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.