Chaffee County Rock and Mudslides - July 22,2002
Cottonwood Pass Road  (County Road 306)  and County Road 162 ("The Dip")

Mt. Princeton Map (Click to Enlarge)Between 6:00 p.m. and 7:00 p.m. on 22 July 2002 (0000-0100 UTC 23 July), a rock / mudslide occurred on the Cottonwood Pass Road (County Road 306) in the valley north of Mt. Princeton (14,197'), west southwest of Buena Vista, Colorado.  County Road 162, in the valley south of Mt. Princeton, also had significant rainfall and experienced rock and mudslides as well.  One main reason for these rock and mudslides is the type of soils and rock in this region.

(Click Map for Larger View)

 

 

Geology of the Mt. Princeton Area 

Some areas around Mt. Princeton are composed of granite and calcite.  The granite is a different composition from that of the Pikes Peak massif, being primarily a mix of quartz monzonite (quartz, triclinic feldspars (sodium, potassium, and calcium aluminum silicates), and mica.  When the granite contains very little mica, it tends to be very hard, and large rocks, giant boulders, and even solid granite cliffs are the rule.  When the granite contains quite a bit of mica, the granite tends to break up quite easily into smaller pieces.  The calcite can be a flaky, white rock, which can break apart easily and turn into a fine white powdery soil.  This calcite is what gives the lower southeast slopes of Mt. Princeton (in and near the hot springs) their white color.  This area is known as the Chalk Cliffs.  

Steep cliffs in this area contain a curious mixture of fine powdery soil, small chunks of decomposed granite, and large rocks and boulders. When an especially heavy rain occurs, the soil can become saturated quickly and will begin to flow downhill.  The embedded rocks and boulders then will be loosened and either flow (on gentler slopes) or tumble (on steeper slopes) downhill.  These rock and mudslides can develop very quickly.  Once the flows and slides cease moving and drier weather prevails, the "slop" sets up like concrete. 

Two troublesome areas exist on CR 306 and 162.  On CR 306 (see map) between the Cottonwood Pass Hot Springs Resort and one of our spotters, the terrain on the north side of the valley is  particularly steep right  down to the road.  On the evening of 22 July, there could have been between 3 and 4 inches of rain in an hour, along with large volumes of small hail on the slopes. There were several massive rock and mudslides in a one-mile long stretch.  The debris was up to 15 feet deep on the highway.  Looking again at the mountainside north of CR 306, we see steep slopes consisting of soil, pebbles, rocks, and boulders. 

 

Cliffs

Cottonwood Canyon, Looking North

Debris Fan Cottonwood Canyon, 
Looking North
Rail with Debris

County Road (CR) 306, Looking East *

The landslide covered  the road.  Once the debris hits the guard rails, it is diverted downhill along the roadway.  

 The most life threatening event during the landslide was when a minivan and camper  were inundated with  rocks and mud.  An elderly couple got stuck in their van in the debris, and were nearly crushed in their vehicle.  They had some scratches and minor hypothermia. It took around two weeks to completely clear the road.  The following are some images of the trapped van and clean up.  Photos are on CR 306, between 6 and 7 miles west southwest of Buena Vista. 

(*Photo Credits  - C. Hasselbrink, Chaffee County EM )

Van and Camper in Mud

Van and Camper in Mud

From Van - Looking East

From Van - Looking East

Van in Mud  - Looking West

Looking West

Van in Mud - from Top

Looking Down on Van

Van in Mud - View from Road

Van in Mud - View from Road

Bulldozer Digging out Debris

Bulldozer Digs out Debris

Van in Debris - Looking East

Van in Debris - Looking East

Digging out the Road

Digging out the Road - note Yellow Line

Almost to the Van

Almost to the Van...

Mudslide dug out

Road Dug out from Mudslide - Notice Debris on Either side

 

  Debris on Rail

Mud and Debris on Guardrail

The debris that made it over the guard rails tumbled into Cottonwood Creek.   After the creek gets "dammed up" for a time, it cuts through the debris.  It took Cottonwood Creek several hours to cut completely through the debris.  

The spotters property was also hard hit.  Rock and mud flowed and tumbled  down the mountainside toward his home and outbuildings.  The original mountain road (Stage Road) which came down from the summit of Cottonwood Pass in the 1800s, runs through his property. Rock and Mud flows and deep channels ran across the old road, and then tumbled onto CR 306 below. 

 

Rockslide

Cottonwood Canyon, North Side

(T. Magnuson)

Rockslide - Exposed Cable

Cottonwood Canyon, North Side

(W. Fortune)

The guard rails along CR 306 kept much of the larger rocks and tree debris banked up on the road, and allowed the debris to flow downhill on the highway.  Therefore, there was debris along a one-mile stretch of the roadway.    To give some perspective, spotter said that a rock and mudslide of this magnitude has never occurred at his place (built in 1970).  

On CR 162 (see map), west of the Mt. Princeton Hot Springs Pools, there is a very dangerous dip in the road.  The county has tried all sorts of tricks to make this dip safe:  culvert, unpaved dip, etc.  This configuration is the best solution. Looking down toward Chalk Creek the unsorted debris cascades into the creek.  

 

Entering The Dip

Entering "The Dip" on CR 162, looking Southwest

(K. Craven)

Looking Down the Dip into Chalk Creek

Looking down The Dip into Chalk Creek

(K. Craven)

Looking Up the Dip Looking up The Dip, to the Chalk Cliffs


(K. Craven)

Our spotter who lives next to the dip, says that in the 22 years he has lived there, the dip has to be cleared 2 to 3 times a year.  He has seen minor blockage of the dip, and rarely, large boulders flying across the dip at 40 mph!  The last close call at the dip occurred in 2000, when a motorist tried to go through the debris during a heavy rain.  He got out of his truck but left a passenger in the vehicle, which then tumbled down into the creek bed. Fortunately, she survived.  There are other flow and slide areas upvalley between the dip and an area called Alpine.   On CR 162 at Cascade Falls, you can see the steep, unstable cut  along the roadside. 

Cascade Falls

Looking East on CR 162

(T. Magnuson)


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.