I've posted a couple photos below from Nick's trip along with photos taken two weeks ago when I spent a morning hiking around Sentinel Gap and the Mattawa boulder bar (field) on my way north to hike near Frenchman Coulee.
The area I know as "Mattawa Bar" is part of a much larger bar known by two names - Wahluke Bar or Priest Rapids Bar.
With Sentinel Gap in the distance, Ellensburg IAFI Chapter President Nick Zentner describes how Ice Age Floodwaters moved through the Othello Basin. Photo by William Meyer-Ellensburg.
Westbound traffic on I-90 gets a good look at Sentinel Gap while descending to Vantage Bridge.
Below Sentinel Gap is a huge boulder field that I enjoy visiting every couple of years. A few ice-rafted erratic boulders sit among thousands of basalt boulders. As the floodwaters flowed through Sentinel Gap into the Pasco basin (Lake Lewis), the velocity decreased and the largest bedload material settled out.
Not sure what the story is with the big erratic ... maybe rafted to a point in or near the main Columbia channel above the gap and then tumbled through the gap during one of the later floods or rafted to this exact spot?
Google Earth view showing Sentinel Gap and the huge boulder field. The big erratic I'm standing by in previous image stands out in this shot ... light colored boulder (bottom-center).
The boulder field west of Mattawa is pretty cool and I feel worth inspection by those interested in Glacial Lake Missoula and the Ice Age Floods but ... the Ephrata Fan below Soap Lake is even more impressive!
Floodwaters moving through the western channels of the Ice Age Floods region were forced to flow around the Saddle Mountains. Water moving down or over Lower Crab Creek Coulee would have been forced to flow in an easterly direction when Sentinel Gap began to restrict the flow.
"Sentinel Gap represents a water gap where erosion by the Columbia River was able to keep pace with folding, faulting and uplifting across the Saddle Mountain anticline. During Ice Age floods this opening was repeatedly reamed out, which probably widened and steepened the walls of the gap. ... If and when floodwater flow ever exceeded the capacity of Sentinel Gap, the floodwaters would automatically self-adjust, sending more water east to Othello Channels to establish equilibrium."
The sand found on lower slopes of the east bluff makes a great playground for ORV riders. If you want to make the short hike to the top of the east bluff, try for an early start. It gets a little noisy around here later in the day.
One of the things I enjoy about hiking around Vantage is the variety of user groups drawn to the area. You don't run into many people, but all seem to appreciate the unique landscape. In addition to the ORV riders you might get the opportunity to visit with hunters, fishermen, boaters, bird watchers, horse riders, rock climbers (that take great care of the Frenchman Coulee area ... THANKS!!!), para gliders and others.
I hiked up the sandy draw on the left, then descended through basalt chute at right. Small pieces of petrified wood and nice views. Talus below chute no fun, best bet up and down sandy draw or perhaps the road on south side.
Links to HUGEfloods.com trip reports for area hikes.
1. John Wayne Trail
2. Ginkgo Petrified Forest State Park Part I
Ginkgo Petrified Forest State Park Part II
3. Lower Crab Creek Coulee
Crab Creek is known for its meandering course through the Columbia Basin. The creek makes one more big "S" (cutting through the Beverly Bar) before joining the Columbia River north of Sentinel Gap. Wildlife that enjoys the streamside habitat in lower Crab Creek Coulee can thank the Columbia Basin Irrigation Project for the year-round flow.
In 1881 (October), Lieutenant Thomas W. Symons noted:
"Crab Creek discharges no water into the Columbia-at this time of the year, at any rate."
Link to Symon's 1881: Report of an Examination of the Upper Columbia River
Symons marked Sentinel Gap as "Sentinel Bluffs" on his map. The first mention Sentinel Bluffs I'm aware of was by Alexander Ross in 1811.
"On the 20th we left the Priest Rapids, and proceeded against a strong ripply current and some small rapids, for ten miles, when we reached two lofty and conspicuous bluffs [Sentinel Bluffs], situate[d] directly opposite to each other, like the piers of a gigantic gate, between which the river flowed smoothly.
Here we staid for the night, on some rocks infested with innumerable rattlesnakes, which caused us not a little uneasiness during the night..."
Alexander Ross 
Several early explorers of the Mid-Columbia region mentioned the healthy rattlesnake population found between Priest Rapids and Sentinel Gap. I didn't cross paths with any snakes as I hiked on this cool October morning, but did meet up with one later the same day while hiking the south rim of Frenchman Coulee.
I'll borrow a photo from that hike and plug him in here along with a short video (below) of the same snake as he tried to let me know that I should just move along.
Click to view short video of Frenchman Coulee rattlesnake.
"...Rattlesnakes are very numerous. At times they may be heard hissing all around, so that we had to keep a sharp look-out to avoid treading on them; but the natives appeared to have no dread of them. As soon as one appears, the Indians fix its head to the ground with a small forked stick round the neck, then extracting the fang or poisonous part, they take the reptile into their hands, put it to their bosoms, play with it , and let it go again."
Alexander Ross 
Washington State Department of Transportation engineers aren't the first to have issues moving traffic between the Columbia River and the steep basalt cliffs at Sentinel Gap.
Palmer found Sentinel Gap to be an obstacle when attempting to establish a Wagon Road between Priest Rapids and Canadian mines. He was forced to unload the wagons and use two canoes side by side to ferry each wagon around the steep rocky bank to Crab Creek. Boards were laid down in the canoes to support wagon wheels. Oxen were driven along a narrow Indian trail through the rocks.
Faint strandlines are visible on both sides of the river at Sentinel Gap.
While hiking down from the east rim, I watched load after load of apples move through the gap. The Mattawa area is known for quality vegetable and fruit production.
Huge boulders have been pushed to the edge of vineyard along SR 243. Sentinel Gap in the distance.
The The Wahluke Slope became Washington's eighth appellation in 2006. Clifton Hill Vineyard has a nice group of boulders displayed along the highway.
I hope the guy living here is excited about the Ice Age Floods. Those are some nice flood-tossed boulders in the front yard and the backyard should be in a geology text book.
Good luck digging a post hole out here. I guess just putting the post in a drum and filling it with basalt will work.
Tales of early steamboats on the Columbia are pretty wild. For many years Priest Rapids created enough of an impediment to prevent the boats from reaching Sentinel Gap and beyond.
Image from Hanford Site Historical Photo Gallery
To support passage of early steamboats on the Snake and Columbia rivers, anchors were often fixed to allow crews to line their vessels through difficult sections. As far as I know, any anchors used for this purpose in the Priest Rapids area are now underwater behind the dam. Photo above was taken on a Snake River boat trip in Hells Canyon where several of the anchors remain.
Randall V. Mill's book "Stern-Wheelers Up Columbia" describes the first successful steamboat navigation of Priest Rapids in the 1880's:
... a new boat, named appropriately the City of Ellensburgh, slid into the river at Pasco, and as soon as the trial runs showed that everything was all right with her, Captain Gray loaded her with cordwood and headed upstream. Priest Rapids broke the river badly, but Gray got out a line, rigged tackle, and by using the capstan, hauled the boat over the rapids to where her wheel could get a grip on Quiet water.
Columbia River Steamer John Gates 1884. Photographer unknown
If you travel between the Vernita Bridge and Sentinel Gap, make sure to check out the 430 foot high Priest Rapids Bar to the east. Photo taken from SR 243 shows Road L-SW climbing to top of the massive flood bar.
Click to view short video clips from top of east bluff and Mattawa boulder bar. I need to find better free music files ... sorry about that.
View Larger Map
Google Maps view of boulder field. Large erratic shown earlier in post is visible at bottom center (just above "2009" in credit) of aerial image. Use your mouse to control map navigation tools at top left.
The Heritage Center at Wanapum Dam provides an excellent introduction to the people who have called this area home for many years.
Great shot from Nick Zentner, showing Ellensburg IAFI field trip members spreading out to explore huge boulder field near Mattawa. Ellensburg IAFI chapter meetings and field trips are free and open to all. Visit chapter page at Ellensburg Ice Age Floods Institute.
The boulder field shown in image is part of the WDFW Columbia Basin Wildlife Area's Priest Rapids Unit.
Link to WDFW area Ownership Map
A WDFW Vehicle Use Permit is required to visit the big erratic.
1. John Wayne Trail
2. Ginkgo State Park
3. West Bar
4. Frenchman Coulee
5. Potholes Coulee
6. Priest Rapids Dam
7. Frenchman Hills
8. Lower Crab Creek Coulee
9. Dry Falls
10. Ephrata Fan
11. Potholes Reservoir
12. Drumheller Channels
13. White Bluffs
14. Hanford Site
Sentinel Gap from Grandfather Cuts Loose the Ponies sculpture.