flat line flood...

Technically, the Portneuf River entered into "moderate" flood stage during the last couple of days. However, the river's rise has been almost imperceptible to the unaided human eye.  Over the next 2-3-4 days, The Portneuf will rise perhaps a half a foot to reach a flat line peak.

This is our first flat line flood.  We're accustomed to floods which produce clearly visible peak flows.  Some floods rise quickly to produce a steep inverted "V" peak.  Other floods produce a slightly rounded peak before flow recession begins.  The Portneuf is educating us on the flat line peak.

When we first realized The Portneuf was going to flood we fully expected to see a rounded peak flow event.  That was before we learned about the unique characteristics of the Portneuf and its major tributary, Marsh Creek. And that was before we learned how a 15,000 year old event continues to play a hand in behaviors of The Portneuf River and Marsh Creek.

Indeed, the epic Bonneville Flood literally laid the groundwork for how The Portneuf River handles spring runoff into the urban Pocatello area.  It's all quite fascinating.

The Bonneville Flood was an ancient catastrophic floods that occurred approximately 14,500 yrs. ago, the result of catastrophic failure of one of the natural dams at Red Rock Pass of Pleistocene Lake Bonneville. The probable peak discharge of the flood was approximately one-third cubic miles per hour (15 million cubic feet per second).

Pleistocene Lake Bonneville covered most of Utah and parts of Idaho and is estimated to have been over twelve times greater in size than the modern Great Salt Lake. When the dam broke, the flood waters involving approximately 4,750 km3 of water, traveled at an estimated 45 miles/hour, carrying 935,000 yds3 of water/sec. eroding the Snake River Canyon to over 500 ft. deep and over a mile wide in some places.

The catastrophic Bonneville Flood was a depositional event in Marsh Valley.  Significant amounts of gravel eroded from alluvial fans that formed the threshold for Lake Bonneville at Red Rock Pass were deposited in Marsh Valley during the Bonneville Flood.  The Booneville Flood flow also made certain its channel upriver from modern Pocatello was practically as flat as could be.

As a result, spring runoff now collects and ponds up in the old Booneville Flood channel. The channel gradient is too faint to send the water any where in a hurry. Likewise, the giant sponge of alluvium before the channel surface is soaking up much of the runoff volume.

So THAT's why it is a flat line flood.  Now, mind you, there have been some humdinger BIG floods come crashing through Pocatello such as the 2,990 cfs peak on February 14, 1962.  Most of those events were heavy rain-on-snow with runoff flowing across solidly frozen ground.  That makes a HUGE difference in the way this game is played.

Although we plan to be at the USGS Pocatello gaging station Sunday, we won't know whether we caught the peak flow until we study the data records later.  For all practical purposes the Portneuf River will look the same Sunday as it did Saturday or it will on Monday.  It's a flat line flood, you see, so forget fluvial fireworks!

Below are some Booneville Flood graphics to help you understand the Portneuf River's watershed in relationship to the flood flow.

The Great Salt Lake is a mere puddle in comparison to ancestral Lake Booneville!

The shaded green area shows the Booneville Flood channel.

Lake Booneville was about half the size of the state of Ohio!

Nice colored graphic showing extent of Booneville alluvium.

The speckled area is the Booneville Flood channel.  It's little wonder such an epic event continues to exert a profound influence in how the Portneuf and Marsh Creek respond to typical spring runoff regimes.

Portneuf River May 2 Update

Slow & Steady...

The Portneuf River's 2023 runoff rise has been very slow but very steady.  Generally, it has risen about 100 cfs per day.  As of May 1 there were no notable impacts from the river's elevated levels.  Both the Northwest River Forecast Center and the National Weather Service continue to peg a Portneuf peak around May 6-7, a few days later than once expected.

So far, Pocatello People are taking the river's rise in stride, as shown by the photo above.  The net flow through Pocatello is definitely damped down by the ancestral Bonneville Flood channel farther upriver.

In this April 30 photo it's easy to see how that old Bonneville Flood channel is capturing the inflow.  There certainly does not appear to be enough runoff capable of causing a classic flood surge through the area.  Interestingly, the same wide, flat areas were credited with damping down the historic 1962 Flood.
That 61-year-old mid-February event wrecked havoc in Lava Hot Springs.  In fact, the hot springs facilities themselves were so damaged the facility had to be rebuilt.  The Idaho Transportation Dept. photo below shows just how vigorous was the 1962 Portneuf flow through Lava Hot Springs.

Not only that but US 30's crossing of the Portneuf floodplain failed.  River flow at nearby Topaz ballooned from about 2,000 cfs to nearly 7,000 cfs in 1.5 hours!  However, that 7K cfs never made it to Pocatello.  The wide parts of the old Bonneville Flood channel captured the flow surge and damped it down to "only" 2,990 cfs at Pocatello.  Of course, 3K cfs was a new all time record and caused considerable damage in the Gate City.  (See additional notes at end of this post.)

The proven "damper" effect upriver from Pocatello is probably the primary reason that both the Northwest River Forecast Center AND the National Weather Service are projecting an extended flat-line peak for the Portneuf River.  However, at 3:49 AM Tuesday,  NWS Pocatello's Area Forecast Discussion stated: "We have had significant snowmelt from well above normal temperatures the last couple of days. These continued warm temperatures will continue. More significant is the expected 

precipitation on Thursday and Friday. Currently the forecast is for 0.25 to 0.75 inches with up to 1 to 2 inches of rain possible. Any areas getting the high end of rain will see even more significant snowmelt, and rivers in these areas will rise even more. The Portneuf and Big Wood rivers are rivers that could see the most significant rises and corresponding impacts."

Even with the possible prospect of potentially significant rain-on-snow, it is unlikely that the Portneuf would see a classic flood surge through urban Pocatello.  Would the river rise?  Certainly it would!  However, the rise would be dampened by the de facto upriver water storage in the wide areas of the old Bonneville Flood channel.

Barring a long duration, high volume rain-on-snow event we believe both the NWRFC and NWS flow forecasts are well constructed and accurate at this time.  The big difference between the 1962 Flood and 2023's runoff is timing.  The 1962 event took place when the Portneuf's watershed soils were thoroughly frozen to an depth estimated by USGS of three feet.  A large rain-on-snow event brought down historic amounts of runoff in a short period of time.  Today, the ground is not frozen and snow melt is proceeding in an orderly manner.

As a result of all of the above, we have delayed out Road Trip to Portneuf Country until May 7.

Above is a 1962 photo of the breech of US 30 downriver from Lava Hot Springs.  Below are excerpts from a subsequent USGS Report that described the circumstances and impacts of th4e 1962 flood event:

"Figure 10 shows a break developing in a high road fill on U.S. Highway 30. The pipe through the fill is in the left background. Flow through the break in right background is around a cattle underpass. Note the vortex above the highway on the left. When the fill went completely out, discharge at the gaging station at Topaz, 2 miles downstream, increased from 2,850 cfs to 6,140 cfs in 1.5 hours. The surge overtopped railroad fills, canal headings, and roads downstream and caused extreme damage."

"Lava Hot Springs was hard hit by the fast- flowing Portneuf River, and damage to buildings, roads, and railroads in the resort town of 5 93 inhabit ants was estimated at more than $1 million. Figure 9 shows the river rampaging through Lava Hot Springs. Water can be seen flowing down the main street in the background. The Union Pacific Railroad and roads were damaged severely from the raging waters between Bancroft and Pocatello."

"Practically the entire town of Bancroft was under several feet of water for days. Flow from the surrounding hills and lowlands flowed into the town much faster than the drainage channels carried it away. Residences, commercial buildings, livestock, highways, railroads, and stocks of grain, feed, groceries, and other commodities were damaged considerably."

Sources: 

Greenbelt walker & old Bonneville Flood channel photos:
https://www.facebook.com/NWSPocatello

1962 flood photos and report excerpts:

https://pubs.usgs.gov/circ/1962/0467/report.pdf

Portneuf flow forecasts:
https://water.weather.gov/ahps2/hydrograph.php?gage=pihi1&wfo=pih
and
https://www.nwrfc.noaa.gov/rfc/

To read our initial post about the Portneuf see:
https://waterandweatherwatching.blogspot.com/2023/04/portneuf-river-spring-2023.html