Saturday, December 3, 2011

Huge Landslide at Whitebluffs

The White Bluffs along the east side of the Columbia River on the Hanford Reach National Monument are located across the river from the former towns of Hanford and Whitebluffs. The bluffs have been formed by the Columbia River down cutting and moving laterally against the deep silts and sands and old soil horizons that make up the bluffs. The northern portions of the bluffs are within the Hanford Reach National Monument created by President Bill Clinton.

I recently visited the northernmost slide area a few miles south of Highway 24. I had previously observed a tumble weed filled canal along Highway 24 leading towards the Hanford Reach Monument and speculated that the canal had previously been used to enhance wildlife populations but was no longer in use.

Tumble weed filled canal

The story of the canal became clearer while I was doing a bit of research on the Pleistocene and Pliocene sediments of the Pasco Basin. Indeed the canal had been used as a water conduit to create a wetland area on the Saddle Mountain National Wildlife Area in an area now part of the Hanford Reach National Monument. However, the introduction of water created perched ground water that flowed towards the steep bluffs along the river and water started to seep out at much greater volumes from seeps and springs along the white bluffs. The water greatly weakened the silt units on the bluff and a huge landslide was triggered.

Satellite overview of canal, wetland and landslide

As can be seen the slide area is two miles wide. This slide really caused some concerns about it being big enough that the river might get diverted toward some of the nuclear reactors that were used for weapons grade nuclear fuel production at the Hanford area. The water supply to the wetlands was cut off once this problem became apparent.  

View of large slide 

More typical slides into the river south of large slide
These slides were triggered by the river undermining the steep bluff

Another view with the typical slides in the foreground and the large slide in the distance
Buff colored layered silts at the top of the bluff are Missoula Flood deposits

Besides the landslides there are some other geologic features. One is that the entire White Bluffs area was submerged by flood waters from The Missoula Floods. This is one of those mind bending flood locations because the river is so far below. But as can be seen by the presence of Missoula Flood silts this was a quiet water area. The area flooded when water backed up at the narrow constriction at Wallula Gap model-of-ice-age-flood. Besides silt, the flood waters had ice bergs and the ice bergs contained rocks. Hence, while traversing across the top of the bluffs I came across angular ice rafted boulders and cobbles.

One-foot diameter granite boulder

Angular cobbles with silt.

The top of the large landslide area is lined with large sand dunes. The loose landslide sediments provide a supply of sand that is blown up the bluff face to the top of the bluff. The bluff itself acts as a barrier that causes the wind to rapidly accelerate as the air flows up and over the top edge of the bluff.

Sand dunes line the top of the bluff above the large  landslide complex

Sam and I look down the face of the dune at our shadows

Sam charges down the dune face

Sam takes in the view between runs down the dune

Mushroom growing within the dune field


David Hopkinson said...

It is such a treat to look at the landscape through your eyes, and see the history, as well as the current terrain.

jeanrw said...

Fascinating, Dan. Ditto to what David said.

Dan McShane said...

Thanks. This trip to Whitebluffs was great fun. Will have a couple more posts from that trip.