Wednesday, February 18, 2009

Rapids, Canyons and Bars

A mid-February trip into the the Cheney-Palouse scabland tract led me to three of Bruce Bjornstad's Ice Age Floods geocache sites.

Click links to open Bruce's geocache pages.

Click any image to enlarge.

Staircase Rapids are a series of basalt cataracts and ledges between Rattlesnake Flat and Washtucna Coulee. Several excellent examples of streamlined Palouse hills are nearby. Note the flood scarped loess hills in the distance.

The streamlined hills of loess are remnants of the loess blanket that covered the entire area prior to the Ice Age Floods.

Huge rock basin cut by floodwaters below the rapids.

In his book "On the Trail of the Ice Age Floods", geologist Bruce Bjornstad quotes J Harlen Bretz:

There is a great extinct rapids extending over three prominent basalt ledges. The water of this cascade (Staircase Rapids) fell 300 feet in three miles."

Bretz (1928)

Rattlesnake Flat is pretty much featurless. The interesting scabland formations are just a short walk south of the trailhead.

This scabland resident didn't make it through the winter.

Trailhead parking on Rattlesnake Flat.

The face of the Palouse hills at left were eroded by the floods. Note the streamlined palouse hill on the right.

The scabland tract is bordered on both sides by rolling hills of Palouse loess.

I was about ready to give up! My GPS told me I was in the right place but I saw no sign of a geocache container. I knew that a cache placed by Bruce would be decent size as he always fills them with interesting pictures and descriptions of Ice Age Floods features in the immediate area. Right after I snapped this photo I noticed a sliver of white plastic in the nearby talus.

I didn't leave it like this. I covered it up just like the last visitor ... I only left a small piece in view. Don't give up!

Photo by Bruce Bjornstad

After visiting Staircase Rapids, I drove south to Washtucna Coulee, then down the coulee to the town of Kahlotus. I wanted to visit Bruce's geocache in Devil's Canyon. This canyon was cut when floodwaters overtopped the south rim of Washtucna Coulee and flowed south to the Snake River. This is very similar to the Palouse River capture a few miles east.

Unfortunatly for the railroad builders, the Ice Age Floods failed to cut the south rim of Washtucna Coulee all the way to the coulee floor. They were forced to bore a 2,000'+ tunnel through the basalt to run trains between Washtucna Coulee and Devil's Canyon. This flood cut canyon is impressive! Five miles long and dropping 400 feet from Washtucna Coulee to the Snake River in a straight line.

It's a long haul to the light at the end of this abandoned railroad tunnel. Once through the tunnel it was just a short hike and scramble up a slope to the second Bjornstad geocache of the day.

Upper Devil's Canyon. (View from road)

Off to geocache #3 near the Lake Sacajawea Flood Bar

This aerial photo by Bruce Bjornstad shows one of largest depositional features left by the Ice Age Floods. The massive Lake Sacajawea Bar is divided into two parts, a pendant bar and an eddy bar. For those that appreciate huge gravel bars ... You've got to see it to believe it ... This thing is 400 feet tall.

Another image by Bruce Bjornstad of the Lake Sacajawea Bar.

The light-colored band shown in photo is exposed at several locations along the bar. This material would have settled out during slackwater conditions.

Image by Bjornstad. This photo and caption below are part of a document Bruce has posted on the Ice Age Floods Institute site: Lake Sacajawea Flood Bar

Near the top of the eddy bar is a sequence of slackwater flood rhythmites containing the Mount St. Helens “S”ash layer, dated at 15,000 calendar years B.P. Flow-direction indicators (arrow) again suggest these deposits were laid down as the last Ice-Age floods swirled around in a large eddy at this location.

Some of the material plastered into the flood bar is colorful.

The third and final geocache of the day is located just west of the huge bar along the Snake River.

Bruce is always good about keeping the containers stocked with Ice Age Floods Institute brochures.

Blue bubbles on map below mark geocache sites. Click bubbles for more info and images.

The map is active. Use controls to move around and zoom. The Streamlined Palouse Hills are interesting if you zoom in on the Staircase Rapids area in the terrain view.

Here's another shot at the same links I posted above. Click links to learn more about each of these Ice Age Floods features.

Click links to open Bruce's geocache pages.

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